Great Reads: 06/25/08

1. Introduction -What is it? -Why learn it? -What will this tutorial teach you?
2. Memory -Number Systems -Decimal -Binary -Hexadecimal -Bits, Nybbles, Bytes, Words, Double Words -The Stack -Segment:Offset -Registers
3. Getting started -Getting an assembler -Program layout -.COM -.EXE
4. Basic ASM -Basic Register operations -Stack operations -Arithmetic operations -Bit wise operation -Interrupts
5. Tools -Debug -CodeView
6. More basics -.COM file format -Flow control operations -Loops -Variables -Arrays -String Operations -Sub-Procedures -User Input
7. Basics of Graphics -Using interrupts -Writing directly to the VRAM -A line drawing program
8. Basics of File Operations -File Handles -Reading files -Creating files -Search operations
9. Basics of Win32 -Introduction -Tools -A Message Box -A Window
Appendix A -Resources
Appendix B -Credits, Contact information, Other shit

1. Introduction================
What is it?-----------Assembly language is a low-level programming language. The syntax is nothing like C/C++, Pascal, Basic, or anything else you might be used to.
Why learn it?-------------If you ask someone these days what the advantage of assembly is, they will tell you it's speed. That might have been true in the days of BASIC or Pascal, but today a C/C++ program compiled with an optimized compiler is as fast, or even faster than the same algorithm in assembly. According to many people assembly is dead. So why bother learning it?1. Learning assembly will help you better understand just how a computer works.2. If windows crashes, it usually returns the location/action that caused the error. However, it doesn't return it in C/C++. Knowing assembly is the only way to track down bugs/exploits and fix them.3. How often do you whish you could just get rid of that stupid nag screen in that shareware app you use? Knowing a high-level language wont get you very far when you open the shit up in your decompiler and see something like CMP EAX, 7C0A4. Certain low level and hardware situations still require assembly5. If you need precise control over what your program is doing, a high level language is seldom powerful enough.6. Anyway you put it, even the most optimized high level language compiler is still just a general compiler, thus the code it produces is also general/slow code. If you have a specific task, it will run faster in optimized assembly than in any other language.7. "Professional Assembly Programmer" looks damn good on a resume.My personal reason why I think assembly is the best language is the fact that you'rein control. Yes all you C/C++/Pascal/Perl/etc coders out there, in all your fancyhigh level languages you're still the passenger. The compiler and the language itselflimit you. In assembly you're only limited by the hardware you own. You control theCPU and memory, not the otherway around.
What will this tutorial teach you?----------------------------------I tryed to make this an introduction to assembly, so I'm starting from the beginning.After you've read this you should know enough about assembly to develop graphicsroutines, make something like a simple database application, accept user input, make Win32 GUIs, use organized and reuseable code, know about different data typesand how to use them, some basic I/O shit, etc.
2. Memory==========In this chapter I will ask you to take a whole new look at computers. To many theyare just boxes that allow you to get on the net, play games, etc. Forget all that today and think of them as what they really are, Big Calculators. All a computer doesis Bit Manipulation. That is, it can turn certain bits on and off. A computer can'teven do all arithmetic operations. All it can do is add. Subtraction is achievedby adding negative numbers, multiplication is repeaded adding, and dividing is repeaded adding of negative numbers.
Number systems--------------All of you are familiar with at least one number system, Decimal. In this chapter I will introduce you to 2 more, Binary and Hexadecimal.
DecimalBefore we get into the other 2 systems, lets review the decimal system. The decimalsystem is a base 10 system, meaning that it consists of 10 numbers that are used to makeup all other number. These 10 numbers are 0-9. Lets use the number 125 as an example: Hundreds Tens UnitsDigit 1 2 5Meaning 1x10^2 2x10^1 5x10^0Value 100 20 5NOTE: x^y means x to the power of y. ex. 13^3 means 13 to the power of 3 (2197)Add the values up and you get 125.
Make sure you understand all this before going on to the binary system!
BinaryThe binary systems looks harder than decimal at first, but is infact quite a bit easiersince it's only base 2 (0-1). Remember that in decimal you go "value x 10^position" toget the real number, well in binary you go "value x 2^position" to get the answer. Sounds more complicated than it is. To better understand this, lets to some converting.Take the binary number 10110:1 x 2^4 = 160 x 2^3 = 01 x 2^2 = 41 x 2^1 = 20 x 2^0 = 0Answer: 22
NOTE: for the next example I already converted the Ax2^B stuff to the real value: 2^0 = 1 2^1 = 2 2^2 = 4 2^3 = 8 2^4 = 16 2^5 = 32 etc....
Lets use 111101:1 x 32 = 321 x 16 = 161 x 8 = 81 x 4 = 40 x 2 = 01 x 1 = 1Answer: 61
Make up some binary numbers and convert them to decimal to practise this. It is very important that you completely understand this concept. If you don't, check Appendix Bfor links and read up on this topic BEFORE going on!
Now lets convert decimal to binary, take a look at the example below:238 / 2 remainder: 0119 / 2 remainder: 159 / 2 remainder: 129 / 2 remainder: 114 / 2 remainder: 07 / 2 remainder: 13 / 2 remainder: 11 / 2 remainder: 10 / 2 remainder: 0Answer: 11101110
Lets go through this:1. Divide the original number by 2, if it divides evenly the remainder is 02. Divide the answer from the previous calculation (119) by 2. If it wont divide evenly the remainder is 1.3. Round the number from the previous calculation DOWN (59), and divide it by 2. Answer: 29, remainder: 14. Repeat until you get to 0....The final answer should be 011101110, notice how the answer given is missing the 1st 0?That's because just like in decimal, they have no value and can be omitted (023 = 23).
Practise this with some other decimal numbers, and check it by converting your answerback to binary. Again make sure you get this before going on!
A few additional things about binary:* Usually 1 represents TRUE, and 0 FALSE* When writing binary, keep the number in multiples of 4 ex. DON'T write 11001, change it to 00011001, remember that the 0 in front are not worth anything* Usually you add a b after the number to signal the fact that it is a binary number ex. 00011001 = 00011001b
HexadecimalSome of you may have notice some consistency in things like RAM for example. They seemto always be a multiple of 4. For example, it is common to have 128 megs of RAM, butyou wont find 127 anywhere. That's because computer like to use multiples of 2, 4, 8, 16, 32, 64 etc. That's where hexadecimal comes in. Since hexadecimal is base 16, it is perfect for computers. If you understood the binary section earlier, you should haveno problems with this one. Look at the table below, and try to memorize it. It's notas hard as it looks.Hexadecimal Decimal Binary0h 0 0000b1h 1 0001b2h 2 0010b3h 3 0011b4h 4 0100b5h 5 0101b6h 6 0110b7h 7 0111b8h 8 1000b9h 9 1001bAh 10 1010bBh 11 1011bCh 12 1100bDh 13 1101bEh 14 1110bFh 15 1111b
NOTE: the h after each hexadecimal number stands for
Now lets do some converting:Hexadecimal to Decimal2A4F
F x 16^0 = 15 x 1 = 154 x 16^1 = 4 x 16 = 64 A x 16^2 = 10 x 256 = 25602 x 16^3 = 2 x 4096 = 8192Answer: 10831
1. Write down the hexadecimal number starting from the last digit2. Change each hexadecimal number to decimal and times them by 16^postion3. Add all final numbers up
Confused? Lets do another example: DEADD x 1 = 13 x 1 = 13A x 16 = 10 x 16 = 160E x 256 = 14 x 256 = 3584D x 4096 = 13 x 4096 = 53248Answer: 57005
Practise this method until you get it, then move on.
Decimal to HexadecimalStudy the following example:1324
1324 / 16 = 82.75 82 x 16 = 13121324 - 1312 = 12, converted to Hexadecimal: C
82 / 16 = 5.125 5 x 16 = 8082 - 80 = 2, converted to Hexadecimal: 2
5 / 16 = 0.31250 x 16 = 05 - 0 = 5, converted to Hexadecimal: 5
Answer: 52C
I'd do another example, but it's too much of a pain in the ass, maybe some other time.
Learn this section you WILL need it!This was already one of the hardest parts, the next sections should be a bit easier
Some additional things abot hexidecimal1. It's not uncommon to say "hex" instead of "hexidecimal" even thechnicaly speaking "hex" means 6, not 16.2. Keep hexidecimal numbers in multiples of 4, adding zeros as necessary3. Most assemblers can't handle numbers that start with a "letter" because they don't know if you mean a label, instruction, etc. In that case there are a number of other ways you can express the number. The most common are: DEAD = 0DEADh (Usually used for DOS/Win) and DEAD = 0xDEAD (Usually used for *Nix based systems) Consult your assembler's manual to see what it uses.
By the way, does anyone think I should add Octal to this...?
Bits, Nibbles, Bytes, Words, Double Words-----------------------------------------Bits are the smallest unit of data on a computer. Each bit can only represent 2 numbers,1 and 0. Bits are fairly useless because they're so damn small so we got the nibble.A nibble is a collection of 4 bits. That might not seem very interesting, but rememberhow all 16 hexadecimal numbers can be represented with a set of 4 binary numbers? That's pretty much all a nibble is good for.
The most important data structure used by your computer is a Byte. A byte is the smallest unit that can be accessed by your processor. It is made up of 8 bits, or2 nibbles. Everything you store on your hard drive, send with your modem, etc is in bytes. For example, lets say you store the number 170 on your hard drive, it would looklike this:
+---+---+---+---+---+---+---+---+ 1 0 1 0 1 0 1 0 +---+---+---+---+---+---+---+---+ 7 6 5 4 3 2 1 0 H.O Nibble L.O Nibble
10101010 is 170 in binary. Since we can fit 2 nibbles in a byte, we can also referto bits 0-3 as the Low Order Nibble, and 4-7 as the High Order NibbleNext we got Words. A word is simply 2 bytes, or 16 bits. Say you store 43690, it wouldlook like this:
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 High Order Byte Low Order Byte
Again, we can refer to bits 0-7 as Low Order Byte, and 7-15 as High Order Byte.Lastly we have a Double Word, which is exactly what it says, 2 word, 4 bytes, 8 nibblesor 32 bits.
NOTE: Originally a Word was the size of the BUS from the CPU to the RAM. Today most computers have at least a 32bit bus, but most people were used to 1 word = 16 bits so they decided to keep it that way.

The Stack---------You have probably heard about the stack very often. If you still don't know what it means, read on. The stack is a very useful Data Structure (anything that holds data).Think of it as a stack of books. You put one on top of it, and that one will be the first one to come of next. Putting stuff on the stack is called Pushing, getting stufffrom the stack is called Poping. For example, say you have 5 books called A, B, C, D,and E stack on top of each other like this:ABCDENow you add (push) book F to the stack:FABCDEIf you pop the stack, you get book F back and the stack looks like this again:ABCDEThis called LIFO, Last In, First Out.So what good is all this? The stack is extremely useful as a "scratchpad" to temporarily hold data.
Segment:Offset--------------Everything on your computer is connected through a series of wires called the BUS. TheBUS to the RAM is 16 bits. So when the processor needs to write to the RAM, it doesso by sending the 16 bit location through the bus. In the old days this meant that computers could only have 65535 bytes of memory (16 bits = 1111111111111111 = 65535). That was plenty back than, but today that's not quite enough. So designers came up with a way to send 20 bits over the bus, thus allowing for a total of 1 MB of memoryIn this new design, memory is segmented into a collection of bytes called Segments, and can be access by specifying the Offset number within those segments. So if the processor wants to access data it first sends the Segment number, followed by the Offset number. For example, the processor sends a request of 1234:4321, the RAM would send back the 4321st byte in segment number 1234.This all might sound a bit complicated, but study it carefully and you should be ableto master segment:offset.The best way to picture seg:off is with a 2 dimensional array. Remember that X,Y shit you had to learn in grade 9 math?Look at the diagram below, the * is located at 4:3. The Y-axis is equal to the segment,and the X-axis is the offset.
+--+--+--+--+--+ 5 +--+--+--+--+--+ 4 * Y axis +--+--+--+--+--+ 3 +--+--+--+--+--+ 2 +--+--+--+--+--+ 1 +--+--+--+--+--+ 1 2 3 4 5 X axis
To get the physical address do this calculation:Segment x 10h + Offset = physical addressFor example, say you have 1000:1234 to get the physical address you do:1000 X 10h = 10000
10000+ 1234------ 11234*#$#$%^&**(((()())))))*&^%!@#$%^ REaD TIlL HerE ^^^^^^^^&**&^%!@#$%^&**&^%$$##@#@
This method is fairly easy, but also fairly obsolete. Starting from the 286 you can work in Protected Mode. In this mode the CPU uses a Look Up Table to compute the seg:off location. That doesn't mean that you cannot use seg x 10h + off though, you will only be limited to working in Real Mode and your programs can't access more than 1 MB. However by the time you know enough to write a program even close to this limit, you already know how to use other methods (for those comming from a 50 gig hard drive world, a program that's 1 MB is about 15x bigger than this text file is).
Registers---------A processor contains small areas that can store data. They are too small to storefiles, instead they are used to store information while the program is running.The most common ones are listed below:
General Purpose:NOTE: All general purpose registers are 16 bit and can be broken up into two 8 bit registers. For example, AX can be broken up into AL and AH. L stands for Low and H for High. If you assign a value to AX, AH will contain the first part of that value, and AL the last. For example, if you assign the value DEAD to AX, AH will contain DE and AL contains AD. Likewise the other way around, if you assign DE to AH and AD to AL, AX will contain DEADAX - Accumulator. Made up of: AH, AL Common uses: Math operations, I/O operations, INT 21BX - Base Made up of: BH, BL Common uses: Base or PointerCX - Counter Made up of: CH, CL Common uses: Loops and RepeatsDX - Displacement Made up of: DH, DL Common uses: Various data, character outputWhen the 386 came out it added 4 new registers to that category: EAX, EBX, ECX, and EDX.The E stands for Extended, and that's just what they are, 32bit extensions to theoriginals. Take a look at this diagram to better understand how this works:
EAX
+----+----+----+----+ AH AL +----+----+----+----+
AX
Each box represents 8 bitsNOTE: There is no EAH or EAL
Segment Registers:NOTE: It is dangerous to play around with these!CS - Code Segment. The memory block that stores codeDS - Data Segment. The memory block that stores dataES - Extra Segment. Commonly used for video stuffSS - Stack Segment. Register used by the processor to store return addresses from routines
Index Registers:SI - Source Index. Used to specify the source of a string/arrayDI - Destination Index. Used to specify the destination of a string/arrayIP - Instruction Pointer. Can't be changed directly as it stores the address of the next instruction.
Stack Registers:BP - Base pointer. Used in conjunction with SP for stack operationsSP - Stack Pointer.
Special Purpose Registers:IP - Instruction Pointer. Holds the offset of the instruction being executedFlags - These are a bit different from all other registers. A flag register is only 1 bit in size. It's either 1 (true), or 0 (false). There are a number of flag registers including the Carry flag, Overflow flag, Parity flag, Direction flag, and more. You don't assign numbers to these manually. The value automatically set depending on the previous instruction. One common use for them is for branching. For example, say you compare the value in BX with the value in CX, if it's the same the flag would be set to 1 (true) and you could use that information to branch of into another area of your program.
There are a few more registers, but you will most likely never use them anyway.
Exercises:1. Write down all general purpose registers and memorize them2. Make up random numbers and manually convert them into Binary and hexadecimal3. Make a 2D graph of the memory located at 0106:01004. Get the physical address of 107A:0100

3. Getting Started===================Now finally on to real assembler! Believe me, I'm getting sick of all this backgroundshit :)
Getting an Assembler--------------------There are quite a few available these days. All code in this tutorial has been testedwith TASM, so you should have no problems if you have it. A86 should also work withthise code, but I can't guarentee that.A86 - Available from: http://eji.com/a86/index.htm License: Shareware Price: A86 only - US$50+Tax/SH A86 + Manual + D86 + 32bit version of each - US$80+Tax/SH Manual - US$10+Tax/SHTASM - Available from: http://www.borland.com/borlandcpp/cppcomp/tasmfact.html License: Gotta buy it Price: US$129.95+Tax/SHThere are tons more out there, check www.tucows.com, I know they have a few. However assaid before, all programs in this tutorial have only been tested with TASM. Ifyou are low on cash, just get A86 and evaluate for longer than you're supposed to.
!@#$%^&&&&&&&&&&&&&&&&&&&&&&&&&&&**&^%$#@!!@#$%^&*&^%$#@!Program Layout--------------It is good programming practise to develop some sort of standard by which you write yourprograms. In this chapter you will learn about a layout of .COM and .EXE files that is excepted by both, TASM and A86.
.COMLets look at the source code to a very simple program:
MAIN SEGMENT ASSUME CS:MAIN,DS:MAIN,ES:MAIN,SS:MAIN
ORG 100h
START: INT 20MAIN ENDSEND START
This program does absolutely nothing, but it does it well and fast. Lots of code for something that doesn't do shit. Lets examine that more closely:
MAIN SEGMENT - Declares a segment called MAIN. A .COM file must fit into 1 segment of memory (aka. 65535 bytes - stack - PSP)
ASSUME CS:MAIN,DS:MAIN,ES:MAIN,SS:MAIN - This tells the assembler the initial values of the CS,DS,ES, and SS register. Since a .COM must fit into one segment they all point to the segment defined in the line above.ORG 100h - Since all .COM files start at XXXX:0100 you declare the entry point to be 100h. More on this later.
START: - A label
INT 20 - Returns to DOS
MAIN ENDS - Ends the MAIN segment
END START - Ends the START label
NOTE: This is the kind of layout we will use most of the time. Maybe later (Chapter 9+) We get into the next one...
Now how do you make this shit into an actual program? First, type this program out inyour favourite editor (notepad, dos edit, etc). If you have A86, just get into DOS, than into the directory A86 is in and type "a86 filename.asm". If you have TASM, get into DOS and into the tasm directory and type "tasm filename.asm", then type"tlink /t filename.obj". In both cases you will get a file called Filename.com. Moreon what .com is and does later.
*****************************************###################************************************************
.EXETake a look at the following code:
DOSSEG .MODEL SMALL .STACK 200h .DATA .CODE
START: INT 20END START
Again, his program does absolutely nothing. Lets examine each line in detail:
DOSSEG - sorts the segments in the order: Code Data StackThis is not required, but recommended while you're still learning
.MODEL SMALL - selects the SMALL memory model, available models are:TINY: All code and data combined into one single group called DGROUP. Used for .COM files.SMALL: Code is in a single segment. All data is combined in DGROUP. Code and data are both smaller than 64k. This is the standard for standalone assembly programs.MEDIUM: Code uses multiple segments, one per module. All data is combined in DGROUP Code can be larger than 64k, but data has to be smaller than 64kCOMPACT: Code is in a single segment. All near data is in DGROUP. Data can be more than 64k, but code can't.LARGE: Code uses multiple segments. All near data is in DGROUP. Data and code can be more than 64k, but arrays can't.HUGE: Code uses multiple segments. All near data is in DGROUP. Data, code and arrays can be more than 64kMost of the time you want to use SMALL to keep the code efficient.
.STACK 200h - sets up the stack size. In this case 200h bytes
.DATA - The data segment. This is where all your data goes (variables for example).
.CODE - The code segment. This is where your actually program goes.
START: - Just a label, more on that later
INT 20 - exits the program, more on that later
END START - Take a wild guess on what this does! Not required for all assemblers, I know TASM needs it, and A86 doesn't.
NOTE: For .EXE files, DON'T use the /t switch when linking!
Exercises:1. Make a program that uses the LARGE memory model, sets up a 100h long stack, and exits to DOS.

4. Basic ASM=============In this chapter we actually start making some working code
Basic Register operations-------------------------You already know what registers are, but you have yet to learn how to modify them.To assign a value to a register:
MOV DESTINATION, VALUE
For example, say you want AX to equal 56h:MOV AX,56hYou can also use another register as the value:MOV AX,BXRemember how all general purpose registers are made up of a H and a L register? Now youcan actually use that info:MOV AL,09Now AL equals 09 and AX equals 0009
The next register operator is XCHG, which simply swaps 2 registers. The syntax is:
XCHG REGISTER1, REGISTER2
For example, consider the following code:
MOV DX,56hMOV AX,3FhXCHG DX,AX
1. DX is equal to 3Fh2. AX is equal to 56h3. DX and AX get swapped and AX now equals 56h, and DX equals 3Fh
NOTE: NEVER try to exchange a 8 bit (h/l) register with a 16 bit (X)!! The following code is invalid: XCHG AH,BX
Next we got 2 simple operations, INC and DEC.INC increment a register's value and DEC decrements it.
Example:
MOV DX,50hINC DX
DX is now equal to 51h (50h + 1h = 51h).
Example:
MOV DX,50hDEC DX
DX is now equal to 4F (50h - 1h = 4Fh).

Stack operations----------------Now it's time to put that stack shit to some actual use. And it's very easy to. Thereare 6 stack operators, 2 of which you will use most of the time. The syntax is:POP REGISTERPUSH REGISTERLets say you want to temporarily store the value in AX on the stack for later use, simply do:PUSH AXNow you played around with AX and want to restore the original value:POP AX
NOTE: The stack will only accept 16 bit registers! That shouldn't be a problem though since the 16 bit registers include the value of the 8 bit.
This next bit of code does some poping and pushing, take a guess on what BX and AX areequal to at the end.
MOV AX,51hMOV BX,4Fh
XCHG AX,BX
PUSH AXMOV AX,34h
POP BX
PUSH BX
POP AX
First AX is equal to 51h and BX to 4Fh, than the 2 get exchanged. Now we gotAX = 4Fh and BX = 51h. AX gets pushed on the stack, then set to 34h:AX = 34h and BX = 51h. BX gets poped, than pushed:AX = 34h and BX = 4Fh. Finally AX gets poped. So the final result is:AX = 4Fh and BX = 4Fh.
Next we got the two variations of the stacks registers, POPF and PUSHF. These twoplace the flag register on the stack. Sounds more complicated than POP and PUSH, butit's actually easier. The syntax is:POPFPUSHFNo operand is required. For example, say you want AX to hold the current flag registervalue:
PUSHFPOP AX
PUSHF puts it on the stack, POP AX places it into AX.
The last two stack operators are PUSHA and POPA.PUSHA puts all general purpose registers on the stackPOPA retrieves all general purpose registers from the stackNOTE: These 2 are 32bit instructions, so they only work on a 386+ and will not work with .COM files.
Example:MOV AX,1hMOV BX,2hMOV CX,3hMOV DX,4h
PUSHA
MOV AX,5hMOV BX,6hMOV CX,7hMOV DX,8h
POPA
At the end of this program, all registers are restored to their initial value
Practise some of these instructions! If you make a program containing everything you've learned so far it won't do anything, but if it doesn't crash it most likely worked. So code some simple programs and play around with the values and registers.

Arithmetic operations---------------------Everyone loves arithmetic. Especially if you do it in hex or binary. For those who don't know what arithmetic is, it's just adding and subtracting. Multiplying and dividing are really just repeated additions and subtractions. So in short, it's a fancy name for grade 3 math. In this chapter I will introduce you to the 4 basic arithmetic operators, ADD, SUB, MUL, DIV . There are a few more that I will cover later.Lets start with ADD. The syntax is:ADD REGISTER1, REGISTER2ADD REGISTER, VALUE
Example 1:MOV AX,5hMOV BX,4Fh
ADD AX,BX
This adds AX and BX and stores the resulting value in AX. So after running this program AX = 9h
Example 2:MOV AX,5h
ADD AX,4Fh
The result is the same as in example 1. AX is set to 5h, and 4Fh is added to it.
Now lets go on to SUB. The syntax is:SUB REGISTER1, REGISTER2SUB REGISTER, VALUE
Example 1:z
This will subtract the value of BX from the value of AX. In this case the result wouldbe 4A.
NOTE: If you still don't completely get hexadecimal, you can easily check this by converting 5, 4F, and 4A to decimal. 4F = 79 4A = 74 5 = 5
As with ADD you can also use a value:MOV BX,4Fh
SUB BX,5h
Which leaves you with BX = 4A
Next in line is the MUL operator. Syntax:MUL REGISTER
Notice that only one operant is required. That's because the processor assumes that you want to multiply the give register with AX or AH.
Example:MOV AX,5hMOV BX,4Fh
MUL BX
This leaves AX equal to 18B (4Fh x 5h = 18B). Notice that the result is stored in AX,or AH, depending on what was used for the operation.
Finally we have the DIV operator. Syntax:DIV REGISTER
Just like the MUL operator, there is only one operand, and AX is assumed to be the second one. Example:MOV AX,5hMOV BX,4Fh
DIV BX
Now AX equals Fh since 4Fh / 5h = Fh.NOTE: The result is rounded to the next lowest number: 4Fh = 79 5h = 5 79 / 5 = 15.8 15 = Fh
NOTE: For now it's fine if you use MUL and DIV, but they are very slow operators. That means if you need speed (in graphics for example), NEVER use MUL/DIV! You can use Shifting combined with addition/subtraction to achieve code that can sometimes be 3000% faster! However shifting is a bit difficult to understand if you don't know much about assembly yet, I will completly discuss them in the graphics part of this tutorial.

Bit wise operation-----------------Sounds hard but is very easy. There are 4 bit wise operators: AND, OR, XOR, and NOT.What these do is compare two values bit for bit. This can be extremely useful!
AND syntax:AND REGISTER1, REGISTER2AND REGISTER, VALUE
AND returns 1 (TRUE) only if BOTH operands are 1 (TRUE)
Example 1:MOV AX,5hMOV BX,6h
AND AX,BX
The result is stored in AX. So for this example AX = 4. Lets look at that result moreclosely:5h = 101b6h = 110b
101b110b---100b
100b = 4h
Example 2:MOV AX,5h
AND AX,6h
The result is the same as in Example 1 (AX = 4h).
AND truth table:0 AND 0 = 01 AND 0 = 00 AND 1 = 01 AND 1 = 1
OR syntax:OR REGISTER1, REGISTER2OR REGISTER, VALUE
OR returns 1 (TRUE) if either operand is 1 (TRUE).
Example 1:MOV AX,5hMOV BX,6h
OR AX,BX
AX is now equal to 7h
5h = 101b6h = 110b
101b110b----111b
111b = 7h
OR truth table:0 OR 0 = 01 OR 0 = 10 OR 1 = 11 OR 1 = 1
XOR syntax:XOR REGISTER1, REGISTER2XOR REGISTER, VALUE
XOR returns 1 (TRUE) if one or the other operand is 1 (TRUE), but not both
Example:MOV AX,5hMOV BX,6h
XOR AX,BX
AX is now equal to 3h
5h = 101b6h = 110b
101b110b----011b
11b = 3h
XOR truth table:0 XOR 0 = 01 XOR 0 = 10 XOR 1 = 11 XOR 1 = 0
And finally we have NOT. NOT is the easiest one as it simply inverts each bit.
NOT syntax:NOT REGISTERNOT VALUE
Example:MOV AX,F0h
NOT AX
AX is now equal to F since
F0h = 11110000Invert it:00001111which is:F
NOTE: The windows calculator won't work for this, do it by hand.
NOT truth table:NOT 1 = 0NOT 0 = 1

Interrupts---------Interrupts are one of the most useful things in assembly. An interrupt is just what itsays, a interruption to the normal execution of a program. The best way to illustrate this is one of those "Press any key to continue" things. The program is running but when you press a key it stops for a split second, check what key you pressed and continues. This kind of interrupt is known as a Hardware Interrupt because it uses hardware (the keyboard). The kind of interrupts you will use in your assembly programsare know as Software Interrupts because they are caused by software, not hardware. An example of a software interrupt is reading and writing to a file. This is a DOS interrupt because it is done by DOS, than there are other interrupts done by other things. For example your BIOS or Video Card all have build in interrupts at your exposure. So how does the computer know what interrupt is what? Each interrupt is assigned a number and stored in the Interrupt Vector Table (IVT for short). The IVT islocated at 0000:0000 (remember the segment:offset shit. This location would be the origin if plotted on a 2D graph). All interrupt handlers are 1 DWORD in size (double word, 32bit, or 4 bytes). So the handler for interrupt 1h can be found at 0000:0004 (since it's a DWORD it goes up by 4 bytes). The most common interrupt is 21h and can be found at 0000:0084.
So how do you use interrupts? Very simple:INT interrupt
For example, in the Program Layout section earlier the program contain the lineINT 20hThe interrupt 20h returns to DOS.
Some interrupts like this one only have one function, but other have many more. So howdoes the operating system know what function you want? You set the AX register up.
Example: MOV AH,02MOV DL,41
INT 21INT 20
This program is quite amazing. It prints the character A. Lets make it even betterby plugging it into our layout:
MAIN SEGMENT ASSUME DS:MAIN,ES:MAIN,CS:MAIN,SS:MAIN
START: MOV AH,02h MOV DL,41h
INT 21h INT 20hMAIN ENDSEND START
Save it and assemble it. Refer back to chapter 2 if you forgot how to do that.So what is happening here?First it does the familiar set up, than it set AH to 02, which is the character outputfunction of interrupt 21. Then it moves 41 into DL, 41 is the character A. Finallyit calls interrupt 21 which displays the A and quits with interrupt 20.How do you know what you have to set all those registers to? You get a DOS interruptlist. Check Appendix B for urls.Quite an accomplishment there, after reading 970 lines of boring text you can finallymake a 11 line program that would take 1 line to do in Perl! Pad yourself on that backand lets move on.
Exercises:1. Make a program that gets the value from AX, puts it into DX and BX, then multiplies the values in DX and BX and stores the result in CX. This sounds easier than it really is, use the stack to help you out.2. Make a program that prints out the string ABC, than quits to DOS Hint: A = 41, B = 42, C = 433. Make a program that performs ALL bit wise operations using the values 5h and 4Fh

5. Tools=========Throughout this tutorial you have been using no software other than your assembler.In this chapter you will learn how to master other software that can be of tremendoushelp to you.

Debug-----Lets start with something that's not only very useful, but also free and alreadyon your computer.Get into dos and type "debug", you will get a prompt like this:-now type "?", you should get the following response: assemble A [address] compare C range address dump D [range] enter E address [list] fill F range list go G [=address] [addresses] hex H value1 value2 input I port load L [address] [drive] [firstsector] [number] move M range address name N [pathname] [arglist] output O port byte proceed P [=address] [number] quit Q register R [register] search S range list trace T [=address] [value] unassemble U [range] write W [address] [drive] [firstsector] [number] allocate expanded memory XA [#pages] deallocate expanded memory XD [handle] map expanded memory pages XM [Lpage] [Ppage] [handle] display expanded memory status XS
Lets go through each of these commands:
Assemble:-a107A:0100At this point you can start assembling some programs, just like using a assembler. However the debug assembler is very limited as you will probably notice. Lets tryto enter a simple program:-a107A:0100 MOV AH,02107A:0102 MOV DL,41107A:0104 INT 21107A:0106 INT 20-gAProgram terminated normally
That's the same program we did at the end of the previous chapter. Notice how yourun the program you just entered with "g", and also notice how the set-up part is not there? That's because debug is just too limited to support that.Another thing you can do with assemble is specify the address at which you want to start,by default this is 0100 since that's where all .COM files start.
Compare:Compare takes 2 block of memory and displays them side by side, byte for byte. Lets doan example. Quite out of debug if you haven't already using "q".Now type "debug c:\command.com"-c 0100 l 8 020010A3:0100 7A 06 10A3:0200This command compared offset 0100 with 0200 for a length of 8 bytes. Debug respondedwith the location that was DIFFERENT. If 2 locations were the same, debug would just omit them, if all are the same debug would simply return to the prompt without anyresponse.
Dump:Dump will dump a specified memory segment. To test it, code that assembly program again:C:\>debug-a107A:0100 MOV AH,02107A:0102 MOV DL,41107A:0104 INT 21107A:0106 INT 20-d 0100 l 8107A:0100 B4 02 B2 41 CD 21 CD 20 ...A.!.
The "B4 02 B2 41 CD 21 CD 20" is the program you just made in machine language.B4 02 = MOV AH,02B2 41 = MOV DL,41CD 21 = INT 21CD 20 = INT 20The "...A.!." part is your program in ASCII. The "." represent non-printable characters.Notice the A in there.
Enter:This is one of the hard commands. With it you can enter/change certain memory areas.Lets change our program so that it prints a B instead of an A.-e 0103 <-- edit program at segment 0103107A:0103 41.42 <-- change 41 to 42-gBProgram terminated normally-Wasn't that amazing?
Fill:This command is fairly useless, but who knows....It fills the specified amount of memory with the specified data. Lets for example clearout all memory from segment 0100 to 0108, which happens to be our program.-f 0100 l 8 0 <-- file offset 0100 for a length of 8 bytes with 0-d 0100 l 8 <-- verify that it worked107A:0100 00 00 00 00 00 00 00 00 .......Yep, it worked.
Go:So far we used go (g) to start the program we just created. But Go can be used for muchmore. For example, lets say we want to execute a program at 107B:0100:-r CS <-- set the CS register to point to 107BCS 107A:107B-g =100You can also set breakpoints.-a <-- enter our original program so we have something107A:0100 MOV AH,02 to work with107A:0102 MOV DL,41107A:0104 INT 21107A:0106 INT 20-g 102 <-- set up a break point at 107A:0102At this point the program will stop, display all registers and the current instruction.
Hex:This can be very useful. It subtracts and adds to hexadecimal values:-h 2 10003 0001 <-- 2h + 1+ = 3h and 2h - 1h = 1hThis is very useful for calculating a programs length, as you will see later.
Input:This is one of the more advanced commands, and I decided not to talk about it too muchfor now. It will read a byte of data from any of your computers I/O ports (keyboard,mouse, printer, etc).-i 3FD60-Your data may be different.In case you want to know, 3FD is Com port 1, also known as First Asynchronous Adapter.
Load:This command has 2 formats. It can be used to load the filename specified with thename command (n), or it can load a specific sector.-n c:\command.com-lThis will load command.com into debug. When a valid program is loaded all registers will be set up and ready to execute the program.The other method is a bit more complicated, but potential also more usefull. The syntaxis L

-l 100 2 10 20This will load starting at offset 0100 from drive C (0 = A, 1 = B, 2 = C, etc), sector10h for 20h sectors. This can be useful for recovering files you deleted.
Move:Move takes a byte from the starting address and moves it to the destination address.This is very good to temporary move data into a free area, than manipulate it withouthaving to worry about affecting the original program. It is especially useful ifused in conjunction with the r command to which I will get later. Lets try an example:-a <-- enter our original program so we have something107A:0100 MOV AH,02 to work with107A:0102 MOV DL,41107A:0104 INT 21107A:0106 INT 20-m 107A:0100 L 8 107B:0100 <-- more 8 bytes starting from 107A:0100 into 107B:0100-e 107B:0103 <-- edit 107B:0103107B:0103 41.42 <-- and change it 42 (B) -d 107A:0100 L 8 <-- make sure it worked107A:0100 B4 02 B2 41 CD 21 CD 20 ...A.!.-d 107B:0100 L 8107A:0100 B4 02 B2 42 CD 21 CD 20 ...B.!.-m 107B:0100 L 8 107A:0100 <-- restore the original program since we like the changes.
Name:This will set debug up with a filename to use for I/O commands. You have to includethe file extension, and you may use addition commands:-n c:\command.com
Output:Exactly what you think it is. Output sends stuff to an I/O port. If you have anexternal modem with those cool lights on it, you can test this out. Find out what portyour modem is on and use the corresponding hex number below:Com 1 = 3F8 - 3FF (3FD for mine)Com 2 = 2F8 - 2FFCom 3 = ??? - ??? (if someone knows, please let me know, I would assume though that it's0F8 - 0FF.)Now turn on the DTA (Data Terminal Ready) bit by sending 01h to it:-o XXX 1 <-- XXX is the com port in hexAs soon as you hit enter, take a look at your modem, you should see a light light up.You can have even more fun with the output command. Say someone put one of those BIOSpasswords on "your" computer. Usually you'd have to take out the battery to get rid ofit, but not anymore:AMI/AWARD BIOS-o 70 17-o 71 17
QPHOENIX BIOS-o 70 FF-o 71 17
QGENERIC-o 70 2E-o 71 FF
These commands will clear the BIOS memory, thus disabling the password. Please notehowever that these are fairly old numbers and BIOS makes constantly change them, sothey might not work with your particular BIOS.
Proceed:Proceeds in the execution of a program, usually used together withy trace, which I will cover later. Like the go command, you can specify an address from which to startusing =address-p 2Debug will respond with the registers and the current command to be executed.
Quite:This has got to be the most advanced feature of debug, it exits debug!-q
Register:This command can be used to display the current value of all registers, or to manuallyset them. This is very useful for writing files as you will see later on. -r AXAX: 011B:5-
Search:Another very useful command. It is used to find the occurrence of a specific byte, orseries of bytes in a segment. The data to search for can by either characters, or ahex value. Hex values are entered with a space or comma in between them, and charactersare enclosed with quotes (single or double). You can also search for hex and characterswith the same string:-n c:\command.com <-- load command.com so we have some data to search in-l-s 0 l 0 "MS-DOS" <-- search entire memory block for "MS-DOS"10A3:39E9 <-- found the string in 10A3:39E9NOTE: the search is case sensitive!
Trace:This is a truly great feature of debug. It will trace through a program one instructionat a time, displaying the instruction and registers after each. Like the go commandyou can specify where to start executing from, and for how long.-a <-- yes, this thing again107A:0100 MOV AH,02107A:0102 MOV DL,41107A:0104 INT 21107A:0106 INT 20-t =0100 8If you leave out the amount of instructions that you want to trace, you can use the proceed (p) to continue the execution as long as you want.
Unassemble:Unassembles a block of code. Great for debugging (and cracking)-u 100 L 8 <-- unassembles 8 bytes starting at offset 100107A:0100 MOV AH,02 <-- debut's response107A:0102 MOV DL,41107A:0104 INT 21107A:0106 INT 20

Write:This command works very similar to Load. It also has 2 ways it can operate: using name,and by specifying an exact location. Refer to back to Load for more information.NOTE: The register CX must be set the file size in order to write!NOTE: Write will not write files with a .EXE or .HEX extension.
Enough about debug, lets move on to CodeView.

CodeView--------CodeView is another program that might come in handy sometimes. However it is not free.There are many debuggers similar to CodeView out there, but it is enough for you to understand one.CodeView has a number of different windows, Help, Locals, Watch, Source 1, Source 2, Memory 1, Memory 2, Registers and a few more, depending on the version number.
The Source WindowsSource 1 and 2 let you view 2 different source code segments at the same time. This isvery useful for comparing.
Memory WindowsThese windows let you view and edit different sections of memory. On the left sideyou have the memory location in segment:offset form, in the middle the hex value of theinstructions, and on the right side the ASCII value. Again, non-printable charactersare represented by a ".". You can switch between multiple menus using F6. You can also press Shift+F4 to switch between hexadecimal, ASCII, words, double words, signedintegers, floating values, and more.
RegisterThis menu lets you view and change the value in each register. The FL register nearthe bottom stands for Flags. At the very bottom you should see 8 different values.They are the specific flag values.OV/NV = Overflow (OVerflow/No oVerflow)DN/UP = Direction (DowN/UP)DI/EI = Interrupt (????)PL/NG = Sign (????)NZ/ZR = Zero (Not Zero/ZeRo)NA/AC = Auxiliary Carry (No Auxiliary carry/Auxiliary Carry)PO/PE = Parity (????)NC/CY = Cary (????)
CommandThis window lets you pass commands to CodeView. I will not explains these as they arealmost identical to the ones Debug uses, however a bit more powerful.
This chapter went through a lot of material. Make sure you actually get it all, or atleast most of it. Debug will be insanely useful later on, so learn it now! The keyis practise, lots of practise!
Exercises:1. Make a program that prints an A on the screen using debug, save it to C drive as cow.com. Quite debug and delete it. Now get back into debug and restore it again. HINT: If you delete a file in DOS, DOS simply changes the first character to E5
It's not as hard as it sounds, basically here's what you do: I) Load as many sectors of your drive as you think you will need II) Search those sectors for the hex value E5 and the string "ow" III) Dumb the offset of the location the search returned IV) Edit that offset and change the E5 instruction to a letter of your choice (41) V) Write the sectors you loaded into RAM back to C drive2. Use debug to get your modem into CS (Clear to Send) mode. The hex value is 2.3. Make a program called cursor.com using debug that will change the cursor size. I) Move 01 into AH II) Move 0007 into CX III) Call interrupt 10 IV) Call interrupt 20

6. More basics===============Before reading this chapter, make sure you completely understood EVERYTHING I talked about so far.
.COM File Format----------------COM stands for COre iMage, but it is much easier to memorize it as Copy Of Memory, as that description is even better. A COM file is nothing more than a binary image of what should appear in the RAM. It was originally used for the CP/M and even though CP/M were used in the Z80/8080 period, COM files have still the same features as they did back in the 70's. Let's examine how a COM file is loaded into memory:1. You type in the file name, DOS searches for filename + .com, if found that file gets executed. If not DOS will search for filename + .exe, if it can't find that it will search for filename + .bat, and if that search fails it will display the familiar "Bad command or filename" message.2. If it found a .com file in step 1, DOS will check its records and make sure that a 64k block of memory is found. This is necessary or else the new program could overwrite existing memory.3. Next DOS builds the Program Segment Prefix. The PSP is a 256 byte long block of memory which looks like the table below: Address Description 00h-01h Instructions to terminate the program, usually interrupt 20h 02h-03h Segment pointer to next available block 04h Reserved, should be 0 05h-09h Far call to DOS dispatcher 0Ah-0Dh INT 22h vector (Terminate program) 0Eh-11h INT 23h vector (Ctrl+C handler) 12h-15h INT 24h vector (Critical Error) 16h-17h PSP segment of parent process 18h-2Bh Pointer to file handler 2Ch-2Dh DOS environment segment 2Eh-31h SS:SP save area 32h-33h Number of file handles 34h-37h Pointer to file handle table 40h-41h DOS version 5Ch-6Bh File control block 1 6Ch-7Bh File control block 2 7Ch-7Fh Reserved 80h Length of parameter string 81h-FFh Default DTA (Disk Transfer Area)4. DS,ES, and SS are set to point to block of memory5. SP is set to FFFFh6. 0000h is pushed on the stack (stack is cleared)7. CS is set to point to memory (segment), IP is set to 0100h (offset, remember debug?)
The PSP is exactly 255 bytes long, meaning that to fit into one segment (aka. to be a valid.com file your program cannot be larger than 65280 bytes). However as I mentioned before, by the time you can code a program in assembly that is that large, you already know well more than enough to make a .EXE file.So what do you need this information for? Well like all other memory, you can viewand edit the PSP. So you could play around with it. For example, later when we getinto file operations you will be working with the DTA. Or maybe you need to know theDOS version, you can just check 40h-41h, etc.

Flow control operations-----------------------Flow control operations are just what the name says, operations that control the flowof your program. If you have worked with another language before, those are the if/thenstatements. From what you've hear about assembly, you might think that this is fairlydifficult, but it's not. To do the equivalent of a if/then I will have to introduce youto 3 new things, labels, the compare command and jump instructions. First things first,maybe you recall the simple program that prints A from the interrupts section. Notice how our layout contains the line START:? START: is a label. If you come from C/C++/Pascal you can think of a label almost like a function/procedure. Take a look at the following code, by now you should know what's happening here:
MAIN SEGMENT ASSUME CS:MAIN,DS:MAIN,SS:NOTHING
ORG 100h
START: INT 20
MAIN ENDSEND START
Notice the line that saying START:, that's a label. So what's the point of putting labels in your code? Simple, you can easily jump to any label in your program using the JMP operator. For example, consider the following code:
MAIN SEGMENT ASSUME CS:MAIN,DS:MAIN,ES:MAIN,SS:MAIN
ORG 100h
START: MOV AH,02h MOV DL,41h INT 21h
JMP EXIT
MOV AH,02h MOV DL,42h INT 21h
EXIT: INT 20h
MAIN ENDSEND START
First the program prints an A using the familiar routine, but instead of using INT 20hto exit, it jumps to the label EXIT and calls INT 20h from there. The result is that itcompletely skips everything after the JMP EXIT line, the B doesn't get printed at all.So by using a label you can easily close the program from any location.This is fairly useless so far though. It gets interesting when you start using some of the other jump commands. I will only explain a few here as there are just too many.Below is a alphabetical list of most of them:JA - Jump if AboveJAE - Jump if Above or EqualJB - Jump if BelowJBE - Jump if Below or EqualJC - Jump on CarryJCXZ - Jump if CX is ZeroJE - Jump if EqualJG - Jump if GreaterJGE - Jump if Greater than or EqualJL - Jump if Less thanJLE - Jump if Less than or EqualJMP - Jump unconditionallyJNA - Jump if Not AboveJNAE - Jump if Not Above or EqualJNB - Jump if Not BelowJNE - Jump if Not EqualJNG - Jump if Not GreaterJNGE - Jump if Not Greater or EqualJNL - Jump if Not LessJNLE - Jump if Not Less or EqualJNO - Jump if No OverflowJNP - Jump on No ParityJNS - Jump on No SignJNZ - Jump if Not ZeroJO - Jump on OverflowJP - Jump on ParityJPE - Jump on Parity EvenJPO - Jump on Parity OddJS - Jump on SignJZ - Jump on Zero
Some of these are fairly self-explanatory (like JCXZ), but others require some moreexplanation. What is being compared to what, and how does the jump know the result?Well almost anything can be compared to almost anything. The result of that comparisonis stored in the flags register. The jump command simply checks there and responseaccordingly. Let's make a simple if/then like structure:
MAIN SEGMENT ASSUME CS:MAIN,DS:MAIN,ES:MAIN,SS:MAIN
ORG 100h
START: MOV DL,41h MOV DH,41h CMP DH,DL JE TheyAreTheSame JMP TheyAreNotSame
TheyAreNotSame: MOV AH,02h MOV DL,4Eh INT 21h INT 20h
TheyAreTheSame: MOV AH,02h MOV DL,59h INT 21h INT 20h
This code is fairly straight forward, it could be expressed in C++ as:void main () {int DH,DLDL = 41DH = 41if (DH == DL) { cout << "Y"; } else { cout << "N";}
In this case the program will return Y, but try changing either DH, or DL to some othervalue. It should display N.
HINT: Tired of constantly typing "tasm blah.asm", "tlink /t blah.obj"? Make a simple batch file containing the following 3 lines and save it as a.bat in your tasm dir. @ECHO OFF TASM %1.ASM TLINK /T %1.OBJ Now you can just type "a blah", even without the file extension. If you have A86 and are sick of typing "a86 blah.asm", just rename a86.exe to something like a.exe.
Loops-----Loops are a essential part of programming, in fact loops make the difference betweenbeing a programming language and being something like HTML. If you don't know what loops are, they are just the repeaded execution of a block of code. The 2 most common types of loops are For and While. A for loop repeads a block of code until a certain condition is met. Look at thefollowing C++ code:
main(){ for (int counter = 0; counter < 5; counter++) { cout << "A"; }return 0;
This will produce the following output:AAAAA
This code is fairly easy, it initializes a variable and sets it equal to zero. It willloop until the varialbe is less than 5, and after each execution the variable getsincremented. Now lets make an exact copy of that program in assembly:
blah segmentassume cs:blah, ds:blah, ss:blah, es:blah, ss:blah ;do the usual setuporg 0100h
start: ;label for start of program MOV CX,5 ;cx is always the counter
LOOP_LABEL: ;label to loop MOV AH,02h ;do the familiar A shit (this printed some wierd MOV DL,41h ;character instead for me, anyone know why?) INT 21h
LOOP LOOP_LABEL ;loop everything in between loop_label: and the ;loop statement as many times as specified in CX INT 20h ;usual ending shitblah endsend Start
Output should be:AAAAAC:\>But as I said, it printed some other shit for me. Well who cares, as long as it looped.This code is basicly doing this:1. Set CX to 52. Print an A3. Check of CX = 0, if not decrement CX4. Go back to loop_label5. Check if CX = 0, if not decrement CX6. etc
Next we have the While loop. It also repeads a block of code as long as a condition is true, but the condition is not changed during in the loop declaration as with the Forloop. Take a look at a simple C++ While loop:
main(){ int counter = 0; while(counter < 5) { counter++ cout << "A"; }return 0;
Notice how the condition is being changed in the actual loop. This is very important asyou may already know. Let's convert that piece of code to assembly:
blah segmentassume cs:blah, ds:blah, ss:blah, es:blah ;do the usual setuporg 0100h
start: MOV CX, 5 ;set CX equal to 5loop_label: MOV AH,02h ;print the A MOV DL,41h INT 21h
DEC CX ;decrement CX CMP CX,0 ;check if CX is zero JNZ loop_label ;no? go back to loop_label
INT 20h ;yes? terminate program ;usual ending shitblah endsend Start
Output:AAAAAC:\>
They look almost identical, so what's different? And why use em? Well the For loop isgood for loops that have a set number of repetitions, while the while loop can changethe amount or repedition during the loop execution. This is useful for user input for example. It is also possible to make a for loop without using the loop statement, just like you would do a while loop. That might not look as pretty, but it canpotentially be a bit faster.
Sometimes interrupts can modify the CX register, in which case your loop would loop anunpredictable number of times. That's not good. To stop that you can make use of thestack:
MAIN SEGMENT ASSUME CS:MAIN,DS:MAIN,ES:MAIN,SS:MAIN
ORG 100h
START: MOV CX,5
Loop_Label: PUSH CX ;store CX on the stack MOV AH,02h MOV DL,41h INT 21h POP CX ;restore it after the pass is completed
LOOP Loop_Label INT 20h ;usual ending shitMAIN ENDSEND START

Variables---------Yeah, yeah, yeah, I know there are no variables in assembly, but this is close enough for me.You may be familiar with variables if you've come from another language, if not variables are simply a name given to a memory area that contains data. To access that data you don't have to specify that memory address, you can simply refer to that variable. In this chapter I will introduce you to the 3 most common variables types: bytes, words, and double words. You declare variables in this format:NAME TYPE CONTENTSWere type is either DB (Declare Byte), DW (Declare Word), or DD (Declare DoubleWord).Variables can consist of number, characters and underscores (_), but must begin witha character.
Example 1:A_Number DB 1
This creates a byte long variable called A_Number and sets it equal to 1
Example 2:A_Letter DB "1"
This creates a byte long variable called A_Letter and sets it equal to the ASCII value1. Note that this is NOT a number.
Example 3:Big_number DD 1234
This declares a Double Word long variable and sets it equal to 1234.
You can also create constants. Constants are data types that like variables can be usedin your program to access data stored at specific memory locations, but unlike variablesthey can not be changed during the execution of a program. You declare constants almostexactly like variables, but instead of using D?, you use EQU. Well actually EQUdeclares a Text Macro. But since we haven't covert macros yet and the effect is basicly the same, we will just tread it as a constant.
Example 4:constant EQU DEADh
So how do you use variables and constants? Just as if they were data. Take a look at the next example:
Example 5:constant EQU 100mov dx,constantmov ax,constantadd dx,ax
This declares a constant called constant and sets it equal to 100, then it assigns thevalue in constant to dx and ax and adds them. This is the same asmov dx,100mov ax,100add dx,ax
The EQU directive is a bit special though. It's not really a standard assembly instruction. It's assembler specific. That means that we can for example do thefollowing:
bl0w EQU PUSHsUcK EQU POP
bl0w CXsUcK CX
When you assemble this, the assembler simply substitues PUSH and POP with every occurance of bl0w and sUcK respectivly.
Arrays------Using this knowledge it is possible to create simple arrays.Example 1:A_String DB "Cheese$"
This creates a 5 byte long array called A_String and sets it equal to the string Cheese. Notice the $ at the end. This has to be there, otherwise your CPU will start executing instructions after the last character, which is whatever is in memory at that particular location. There probably won't be any damage done, but who knows what's hidden in those dark corners... To use quotes (single or double) within a string you can use a littletrick:
Example 2:Cow DB 'Ralph said "Cheese is good for you!"$'orCow DB "Ralph said 'Cheese is good for you!'$"Use whichever you think looks better. What if you have to use both types of quotes?
Example 3:Cow DD 'Ralph said "I say: ""GNAAAARF!""$'Use double double/single quotes.
What if you don't know what the variable is going to equal? Maybe it's user-inputed.
Example 4:Uninitialized_variable DB ?
Now lets use a variable in an actual program:
MAIN SEGMENT ASSUME CS:MAIN,DS:MAIN,ES:MAIN,SS:MAIN
ORG 100h
START:COW DB "Hello World!$" MOV AH,09h MOV DX,OFFSET COW INT 21h INT 20hMAIN ENDSEND START
Yes, even in assembly you finally get to make a Hello World program! Here we're using interrupt 21h, function 9h to print a string. To use this interruptyou have to set AH to 9h and DX must point to the location of the string.
NOTE: VERY important! ALWAYS declare unitialized arrays at the VERY END of your program, or in a special UDATA segment! That way they will take up no space at all, regardless of how big you decide to make them. For example say you have this in a program: Some_Data DB 'Cheese' Some_Array DB 500 DUP (?) More_Data DB 'More Cheese' This will automaticly add 500 bytes of NULL characters to your program. However if you do this instead: Some_Data DB 'Cheese' More_Data DB 'More Cheese' Some_Array DB 500 DUP (?) Your program will become 500 bytes smaller.
String Operations-----------------Now that you know some basics of strings, let's use that knowledge. There are a number of string operations available to you. Here I will discuss 4 of them.Lets start with MOVSB. This command will move a byte from one location to another.The source destination is ES:SI and the destination is DS:DI.
Example 1:MAIN SEGMENT ASSUME CS:MAIN,DS:MAIN,ES:MAIN,SS:MAIN
ORG 100h
START: MOV AH,9
MOV DX,OFFSET NEWSTRING INT 21h
MOV DX,OFFSET OLDSTRING INT 21h
MOV CX,9 LEA SI,OLDSTRING LEA DI,NEWSTRING
REP MOVSB
MOV DX,OFFSET NEWSTRING INT 21h
MOV DX,OFFSET OLDSTRING INT 21h
INT 20hOLDSTRING DB 'ABCDEFGHI $'NEWSTRING DB '123456789 $'MAIN ENDSEND START
Output:ABCDEFGHI 123456789 ABCDEFGHI ABCDEFGHI
This little example has a few instructions that you haven't seen before, so letsgo through this thing step by step.1. We do the regular setup2. We use the method from the previous section to print NEWSTRING (ABCDEFGHI)3. We print OLDSTRING (123456789)4. We set CX equal to 9. Remeber that the CX register is the counter.5. Here's a new instruction, LEA. LEA stands for Load Effective Address. This instruction will load the contents of a "variable" into a register. Since DI contains the destination and SI the source, we assign the location of NEWSTRING and OLDSTRING to them respectivly6. MOVSB is the string operator that will move a byte from SI to DI. Since we have an array of 9 characters (well 10 if you count the space, but that is the same in both anyway) we have to move 9 bytes. To do that we use REP. REP will REPeat the given instruction for as many times as specified in CX. So REP MOVSB will perform the move instruction 9 times, ones for each character.7. To see our result we simple print each string again using the same code we used in step 2 and 3.
The next string operator is not only very easy to use, but also very useful. It will scan a string for a certain character and set the EQUAL flag bit if the searchwas successful. The operator is SCASB, the location of the string is in DI, andthe character is stored in AL.
Example 2:MAIN SEGMENT ASSUME CS:MAIN,DS:MAIN,ES:MAIN,SS:MAIN
ORG 100h
START: MOV CX,17h LEA DI, STRING MOV AL, SEARCH
REPNE SCASB
JE FOUND JNE NOTFOUND
NOTFOUND: MOV AH,09h MOV DX,OFFSET NOTFOUND_S INT 21h INT 20h
FOUND: MOV AH,09h MOV DX,OFFSET FOUND_S INT 21h INT 20h
SEARCH DB '!'STRING DB 'Cheese is good for you!'FOUND_S DB 'Found$'NOTFOUND_S DB 'Not Found$' MAIN ENDSEND START
This should be fairly easy to figure out for you. If you can't, I'll explain it:1. We do the usual setup2. We set CX equal to 17h (23 in decimal), since our string is 17h characters long3. We load the location of STRING into DI4. And the value of the constant SEARCH into AL5. Now we repeat the SCASB operation 23x6. And use a jump to signal wether or not we found the string
Finally we have the CMPS instruction. This operator will compare the value of twostrings with each other until they're equal.
Example 3:MAIN SEGMENT ASSUME CS:MAIN,DS:MAIN,ES:MAIN,SS:MAIN
ORG 100h
START: MOV CX,17h LEA SI,STRING1 LEA DI,STRING
REP CMPSB JE EQUAL JNE NOTEQUAL
NOTEQUAL: MOV AH,09h MOV DX,OFFSET NOT_EQUAL INT 21h INT 20h
EQUAL: MOV AH,09h MOV DX,OFFSET EQUAL1 INT 21h INT 20h
STRING1 DB 'Cheese is good for you!' STRING DB 'Cheese is good for you!'EQUAL1 DB 'They''re equal$'NOT_EQUAL DB 'They''re not equal$' MAIN ENDSEND START
By now you should know what's going on. SI and DI contain the two strings to becompared, and REP CMPSB does the comparison 17h times, or until it comes acrosstwo bytes that are not equal (b and g in this case). Then it does a jump commandto display the appropriate message.
The final string operations I will introduce you to are STOSB and and LODSB. STOSBwill store a byte from AL at the location that ES:DI points to. STOSB will get a bytethat ES:DI points into AL. These two instructions are very very powerful as you willsee if you continue learning assembly. Take a look at the next example.
MAIN SEGMENT ASSUME CS:MAIN,DS:MAIN,ES:MAIN,SS:MAIN
ORG 100h
START: MOV AH,9 MOV DI,OFFSET STRING
MOV SI,DI
LODSB INC AL STOSB
MOV DX,DI DEC DX
INT 21h INT 20h
STRING DB "oh33r! $"
MAIN ENDSEND START
This code will return:ph33r!
So what does it do?1. It moves 9 into AH to set it up for interrupt 21's Print String function2. Move the location of STRING into DI for the the LODSB instruction3. Do the same with SI4. Load ES:DI into AL5. Increment AL, thus changing it from o to p6. And put the contents of AL back to ES:DI7. Put DI into DX for interrupt 21's Print String function8. STOSB will increment DI after a successful operation, so decrement it9. Call interrupt 21h10. And terminate the program
And here's a final note that I should have mentioned earlier: All these stringinstructions actually don't always end in B. The B simply means Byte but could bereplaced by a W for example. That is, MOVSB will move a byte, and MOVSW will move aword. If you're using a instruction that requires another register like AL for example,you use that registers 32 or 64 bit part. For example, LODSW will move a word into AX.

Sub-Procedures--------------This chapter should be fairly easy as I will only introduce one new operator, CALL.CALL does just that, it CALLs a sub-procedure. Sub-Procedure are almost exactly likelabels, but they don't end with a : and have to have a RET statement at the end of thecode. The purpose of sub-procedures is to make your life easier. Since you can callthem from anywhere in the program you don't need to write certain sections over andover again.
MAIN SEGMENT ASSUME CS:MAIN,DS:MAIN,ES:MAIN,SS:MAIN
ORG 100h
START: CALL CHEESE CALL CHEESE CALL CHEESE CALL CHEESE CALL CHEESE CALL CHEESE CALL CHEESE INT 20h
CHEESE PROC MOV AH,09 LEA DX,MSG INT 21h RETCHEESE ENDP
MSG DB 'Cheese is good for you! $' MAIN ENDSEND START
1. We use the CALL command to call the sub-procedure CHEESE 7 times.2. We set up a sub-procedure called CHEESE. This is done in the following format: LABEL PROC3. We type in the code that we want the sub-procedure to do4. And add a RET statement to the end. This is necessary as it returns control to the main function. Without it the procedure wouldn't end and INT 20h would never get executed.5. We end the procedure using LABEL ENDP6. The usual...
User Input----------Finally! User Input has arrived. This chapter will discuss simple user input usingBIOS interrupts. The main keyboard interrupt handler is 16h. For the first part of thischapter we will be using the function 0h.Lets start with a simple program that waits for a keypress:
Example 1:MAIN SEGMENT ASSUME CS:MAIN,DS:MAIN,ES:MAIN,SS:MAIN
ORG 100h
START: MOV AH,0 INT 16h INT 20hMAIN ENDSEND START
This program waits for you to press a key, and then just quits. Expected more? Ofcourse. We have the echo the key back. Only than it will be truly 3337. Rememberall those programs you did in the debug part of this tutorial that printed out an A?Remember how we did it? No? Like this:
Example 2:MAIN SEGMENT ASSUME CS:MAIN,DS:MAIN,ES:MAIN,SS:MAIN
ORG 100h
START: MOV AH,2h MOV DL,41h INT 21h INT 20hMAIN ENDSEND START
Notice how the register DL contains the value that we want to print. Well if we useinterrupt 16h to get a key using function 0h, the ASCII scan code gets stored in AL, soall we have to do is move AL into DL, then call the old interrupt 21h, function 2h.
Example 3:MAIN SEGMENT ASSUME CS:MAIN,DS:MAIN,ES:MAIN,SS:MAIN
ORG 100h
START: MOV AH,0h INT 16h MOV AH,2h MOV DL,AL INT 21h INT 20hMAIN ENDSEND START
Isn't this awesome? Well that's not all INT 16 can do. It can also check the statusof the different keys like Ctrl, Alt, Caps Lock, etc. Check Appendix A for links tointerrupt listings and look them up.Let's use our new found t3kn33kz to create another truly 3337 program:
Example 4:MAIN SEGMENT ASSUME CS:MAIN,DS:MAIN,ES:MAIN,SS:MAIN
ORG 100h
START: MOV AH,0h INT 16h MOV KEY,AL CMP KEY,90h JE ITS_A_Z JNE NOT_A_Z
ITS_A_Z: MOV AH,9h MOV DX,OFFSET NOTA INT 21h INT 20h NOT_A_Z: MOV AH,2h MOV DL,KEY INT 21h
INT 20h
KEY DB ?NOTA DB "You pressed Z!!!!!!!!",10,13,"Ph33r! $"MAIN ENDSEND START
Well you should be able to understand this program without any problems. If you don't:1. We set AX to 0h and call interrupt 16h, that waits for the user to press a key2. We move the value of AL (which holds the ASCII value of the key pressed) into a variable. This way we can manipulate the registers without having to worry about destroying it3. We compare KEY with 90h, which is hex for Z (case sensitive)4. If it is a Z we jump to ITS_A_Z which displays the message5. If not, we jump to NOT_A_Z, which simply echos the key back.6. We decalared 2 variables, one which is not initialized yet called KEY, and one that holds the value "You pressed Z!!!!!!!!",10,13,"Ph33r! $" Which looks like this on a DOS computer: You pressed Z!!!!!!!! Ph33r!
Exercises:1. Make a program that will accept a series of keypresses, but when the user enters the following characters, convert them to their real values as shown below: S = Z F = PH PH = F E = 3 I = 1 EA = 33 T = 7 O = 0 A = 4 L = NOTE: This is NOT case sensitive. In other words, you're going to either have to convert lower case to upercase (or the otherway around) as soon as its entered by for example subtracting 20 from the ASCII value, or by making a branch for either case. Also, try using procedures to do this.

7. Basics of Graphics======================Graphics are something we all love. Today you will learn how to create some bad assgraphics in assembly! Well actually I will tell you how to plot a pixel using variousmethods. You can apply that knowledge to create some other graphics routines, likeline drawing shit, or a circle maybe. It's all just grade 11 math.
Using interrupts----------------This is the easiest method. We set up some registers and call an interrupt. The interrupt we will be using is 10h, BIOS video. Before we do anything, we have toget into graphics mode. For the purpose of simplicity I will just cover 320x200x256resolution (that is 320 vertical pixels, 200 horizontal pixels, and 256 shades ofcolors). So how do you get into this mode? You set AH to 00h and AL to 13h. 00htells interrupt 10h that we want to get into graphics mode, and 13h is the mode(320x200x256).
Example 1:MAIN SEGMENT ASSUME DS:MAIN,ES:MAIN,SS:MAIN,CS:MAIN
ORG 100h
START: MOV AH,00h MOV AL,13h INT 10h
INT 20hMAIN ENDSEND START
This ins't too exiting, just looks bigger. Let's plot a pixel.
Example 2:MAIN SEGMENT ASSUME DS:MAIN,ES:MAIN,SS:MAIN,CS:MAIN
ORG 100h
START: MOV AH,00h MOV AL,13h INT 10h
MOV AH,0Ch MOV AL,10 MOV CX,100 MOV DX,100 MOV BX,1h INT 10h
INT 20hMAIN ENDSEND START
First we get into graphics mode, then we set AH to 0Ch which is the Draw Pixel functionof interrupt 10h. In order to use 0Ch we have to set up some other registers as well.AL contains the colors of the pixel, CX the location on the X axis and DX the locationon the Y axis. Finally BX tells interrupt 10h to use page one of the VGA card. Don'tworry about what pages are until you get into more advanced shit.Once in graphics mode you can switch back to text using MOV AH,00h MOV AL,03h INT 10hSo putting it all together, the following program will draw a green pixel at location100,100 on page 1, then switch back to text mode, clearing the pixel along the way.Notice that it sets the AL and AH registers using only 1 move by moving them into AX.This might save you a clock tick or two and makes the executable file a whooping 3 bytes smaller!
Example 3:MAIN SEGMENT ASSUME DS:MAIN,ES:MAIN,SS:MAIN,CS:MAIN
ORG 100h
START: MOV AX,0013h INT 10h
MOV AX,0C04h MOV CX,100 MOV DX,100 MOV BX,1h INT 10h
MOV AX,0003h INT 10h
INT 20hMAIN ENDSEND START
Even though we did a bit of optimization there, it's still very slow. Maybe with onepixel you won't notice a difference, but if you start drawing screen full after screenfull using this method, even a fast computer will start to drag. So lets move on tosomething quite a bit faster.By the way, if you're computer is faster than a 8086, you will see nothing at all because even though the routine is slow, a single pixel can still be drawn fast. Sothe program will draw the pixel and earase is before your eye can comprehend itsexistance.
Writing directly to the VRAM----------------------------This is quite a bit harder than using interrupts as it involves some math. To makethings even worse I will introduce you to some new operators that will make the pixelsappear even faster.When you used interrupts to plot a pixel you were just giving the X,Y coordinates,when writing directly the the VRAM you can't do that. Instead you have to find the offset of the X,Y location. To do this you use the following equation:Offset = Y x 320 + XThe segment is A000, which is were VRAM starts, so we get:A000:Y x 320 + XHowever computers hate multiplication as it is just repeated adding, which is slow.Let's break that equation down into different numers:A000:Y x 256 + Y x 64 + XorA000:Y x 2^8 + Y x 2^6 + XNotice how now we're working with base 2? But how to we get the power of stuff?Using Shifts. Shifting is a fairly simple concept. There are two kinds of shifts,shift left and shift right. When you shift a number, the CPU simply adds a zero toone end, depending on the shift that you used. For example, say you want to shift256 256 = 100000000bShift Left: 1000000000b 512 = 1000000000bShift Right: 0100000000b 256 = 100000000bShifts are equal to 2^n where N is the number shifted by. So we can easily plug shiftsinto the previous equation.A000:Y SHL 8 + Y SHL 6 + XThis is still analog. Let's code that in assembly:
SET_VSEGMENT: ;set up video segment MOV AX,0A000h ;point ES to VGA segment MOV ES,AX
VALUES: ;various values used for plotting later on MOV AX,100 ;X location MOV BX,100 ;Y location
GET_OFFSET: ;get offset of pixel location using X,Y MOV DI,AX ;put X location into DI MOV DX,BX ;and Y into DX SHL BX,8 ;Y * 2^8. same as saying Y * 256 SHL DX,6 ;Y * 2^8. same as sayinh Y * 64 ADD DX,BX ;add the two together ADD DI,BX ;and add the X location
Now all we have to do is plot the pixel using the STOSB instruction. The color of thepixel will be in AL. MOV AL,4 ;set color attributes STOSB ;and store a byte
So the whole code to plot a pixel by writing directly to the VRAM looks like this:
MAIN SEGMENT ASSUME CS:MAIN,ES:MAIN,DS:MAIN,SS:MAIN
ORG 100h
START: MOV AH,00h ;get into video mode. 00 = Set Video Mode MOV AL,13h ;13h = 320x240x16 INT 10h
SET_VSEGMENT: ;set up video segment MOV AX,0A000h ;point ES to VGA segment MOV ES,AX
VALUES: ;various values used for plotting later on MOV AX,100 ;X location MOV BX,100 ;Y location
GET_OFFSET: ;get offset of pixel location using X,Y MOV DI,AX ;put X location into DI MOV DX,BX ;and Y into DX SHL BX,8 ;Y * 2^8. same as saying Y * 256 SHL DX,6 ;Y * 2^8. same as sayinh Y * 64 ADD DX,BX ;add the two together ADD DI,BX ;and add the X location;this whole thing gives us the offset location of the pixel
MOV AL,4 ;set color attributes STOSB ;and store
XOR AX,AX ;wait for keypress INT 16h
MOV AX,0003h ;switch to text mode INT 10h
INT 20h ;and exit
END STARTMAIN ENDS
If you don't understand this yet, study the source code. Remove all comments and addthem yourself in your own words. Know what each line does and why it does what it does.
A line drawing program----------------------To finish up the graphics section I'm going to show you a little modification to the previous program to make it print a line instead of just a pixel. All you have to do isrepeat the pixel ploting procedure as many times as required. It should be commentedwell enough, so I wont bother explaining it.
MAIN SEGMENT ASSUME CS:MAIN,ES:MAIN,DS:MAIN,SS:MAIN
ORG 100h
START: MOV AH,00h ;get into video mode. 00 = Set Video Mode MOV AL,13h ;13h = 320x240x16 INT 10h
SET_VSEGMENT: ;set up video segment MOV AX,0A000h ;point ES to VGA segment MOV ES,AX
VALUES: ;various values used for plotting later on MOV AX,100 ;X location MOV BX,100 ;Y location MOV CX,120 ;length of line. used for REP
GET_OFFSET: ;get offset of pixel location using X,Y MOV DI,AX ;put X location into DI MOV DX,BX ;and Y into DX SHL BX,8 ;Y * 2^8. same as saying Y * 256 SHL DX,6 ;Y * 2^8. same as sayinh Y * 64 ADD DX,BX ;add the two together ADD DI,BX ;and add the X location;this whole thing gives us the offset location of the pixel
MOV AL,4 ;set color attributes REP STOSB ;and store 100 bytes, decrementing CX and ;incrementing DI
XOR AX,AX ;wait for keypress INT 16h
MOV AX,0003h ;switch to text mode INT 10h
INT 20h ;and exit
END STARTMAIN ENDS

8. Basics of File Operations=============================In the old days, DOS did not include interrupts that would handle file operations. Soprogramers had to use some complicated t3kn33kz to write/open files. Today we don'thave to do that anymore. DOS includes quite a few interrupts to simplify this process.
File Handles------------File handles are are numbers assigned to a file upon opening it. Note that openinga file does not mean displaying it or reading it. Take a look at the following code:
MAIN SEGMENT ASSUME CS:MAIN,DS:MAIN,SS:MAIN,ES:MAIN ORG 100h
START: MOV AX,3D00h LEA DX,FILENAME INT 21h
JC ERROR INT 20h
ERROR: MOV AH,09h LEA DX,ERRORMSG INT 21h
INT 20h FILENAME DB 'TEST.TXT',0ERRORMSG DB 'Unable to open [test.txt]$'MAIN ENDSEND START
If you have a file called test.txt in the current directory the program will simply quite. If the file is missing it will display an error message. So what's happeninghere?1. We move 3D00h into AX. This is a shorter way of saying: MOV AH,3Dh MOV AL,00h 3Dh is the interrupt 21h function for opening files. The interrupt checks the AL register to how it should open the file. The value of AL is broken down into the following: Bit 0-2: Access mode 0 - Read 1 - Write 2 - Read/Write Bit 3: Reserved (0) Bit 4-6: Sharing Mode 0 - Compadibility 1 - Exclusiv 2 - Deny Write 3 - Deny Read 4 - Deny None Bit 7: Inheritance Flag 0 - File is inherited by child processes 1 - Prive to current process
Don't worry too much about what all this means. We will only use the Access mode bit.2. We load the address of the file name into DX. Note that the filename has to be an ASCIIZ string, meaning it is terminated with a NULL character (0).3. We call interrupt 21h4. If an error occured while opening the file, the carry flag is set and the error code is returned in AX. In this case we jump to the ERROR label.5. If no error occured, the file handel is stored in AX. Since we don't know what to do with that yet, we terminate the program at this point.

Reading files-------------Having optained the file handle of the file, we can now use the file. For exampleread it. When you use interrupt 21h's file read function, you have to set up theregisters as follows:AH = 3FhBX = File handleCX = Number of bytes to readDX = Pointer to buffer to put file contents inThan you simply print out that buffer using interrupt 21h's print string function.However notice how you have to specify the amount of data to read. That's not goodsince most of the time we don't know how much data is in a file. So we can use a little trick. If an error occured, the error code is stored in AX. The error code 0means that the program has encounter a EOF (End Of File). So we can simply make a while loop that prints a single byte from the text as long as AX is not equal to zero.If it is, we know that the file ended and we can terminate the program. Note that it is good coding practise to use interrupt 21h's function Close File to do just that.Here is the code for this thing:
MAIN SEGMENT ASSUME CS:MAIN,DS:MAIN,SS:MAIN,ES:MAIN ORG 100h
START: MOV AX,3D00h LEA DX,FILENAME INT 21h
JC ERROR
MOV BX,AX
READFILE: MOV AH,3Fh MOV CX,0001h LEA DX,CHARACTER INT 21h
CMP AX,000h JE ENDPROGRAM
MOV AH,02h MOV DL,CHARACTER INT 21h
JMP READFILE
ENDPROGRAM: MOV AH,3Eh INT 21h
INT 20h
ERROR: MOV AH,09h LEA DX,ERRORMSG INT 21h
INT 20h
FILENAME DB 'TEST.TXT',0ERRORMSG DB 'Unable to open [test.txt]$'CHARACTER DB ?MAIN ENDSEND START
This is a fairly big piece of code, but you should be able to understand it.
1. We get the file handle using the method discussed in the previous chapter.2. We move the file handle from AX into BX. This is because interrupt 21h's function to read a file requires the handle to be in BX.3. We move 3Fh into AH, tells interrupt 21h that we want to read a file4. CX contains the bytes to read, we only want one5. The read byte is put into buffer that DX points to. In this case its called CHARACTER. Notice how we set up CHARACTER is an unitialized variable.6. We compare AX to 0, which it would be if a EOF is encountered. If it is, we end the program.7. Otherwise we use interrupt 21h's function Print Character to print the character in the buffer. You should be familiar with that from previous chapters.8. We return to the label READFILE to read another byte.9. If EOF is encountered, we use function 3Eh to close the file and terminate the program.

Creating files--------------To create files you have to:1. Create an empty file2. Move a buffer into the file handleThe following code will do that for us:
MAIN SEGMENT ASSUME CS:MAIN,DS:MAIN,ES:MAIN,SS:MAIN
ORG 100h
START: MOV AH,3Ch XOR CX,CX MOV DX,OFFSET FILE_NAME INT 21h
JC ERROR1
MOV BX,AX MOV AH,40h MOV CX,9 MOV DX,OFFSET SHIT INT 21h
JC ERROR
INT 20h
ERROR: MOV AH,09h MOV DX,OFFSET ERROR_WRITING INT 21h INT 20h
ERROR1: MOV AH,09h MOV DX,OFFSET ERROR_CREATING INT 21h INT 20h
FILE_NAME db "blow.me",0SHIT db "123456789"ERROR_WRITING db "error writing to file$"ERROR_CREATING db "error creating file$"MAIN ENDSEND START
1. We create the file using function 3Ch. Register have to be set up like this: CX - Type of file. 0 - normal, 1 - Read Only, 2 - Hidden DX - Name of the file. Has to be an ASCIIZ string. This function returns the file handle of the new file in AX.2. We check for an error, and jump of necessary3. We move the file handle from AX into BX4. And choose interrupt 21h's function Write File (40h). For this function we need the registers set up like this: BX - File Handle CX - File size to write (9 in our case) DX - Points to buffer to be written5. We check for an error, if so we jump, otherwise we terminate the program.
Search operations-----------------In assembly you have two search functions at your disposal, Search First and SearchNext. Out of those to search first is the more complicated one. As the name implies,Search Next can only be done after a Search First function. So first thing we do tosearch for a file is set up a Search First routine. The register have to be setup asfollows:AH - 4EhCL - File AttributesDX - Pointer to ASCIIZ path/file nameThe file attributes are set up in a wierd way, and I will not get into those. It'senough for you to know that we will be using 6h, which is a normal file. Well actuallyDOS will read 6h as 00000110, and each bit has a different meaning.This function will return an error in AX. If AX is zero the search was successful, otherwise we know it wasn't. If it found the files to search for, DOS will setupa block of memory 43 bytes long in the DTA. DTA stands for Disk Transfer Area and fornow it's enough to think of it as a "scratch pad" for DOS. In this tutorial I willnot get into reading it, but it doesn't hurt telling you what these 43 byte contain:0 - 21: Reserved for the Find Next function. This saves us from having to do the setup again.21 - 22: Attributes of the file found22 - 24: Time the file found was created24 - 26: Date the file found was created26 - 30: Size of the file found (in bytes)30 - 43: File name of the file found.
So our Search First function will look like this:SEARCH: MOV DX,OFFSET FILE_NAME MOV CL,6h MOV AH,4Eh INT 21h OR AL,AL JNZ NOT_FOUNDNotice how we use a bitwise operator instead of a CMP? Bitwise operations are insanlyfast, and CoMParing 2 values is bascily subtracting which is slower. Remember how ORworks?0 OR 0 = 01 OR 0 = 10 OR 1 = 11 OR 1 = 1So OR AL,AL will only return 0 if every single bit in AL is 0. So if it doesn't return0, we know that it contains an error code and the search failed. We wont botherchecking what the error code is, we just jump to a label that will display an errormessage. If the search was successful we move on to the Search Next function to checkif anymore files meet our describtion. Search Next is a fairly easy function. All wehave to do is move 4Fh into AH and call int 21h.FOUND: MOV Ah,4Fh INT 21h OR AL,AL JNZ ONE_FILEThis code will perform the Search Next function, and if it fails jump to the labelONE_FILE. But what happens if it found another file? Well we could do another Search Next function.MORE_FOUND: MOV AH,4Fh INT 21h OR AL,AL JNZ MORE_FILES_MSGThis will check if yet another file is found. Now we should implement a way of knowinghow many files we found. We can do so by setting a register to 1 after the Search Firstfunction was successful, and incremeant it each time it finds another file. So letsput all this together and create a program that will search for a file, search again if it found it and start a loop that keeps searching for files and keeps track of how many it found:
MAIN SEGMENT ASSUME CS:MAIN,DS:MAIN,ES:MAIN,SS:MAIN
ORG 100h
SEARCH: MOV DX,OFFSET FILE_NAME MOV CL,6h MOV AH,4Eh INT 21h OR AL,AL JNZ NOT_FOUND
FOUND: MOV CL,1 ;the counter that keeps track of how many files we found MOV Ah,4Fh INT 21h OR AL,AL JNZ ONE_FILE
MORE_FOUND: INC CL ;here we increment it MOV AH,4Fh INT 21h OR AL,AL JNZ MORE_FILES_MSG JMP MORE_FOUND
MORE_FILES_MSG: MOV AH,02h OR CL,30h ;convert counter to number (see blow) MOV DL,CL ;and display it. INT 21h
MOV AH,9h MOV DX,OFFSET MORE_FILES INT 21h INT 20h
ONE_FILE: MOV AH,9h MOV DX,OFFSET FILE_FOUND INT 21h INT 20h
NOT_FOUND: MOV AH,9h MOV DX,OFFSET FILE_NOT_FOUND INT 21h INT 20h
MORE_FILES DB " FILES FOUND",10,13,'$'FILE_NOT_FOUND DB "FILE NOT FOUND",10,13,'$'FILE_FOUND DB "1 FILE FOUND",10,13,'$'FILE_NAME DB "*.AWC",0 ;this is the file we search for
MAIN ENDSEND SEARCH
Returns:FILE NOT FOUNDIf not files with extension .AWC are found1 FILE FOUNDIf the current directory contains 1 file with the extension .AWCX FILES FOUNDIf more than one file with extension .AWC was found. X stands for the number offiles found. Remember how function 2h will print the ASCII value of a hex number?Well we don't really want that. So to convert it to a number we OR it with 30h.That's because if you look at an ASCII chart you'll notice that the numeric valueof a ASCII number is always 30h more than the hex number. For example, The number5 is equal to 35h, 6 is 36h, etc. So to convert it we OR it with 30h: 5h = 00010130h = 00011035h = 110101 (ASCII Value: "5")
6h = 00011030h = 11000036h = 110110 (ASCII Value: "6")
etc.
Exercises:1. Create a program that will display how many files are in the current directory2. Create a program that will create a new file, write something to it, close it, open it, and read its contents.

Basics of Win32===============
Introduction------------I didn't want to include this as I absolutly HATE microsoft, but I guess I have to facethe fact that it sadly took over all other good operating systems and people havestarted to switch to it. This chapter will be quite a bit different from the previousones as Win32 programing is not really low level. Basicly all you're doing is makingcalls to internal windows .DLL files. But the most signicant differance is the factthat you will be working in Protected Mode. This is the mode a briefly mentioned whereyou have a 4 gig limit instead of the old 64k you've been working with so far. Iwon't heavily get into what protected mode is and does as that is out of the scope ofthis tutorial (my next asm tutorial will though), but you will need to refer back to.EXE file layout I talked about in chapter 3.
Tools-----Well first of all you will have to download a new assembler. That's because my versionof TASM is older and doesn't support Win32. So for this chapter get yourself a copy ofMASM. That's an assembler by microsoft that has now become freeware. Why didn't Imention MASM before since it's free? Well the only thing MASM is now good for is Win32programing. TASM uses something called IDEAL mode which is a much better way of programing in assembly. MASM uses MASM mode which quite frankly blows. Get MASM from:Download and install it, than move on to the next section

A Message Box-------------First of all you have to get familiar with the program layout:
386 .MODEL Flat, STDCALL
.DATA
.DATA?
.CONST
.CODE
LABEL: END LABEL
This should look fairly familiar to you. If it doesn't, let's go over it again:
386 - This declares the processor type to use. You can also use 4 and 586, but for the sake of backwards compadibility you should stick with 386 unless you have to use something higher.
.MODEL FLAT, STDCALL - This declares the memory model to use. In Win32 program you don't have the choices you did before anymore, FLAT is the only one. The STDCALL tells the assembler how to pass parameters. Don't worry about what that means just yet, you will most likely never use anything buy STDCALL in Win32 programming as there is only 1 instruction that needs a different one (C).
.DATA - All your initialized data should go in here
.DATA? - All your uninitialized data should go here
.CONSTS - Constants go here
.CODE - And your code goes here
LABEL: - Just like before, you have to define a starting label
END LABEL - And END it
Now I'm gonna ask you to take a different look at this whole assembly thing. So far youhave been manipulating memory and the CPU, with Win32 you manipulate memory and Windowscomponents. I'm sure you know what Include files are, files that will be included withyour program when you compile it. Well in Win32 programing you're using windows includefiles in the form of DLLs. These files are known as Application Programming Interfaceor API for short. For example, Kernel32.dll, User32.dll, gdi32.dll are APIs. Again,I won't bother getting into details on how APIs work. Assuming you have included allthe .DLL files you need, you call specific Win32 functions in the following format:
INVOKE expression,arguments
So for example, to exit a program by making a call to the exit function you do:
INVOKE ExitProcess,0
So let's make a program that does just that, exits:

386 .MODEL Flat, STDCALL option casemap:none ;turn case sensitivity oninclude \masm32\include\windows.inc ;the include files that we needinclude \masm32\include\kernel32.incincludelib \masm32\lib\kernel32.lib.DATA .CODE
START: INVOKE ExitProcess,0 END START
To get an .EXE out of this, get into your MASM directory and then into BIN. Thenassemble with:ml /c /coff /Cp filename.asmAnd link with:link /SUBSYSTEM:WINDOWS /LIBPATH:c:\masm32\lib filename.objThis will get you a file called filename.exe, run it and ph33r.Now lets make this into a message box. We use the INVOKE command again, but insteadof using the ExitProcess function, we use MessageBox.
INVOKE MessageBox, 0, OFFSET MsgBoxText, OFFSET MsgBoxCaption, MB_OK
Let's disect this thing:MessageBox tells windows what function we want, and add a 0, just like we did withExitProcess. This is done because all ANSI strings in windows must be terminated with a0. Next we put the location of MsgBoxText in there. This is done just like you woulddo it using INT 21h, OFFSET LOCATION. We do the same with MsgBoxCaption and finallyspecify what kind of message box we want. In this case MB_OK is a constant representingthe familiar box where you can only press Ok. Usually this would be a number, but we'reincluding a file that contains defintions of them. So how did I know what goes where?A Win32 refrence will tell you. We also have to define MsgBoxText and MsgBoxCaption.We do this the way we always did:MsgBoxCaption DB "ph33r b1ll g473z!",0MsgBoxText DB "Yes, I ph33r",0
So throwing it all together, the code would look like this:
.386 .MODEL FLAT,stdcall option casemap:none include \masm32\include\windows.inc include \masm32\include\kernel32.inc includelib \masm32\lib\kernel32.lib include \masm32\include\user32.inc includelib \masm32\lib\user32.lib
.DATAMsgBoxCaption DB "ph33r b1ll g473z!",0MsgBoxText DB "Yes, eYe ph33r",0
.CODE START: INVOKE MessageBox, 0, OFFSET MsgBoxText, OFFSET MsgBoxCaption, MB_OK INVOKE ExitProcess, 0 END START
NOTE: Instead of offset you could have use ADDR. ADDR does basicly the same, but it can handle forward refrences and OFFSET can't. In other words, if you would have declared MsgBoxCaption and MsgBoxText after you use them (INVOKE.....), using OFFSET would return an error. So you should get the habbit of using ADDR instead of Win32.
Now assemble and link with:ml /c /coff /Cp filename.asmlink /SUBSYSTEM:WINDOWS /LIBPATH:c:\masm32\lib filename.objBy the way, you should have made a .bat file by now that does this for you. If youhaven't, make a file containing the following lines and save it as whatever.bat:@echo offml /c /coff /Cp %1.asmlink /SUBSYSTEM:WINDOWS /LIBPATH:c:\masm32\lib %1.obj

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