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    A introduction to x86 assembly
    Written by Gavin Estey		          email: gavin@senator.demon.co.uk

    This document is version:

    0.9 Beta 2/19/95

    This will hopefully be the final version of this unless somebody finds some
    mistakes in it. I have spent over 620 minutes creating this document (not
    including writing or testing the code or the original text document) so I hope
    you will find it useful.

    This version is still in testing which means that I cannot be sure that all the code
    will compile and work correctly. I have done my best to make sure that all the
    information contained in this document is correct. If any mistakes are found
    please could you notify me be email.

    If someone who has a computer that doesn�t have a VGA card/monitor could you
    please compile the code on P29 that checks what graphics the computer can handle
    and tell me if it returned DX as 0.

    This document was written by Gavin Estey. No part of this can be reproduced and sold
    in any commercial product without my written permission (email me for more
    information). I am not responsible for any damage caused in anyway from this
    document.

    If you enjoy this document then a donation would be gratefully received. No payment
    is necessary but none will turned down. If any mistakes are noticed then I would like to
    be notified. If you would like any other help or have suggestions for a later version of
    this document.

    Firstly you will need a suitable assembler to compile your programs. All examples have
    been tested with two compilers: A86 and Turbo Assemblerâ  ver 4. A86 is a very
    good shareware assembler capable of producing code up to 80286. This can be found
    on your local simtel mirror (try ftp.demon.co.uk or ftp.cdrom.com) under
    simtel/msdos/asmutil called A86V322.ZIP.

    Firstly I am going to talk about SEGMENTS and OFFSETS. This (for me anyway) is
    probably the hardest part of assembly to understand.

    Segments and offsets:

    The original designers of the 8088 decided that nobody will every need to use more
    that one megabyte of memory. So they built the chip so it couldn't access above that.
    The problem is to access a whole megabyte 20 bits are needed (one bit being either a
    one or a zero). Registers only have 16 bits and they didn't want to use two because
    that would be 32 bits and they thought that this would be too much for anyone. They
    decided to do the addressing with two registers but not 32 bits. Is this confusing?
    Blame the designers of the 8088.

    OFFSET = SEGMENT * 16
    SEGMENT = OFFSET / 16 (the lower 4 bits are lost)

    SEGMENT * 16	|0010010000010000 - ---|  range (0 to 65535) * 16
    OFFSET         		|-- - -0100100000100010|  range (0 to 65535)
    20 bit address		|00101000100100100010|  range 0 to 1048575 (1 MEG)
    		             =====  DS =====
    		                      ====== SI =====

    (note DS and SI overlap). This is how DS:SI is used to make a 20 bit address. The
    segment is in DS and the offset is in SI.
    Segment registers are: CS, DS, ES, SS. On the 386+ there are also FS & GS

    Offset registers  are: BX, DI, SI, BP, SP, IP.  In 386+ protected mode, ANY general
    register (not a segment register) can be used as an  Offset register.  (Except IP, which
    you can't access.)

    Registers:

    AX, BX, CX and DX are general purpose registers. On a 386+ they are 32 bits; EAX,
    EBX, ECX and EDX. The all can be split up into high and low parts, each being 8 bits.

    EAX 	32 bits
    AX 	16 bits
    AH	8 bits
    AL	8 bits

    This means that you can use AH and AL to store different numbers and treat them as
    separate registers for some tasks.

    BX (BH/BL): same as AX
    CX (CH/CL): Same as AX (used for loops)
    DX (DH/DL): Same as AX (used for multiplication/division)

    DI and SI are index registers and can be used as offset registers.

    SP: Stack Pointer. Does just that. Points to the current position in the stack. Don't
    alter unless you REALLY know what you are doing or want to crash your computer.

    The Stack:

    This is and area of memory that is like a stack of plates. The last one you put on is the
    first one that you take off (LOFO). If another piece of data is put on the stack it grows
    downwards.

    Diagram 1: This shows how the stack is organised

    PUSH and POP:

    Push puts a value onto the stack and pop takes it back off. Here's some code. (you
    can't compile this - yet.)

    	push cx         ;put cx on the stack
    	push ax         ;put ax on the stack
    	.
    	.
    	.               ;this is going to pop them back in
    	.			 ;the wrong so they are reversed
    	pop cx          ;put value from stack into cx
    	pop ax          ;put value from stack into ax

    This means that cx is equal to what ax is and ax is equal to what cx was.

    Some instructions:

    MOV: this moves a value from one place to another.

    Syntax:
    MOV destination, source

    for example:
    	MOV ax,10       ;moves an immediate value into ax
    	MOV bx,cx       ;moves value from cx into bx

    INT: calls a DOS or BIOS function, mainly to do something you would rather not
    write a function for e.g. change video mode, open a file etc.

    Syntax:
    INT interrupt number

    Note: an h after a number denotes that that number is in hex.

    	INT 21h         ;Calls DOS standard interrupt 21h
    	INT 10h         ;Calls the Video BIOS interrupt

    All interrupts need a value to specify what subroutine to use. This is usually in AH. To
    print a message on the screen all you need to do is this:

    	MOV ah,9      ;subroutine number 9
    	INT 21h       ;call the interrupt

    But first you have to specify what to print. This function needs DS:DX to be a far
    pointer to where the string is. The string has to be terminated with a dollar sign ($).

    This example shows how it works:
    	MyMessage db    "This is a message!$"
    	.
    	.
    	.
    	mov dx,OFFSET MyMessage
    	mov ax,SEG MyMessage
    	mov ds,ax
    	mov ah,9
    	int 21h

    DB declares a piece of data. This is the same as in BASIC:
    	LET MyMessage$ = "This is a message!"

    DB declares a byte, DW declares a word and DD declares a Dword.

    This is your first assembly program. Cut this in and then assemble it. If you are using
    A86, type:
    	A86 name of file
    If you are using Turbo Assembler then type:
    	tasm name of file
    	tlink name of file minus extention
    With tlink you can make a .COM file by using a /t switch.

    ;THIS IS A SIMPLE PROGRAM TO DISPLAY A MESSAGE ON THE
    ;SCREEN. SHOULD OUTPUT This is a message! TO SCREEN AND
    ;RETURN WITH NO ERRORLEVEL. THIS CAN�T BE MADE INTO A
    .COM FILE WITH TLINK AS IT GENERATES AN ERROR.

    	.MODEL SMALL ;Ignore the first two lines
    			   ;I'll explain what this means later.
    	.STACK	   ;allocate a stack
    	.CODE        ;a directive to start code segment

    	START:       ;generally a good name to use as an
    			   ;entry point

    	JMP BEGIN    ;jump to BEGIN: (like goto)

    	MyMessage db "This is a message!$"

    	BEGIN:       ;code starts here

    	mov dx,OFFSET MyMessage	;put the offset of message
    						;in DX
    	mov ax,SEG MyMessage	;put the segment that the
    						;message is in in AX
    	mov ds,ax				;copy AX into DS
    	mov ah,9				;move 9 into ah to call
    	int 21h				;function 9 of interrupt
    						;21h - display a string to
    						;screen.

    	MOV ax,4c00h ;move the value of 4c00 hex into AX
    	INT 21h      ;call interrupt 21h, subroutine 4C
    ;which returns control to dos, otherwise it would of
    ;crashed. The 00 on the end means what errorlevel it
    ;would return to DOS. This can be checked in a batch file

    	END START    ;end here

    Some instructions that you need to know:

    This is just a list of some basic assembly instructions that are very important and are
    used often.

    ADD	Add the contents of one number to another

    Syntax:

    	ADD operand1,operand2

    This adds operand2 to operand1. The answer is stored in operand1. Immediate data
    cannot be used as operand1 but can be used as operand2.

    SUB	Subtract one number from another

    Syntax:
    	SUB operand1,operand2

    This subtracts operand2 from operand1. Immediate data cannot be used as operand1
    but can be used as operand2.

    MUL	Multiplies two unsigned integers (always positive)
    IMUL	Multiplies two signed integers (either positive or negitive)

    Syntax:
    	MUL register or variable
    	IMUL register or variable

    This multiples the register given by the number in AL or AX depending on the size of
    the operand. The answer is given in AX. If the answer is bigger than 16 bits then the
    answer is in DX:AX (the high 16 bits in DX and the low 16 bits in AX).

    On a 386, 486 or Pentium the EAX register can be used and the answer is stored in
    EDX:EAX.

    DIV	Divides two unsigned integers(always positive)
    IDIV 	Divides two signed integers (either positive or negitive)

    Syntax:
    	DIV register or variable
    	IDIV register or variable

    This works in the same way as MUL and IMUL by dividing the number in AX by the
    register or variable given. The answer is stored in two places. AL stores the answer
    and the remainder is in AH. If the operand is a 16 bit register than the number in
    DX:AX is divided by the operand and the answer is stored in AX and remainder in
    DX.

    The way we entered the address of the message we wanted to print was a bit
    cumbersome. It took three lines and it isn�t the easiest thing to remember

    	mov dx,OFFSET MyMessage
    	mov ax,SEG MyMessage
    	mov ds,ax

    We can replace all this with just one line. This makes the code easier to read and it
    easier to remember. This only works if the data is only in the data is in one segment i.e.
    small memory model.

    	lea dx,MyMessage
    or	mov dx,OFFSET MyMessage
    Using lea is slightly slower and results in code which is larger.

    LEA means Load Effective Address.

    Syntax:
    LEA destination,source

    Desination can be any 16 bit register and the source must be a memory operand (bit of
    data in memory). It puts the offset address of the source in the destination.

    Keyboard input:

    We are going to use interrupt 16h, function 00h to read the keyboard. This gets a key
    from the keyboard buffer.  If there isn't one, it waits until there is. It returns the SCAN
    code in AH and the ASCII translation in AL.

            MOV ah,00h		;function 00h of
       INT 16h		;interrupt 16h

    All we need to worry about for now is the ascii value which is in al.

    Printing a character:

    The problem is that we have the key that has been pressed in ah. How do we display
    it? We can't use function 9h because for that we need to have already defined the string
    which has to end with a dollar sign. This is what we do instead:

    ;after calling function 00h of interrupt 16h
            MOV dl,al       ;move al (ascii code) into dl
            MOV ah,02h      ;function 02h of interrupt 21h
            INT 21h         ;call interrupt 21h

    If you want to save the value of ah then push it before and pop it afterwards.

    Control flow:
    Firstly, the most basic command:
    	JMP label
    This is the same as GOTO in basic.

            	JMP ALabel
         .
    	.
    	ALabel:
         	;code to do something

    What do we do if we want to compare somthing. We have just got a key from the user
    but we want to do something with it. Lets print something out if it is equal to
    somethine else. How do we do that? Its easy. We use the jump on condition
    commands. Here is a list of them:

    Jump on condition commands:

    JA
    jumps if the first number was above the second number

    JAE
    same as above, but will also jump if they are equal

    JB
    jumps if the first number was below the second

    JBE
    same as above, but will also jump if they are equal

    JNA
    jumps if the first number was NOT above  (same as JBE)

    JNAE
    jumps if the first number was NOT above or the same as (same as JB)

    JNB
    jumps if the first number was NOT below (same as JAE)

    JNBE
    jumps if the first number was NOT below or the same as (same as JA)

    JZ
    jumps if the two numbers were equal

    JE
    same as JZ, just a different name

    JNZ
    jumps if the two numbers are NOT equal

    JNE
    same as above

    [NOTE: There are quite a few more but these are the most useful. If you want the full
    list then get a good assembly book]

    They are very easy to use.

    Syntax:
            CMP register containing value, a value
        jump command destination

    An example of this is:
          cmp al,'Y'      ;compare the value in al with Y
          je ItsYES       ;if it is equal then jump to ItsYES

    The following program is an example of how to use control and input and output.

    .MODEL  SMALL
    .STACK 					;define a stack
    .CODE
    Start:					;a good place to start.

          lea dx,StartUpMessage  	;display a message on th e
    						;screen
          mov ah,9                ;using function 09h
          int 21h                 ;of interrupt 21h
          lea dx,Instructions     ;display a message on the
    						;screen
          mov ah,9                ;using function 09h
          int 21h                 ;of interrupt 21h

    	 mov ah,00h              ;function 00h of
          int 16h                 ;interrupt 16h gets a
    						;character from user
          mov bl,al               ;save bl
          mov dl,al               ;move ascii value of key
    				        	;pressed to dl

    	 mov ah,02h              ;function 02h of
          int 21h                 ;interrupt 21h displays a
    				          ;character to screen
          cmp bl,'Y'              ;is al=Y?
          je Thanks    		     ;if yes then goto Thanks
     	 cmp bl,'y'              ;is al=y?
          je Thanks          	;if yes then goto Thanks
          jmp TheEnd
    Thanks:
          lea dx,ThanksMsg        ;display message
          mov ah,9                ;using function 9
          int 21h                 ;of interrupt 21h
    TheEnd:
          lea dx,GoodBye          ;print goodbye message
          mov ah,9                ;using function 9
          int 21h                 ;of interrupt 21h
          mov AX,4C00h            ;terminate program and
    						;return to DOS using
          INT 21h                 ;interrupt 21h function 4CH

    .DATA
    ;0Dh,0Ah adds a enter at the beginning

    StartUpMessage DB "A Simple Input Program$"
    Instructions  DB 0Dh,0Ah,"Just press a Y to continue...$�
    ThanksMsg      DB 0Dh,0Ah,"Thanks for pressing Y!$"
    GoodBye        DB 0Dh,0Ah,"Have a nice day!$"

            END

    Procedures:

    Assembly, like C and Pascal can have procedures. These are very useful for series of
    commands that have to be repeated often.

    This is how a procedure is defined:

    PROC AProcedure
        .
        .          ;some code to do something
        .
        RET		;if this is not here then your computer
    			;will crash
    ENDP AProcedure

    You can specify how you want the procedure to be called by adding a FAR or a
    NEAR after the procedure name. Otherwise it defaults to the memory model you are
    using. For now you are better off not doing this until you become more experianced. I
    usually add a NEAR in as compilers can�t decide between a near and a far very well.
    This means if the jump needs to be far the compiler will warn you and you can change
    it.

    This is a program which uses a procedure.

    ;a simple program with a procedure that prints a message
    ;onto the screen. (Use /t switch with tlink). Should
    ;display Hello There! on the screen.

    .MODEL  TINY
    .CODE
    ORG     100h

    MAIN    PROC
    	   JMP Start 		  ;skip the data
    HI      DB "Hello There!$" ;define a message
    Start:				  ;a good place to start.
            Call Display_Hi    ;Call the procedure
            MOV AX,4C00h       ;terminate program and return
    					  ;to DOS using
            INT 21h            ;interrupt 21h function 4Ch

    Display_Hi PROC            ;Defines start of procedure
            lea dx,HI          ;put offset of message into DX
            mov ah,9           ;function 9 of
            int 21h            ;interrupt 21h
            RET                ;THIS HAS TO BE HERE
    Display_Hi ENDP            ;Defines end of procedure

    Main    ENDP
            END MAIN

    Memory Models:

    We have been using the .MODEL directive to specify what type of memory model we
    use, but what does this mean?

    Syntax:
    	.MODEL MemoryModel

    Where MemoryModel can be SMALL, COMPACT, MEDIUM, LARGE, HUGE,
    TINY OR FLAT.

    Tiny:
    This means that there is only one segment for both code and data. This type of
    program can be a .COM file.

    Small:
    This means that by default all code is place in one physical segment and likewise all
    data declared in the data segment is also placed in one physical segment. This means
    that all proedures and variables are addressed as NEAR by pointing at offsets only.

    Compact:
    This means that by default all elements of code (procedures) are placed in one physical
    segment but each element of data can be placed in its own physical segment. This
    means that data elements are addressed by pointing at both at the segment and offset
    addresses. Code elements (procedures) are NEAR and varaibles are FAR.

    Medium:
    This is the opposite to compact. Data elements are near and procedures are FAR.

    Large:
    This means that both procedures and variables are FAR. You have to point at both the
    segment and offset addresses.

    Flat:
    This isn�t used much as it is for 32 bit unsegmented memory space. For this you need a
    dos extender. This is what you would have to use if you were writing a program to
    interface with a C/C++ program that used a dos extender such as DOS4GW or
    PharLap.

    Macros:

    (All code examples given are for macros in Turbo Assembler. For A86 either see the
    documentation or look in the A86 macro example later in this document).

    Macros are very useful for doing something that is done often but for which a
    procedure can�t be use. Macros are substituted when the program is compiled to the
    code which they contain.

    This is the syntax for defining a macro:

    Name_of_macro	macro
    ;
    ;a sequence of instructions
    ;
    endm

    These two examples are for macros that take away the boring job of pushing and
    popping certain registers:

    	SaveRegs macro
    		pop ax
    		pop bx
    		pop cx
    		pop dx
    	endm
    	RestoreRegs macro
    		pop dx
    		pop cx
    		pop bx
    		pop ax
    	endm
    Note that the registers are popped in the reverse order to they wer popped.

    To use a macro in you program you just use the name of the macro as an ordinary
    macro instruction:

    	SaveRegs
    	;some other instructions
    	RestoreRegs

    This example shows how you can use a macro to save typing in. This macro simply
    prints out a variable to the screen.

    The only problems with macros is that if you overuse them it leads to you program
    getting bigger and bigger and that you have problems with multiple definition of labels
    and variables. The correct way to solve this problem is to use the LOCAL directive for
    declaring names inside macros.

    Syntax:
    	LOCAL name
    Where �name� is the name of a local variable or label.

    If you have comments in a macro everytime you use that macro the comments will be
    added again into your source code. This means that it will become unesescarily long.
    The way to get round this is to define comments with a ;; instead of a ;. This example
    illustrates this.

    	;a normal comment
    	;;a comment in a macro to save space

    Macros with parameters:
    Another useful property of macros is that they can have parameters. The number of
    parameters is only restricted by the length of the line.

    Syntax:
    Name_of_Macro macro par1,par2,par3
    ;
    ;commands go here
    endm

    This is an example that adds the first and second parameters and puts the result in the
    third:

    	AddMacro macro num1,num2,result
    		push ax		;save ax from being destroyed
    		mov ax,num1	;put num1 into ax
    		add ax,num2	;add num2 to it
    		mov result,ax	;move answer into result
    		pop ax		;restore ax
    	endm

    On the next page there is an example of some a useful macro to exit to dos with a
    specified . There are two versions of this program because both A86 and Turbo
    Assembler handle macros differently.

    ;this is a simple program which does nothing but use a
    ;macro to exit to dos. This will only work with tasm and
    ;should be compiled like this: tasm /m2 macro.asm
    ;because tasm needs more than one pass to work out what
    ;to do with macros. It does cause a warning but this can
    ;be ignored.

    .MODEL small
    .STACK
    .CODE        ;start the code segment

    Start:       ;a good a place as any to start this

    ;now lets go back to dos with another macro
    	BackToDOS 0     ;the errorlevel will be 0

    BackToDOS macro errorlevel

    ;;this is a macro to exit to dos with a specified
    ;;errorlevel given. No test is done to make sure that a
    ;;procedure is actually passed or it is within range.

    	mov ah,4Ch          ;;terminate program and return
    					;;to DOS using
    	mov al,errorlevel   ;;put errorlevel into al
    	int 21h             ;;interrupt 21h function 4Ch

    	endm				;;end macro
    end					;end program

    Procedures that pass parametres:
    Procedures can be made even more useful if they can be made to pass parameters to
    each other. There are three ways of doing this and I will cover all three methods:
      In registers,
      In memory,
      In stack.

    In registers:

    Advantages:		Easy to do and fast.
    Disadvantages: 	The is not many registers.

    This is very easy to do, all you have to do is to is move the parameters into registers
    before calling the procedure. This example adds two numbers together and then
    divides by the third it then returns the answer in dx.

    	push ax		;save value of ax
    	push bx		;save value of bx
    	push cx		;save value of cx
    	mov ax,10		;first parameter is 10
    	mov bx,20		;second parameter is 20
    	mov cx,3		;third parameter is 3
    	Call ChangeNumbers ;call procedure
    	pop cx		;restore cx
    	pop bx		;restore bx
    	pop ax		;restore dx
    .....
    ChangeNumbers PROC	;Defines start of procedure
    	add ax,bx		;adds number in bx to ax
    	div cx		;divides ax by cx
    	mov dx,ax		;return answer in dx
    	ret
    ChangeNumbers ENDP	;defines end of procedure

    How to use a debugger:

    This is a good time to use a debugger to find out what your program is actually doing.
    I am going to demonstrate how to use Turbo Debugger to check if this program is
    working properly. First we need to compile this program to either a .EXE or .COM
    file. Then type:
    	td name of file
    Turbo Debugger then loads. You can see the instructions that make up your programs,
    for example the first few lines of this program is shown as:
    	cs:0000 50		push   ax
    	cs:0001 53		push   bx
    	cs:0002 51		push   cx
    (This might be slighly different than is shown on your screen but hopefully you will get
    the main idea)

    This diagram shows what the Turbo Debuggerâ  screen looks like

    The numbers that are moved into the registers are different that the ones that we typed
    in because they are represented in their hex form (base 16) as this is the easiest base to
    convert to and from binary and that it is easier to understand than binary also.

    At the left of this display there is a box showing the contents of the registers. At this
    time all the main registers are empty. Now press F7 this means that the first line of the
    program is run. As the first line pushed the ax register into the stack, you can see that
    the stack pointer (SP) has changed. Press F7 until the line which contains mov
    ax,000A is highlighted. Now press it again. Now if you look at the box which contains
    the contents of the registers you can see that AX contains A. Press it again and BX
    now contains 14, press it again and CX contains 3. Now if you press F7 again you can
    see that AX now contains 1E which is A+14. Press it again and now AX contains A
    again, A being 1E divided by 3 (30/3 = 10). Press F7 again and you will see that DX
    now also contains A. Press it three more times and you can see that CX,BX and AX
    are all set back to their origional values of zero.

    Passing through memory:

    Advantages:		Easy to do and can use more parameters
    Disadvantages: 	Can be slower

    To pass parameters through memory all you need to do is copy them to a variable
    which is stored in memory. You can use a varable in the same way that you can use a
    register but commands with registers are a lot faster. This table shows the timing for
    the MOV command with registers and then variables and then the amount of clock
    cycles (the speed - smaller faster) it takes to do them.

    KEY: 	reg8 means an 8 bit register eg AL
    	mem8 means an 8 bit variable declared with DB
    	reg16 means an 16 bit register eg AX
    	mem16 means an 16 bit variable declared with DW
    	imm8 means an immediate byte eg MOV al,8
    	imm16 means an immediate word eg MOV ax,8

    Instruction
    486
    386
    286
    86

    MOV reg/mem8,reg8
    1
    2/2
    2/3
    2/9

    MOV reg,mem16,reg16
    1
    2/2
    2/3
    2/9

    MOV reg8,reg/mem8
    1
    2/4
    2/5
    2/8

    MOV reg16,reg/mem16
    1
    2/4
    2/5
    2/8

    MOV reg8,imm8
    1
    2
    2
    4

    MOV reg16,imm16
    1
    2
    2
    4

    MOV reg/mem8,imm8
    1
    2/2
    2/3
    4/10

    MOV reg/mem16,imm16
    1
    2/2
    2/3
    4/10

    These timings are taking from the �Borlandâ Turbo Assemblerâ Quick Reference�

    This shows that on the 8086 using variables in memory can make the instuction four
    times as slow. This means that you should avoid using too many variables
    unnecessarily. On the 486 it doesn�t matter as both instructions take the same amount
    of time.

    The method actually used is nearly identical to passing parameters in registers. This
    example is just another version of the example given for passing in registers.

    FirstParam  db 0  ;these are all variables to store
    SecondParam db 0  ;the parameters for the program
    ThirdParam  db 0
    Answer	  db 0
    .....
    	mov FirstParam,10	;first parameter is 10
    	mov SecondParam,20	;second parameter is 20
    	mov ThirdParam,3	;third parameter is 3
    	Call ChangeNumbers
    .....
    ChangeNumbers PROC	     ;Defines start of procedure
    	push ax			;save ax
    	push bx			;save bx
    	mov ax,FirstParam	;copy FirstParam into ax
    	mov bx,SecondParam	;copy SecondParam into bx
    	add ax,bx			;adds number in bx to ax
    	mov bx,ThirdParam	;copy ThirdParam into bx
    	div bx			;divides ax by bx
    	mov Answer,ax		;return answer in Answer
    	pop bx			;restore bx
    	pop ax			;restore ax
    	ret
    ChangeNumbers ENDP		;defines end of procedure

    This way may seem more complicated but it is not really suited for small numbers of
    this type of parameters. It is much more useful when dealing with strings or large
    numbers of big values.

    Passing through Stack:

    This is the most powerful and flexible method of passing parameters. This example
    shows the ChangeNumbers procedure that has been rewritten to pass its parameters
    through the stack.
    	mov ax,10		;first parameter is 10
    	mov bx,20		;second parameter is 20
    	mov cx,3		;third parameter is 3
    	push ax		;put first parameter on stack
    	push bx		;put second parameter on stack
    	push cx		;put third parameter on stack
    	Call ChangeNumbers
    .....
    ChangeNumbers PROC	;Defines start of procedure
    	push bp
    	mov bp,sp
    	mov ax,[bp+8]	;get the parameters from bp
    	mov bx,[bp+6]  ;remember that first it is last out
    	mov cx,[bp+4]	;so number is larger
    	add ax,bx		;adds number in bx to ax
    	div cx		;divides ax by cx
    	mov dx,ax		;return answer in dx
    	ret
    ChangeNumbers ENDP	;defines end of procedure

    This diagram shows the contents of the stack for a program with two parameters:

    To get a parameter from the stack all you need to do is work out where it is. The last
    parameter is at BP+2 and then the next and BP+4.

    Files and how to used them:

    Files can be opened, read and written to. DOS has some ways of doing this which save
    us the trouble of writing our own routines. Yes, more interrupts. Here is a list of
    helpful functions of interrupt 21h that we are going to need to use for our simple file
    viewer.

    Function 3Dh: open file
    Opens an existing file for reading, writing or appending on the specified drive and
    filename.

    INPUT:
            AH = 3Dh
            AL = access mode
                 bits 0-2   000 = read only
                            001 = write only
                            010 = read/write
                 bits 4-6   Sharing mode (DOS 3+)
                            000 = compatibility mode
                            001 = deny all (only current program can access file)
                            010 = deny write (other programs can only read it)
                            011 = deny read (other programs can only write to it)
                            100 = deny none
            DS:DX = segment:offset of ASCIIZ pathname

    OUTPUT:
            CF = 0 function is succesful
            AX = handle
            CF = 1 error has occured
            AX = error code
                    01h missing file sharing software
                    02h file not found
                    03h path not found or file does not exist
                    04h no handle available
                    05h access denied
                    0Ch access mode not permitted

    What does ASCIIZ mean? An ASCIIZ string is a ASCII string with a zero on the end
    (instead of a dollar sign).

    Important: Remember to save the file handle it is needed for later.

    How to save the file handle:
    It is important to save the file handle because this is needed to do anything with the
    file. Well how is this done? There are two methods, which is better?

      Copy the file handle into another register and don't use that register.
      Copy it to a variable in memory.

    The disadvantages with the first method is that you will have to remember not to use
    the register you saved it in and it wastes a register that can be used for something more
    useful. We are going to use the second. This is how it is done:

    	FileHandle DW 0     ;use this for saving the file
    					;handle

         MOV FileHandle,ax   ;save the file handle (same as
    					;FileHandle=ax)

    Function 3Eh: close file
    Closes a file that has been opened.

    INPUT:
            AX = 3Eh
            BX = file handle

    OUTPUT:
            CF = 0 function is sucsessful
            AX = destroyed
            CF = 1 function not sucsessful
            AX = error code - 06h file not opened or unautorized handle.

    Important: Don't call this function with a zero handle because that will close the
    standard input (the keyboard) and you won't be able to enter anything.

    Function 3Fh: read file/device
    Reads bytes from a file or device to a buffer.

    INPUT:
            AH = 3Fh
            BX = handle
            CX = number of bytes to be read
            DS:DX = segment:offset of a buffer

    OUTPUT:
            CF = 0 function is successful
            AX = number of bytes read
            CF = 1 an error has occured
                    05h access denied
                    06h illegal handle or file not opened

    If CF = 0 and AX = 0 then the file pointer was already at the end of the file and no
    more can be read. If CF = 0 and AX is smaller than CX then only part was read
    because the end of the file was reached or an error occured.

    This function can also be used to get input from the keyboard. Use a handle of 0, and it
    stops reading after the first carriage return, or once a specified number of characters
    have been read. This is a good and easy method to use to only let the user enter a
    certain amount of characters.

    Note: If you are using A86 this will cause an error. Change @data to data to make it
    work.

    .MODEL small
    .STACK
    .CODE
    	mov ax,@data     ;base jaddress of data
    	mov ds,ax        ;segment

    	lea dx,FileName  ;put address of fileneame in dx
    	mov al,2         ;access mode - read and write
    	mov ah,3Dh       ;function 3Dh -open a file
    	int 21h          ;call DOS service
    	mov Handle,ax    ;save file handle for later
    	jc ErrorOpening

    	mov dx,offset Buffer  ;address of buffer in dx
    	mov bx,Handle         ;handle in bx
    	mov cx,100            ;amount of bytes to be read
    	mov ah,3Fh            ;function 3Fh - read from file
    	int 21h               ;call dos service
    	jc ErrorReading

    	mov bx,Handle    ;put file handle in bx
    	mov ah,3Eh       ;function 3Eh - close a file
    	int 21h          ;call dos service

    	mov di,OFFSET Buffer+101  ;Where to put the "$"
    	mov byte ptr [di],"$"     ;Put it at es:di
    	mov ah,9                  ;write a string to the
    	mov dx,OFFSET Buffer	 ;screen using function 9h
    	int 21h				 ;of interrupt 21h

    	mov AX,4C00h     ;terminate program and return to
    				  ;DOS using
    	INT 21h          ;interrupt 21h function 4CH

    ErrorOpening:
    	mov dx,offset OpenError ;display an error
    	mov ah,09h       ;using function 09h
    	int 21h          ;call dos service

         mov ax,4C01h ;end program with an errorlevel of 1
    	int 21h

    ErrorReading:
    	mov dx,offset ReadError ;display an error
    	mov ah,09h       ;using function 09h
    	int 21h          ;call dos service

    	mov ax,4C02h ;end program with an errorlevel of 2
            int 21h

    .DATA

    Handle        DW ? ;variable to store file handle
    FileName      DB "C:\test.txt",0 ;file to be opened

    OpenError     DB "An error has occured(opening)!$"
    ReadError     DB "An error has occured(reading)!$"

    Buffer        DB 101 dup (?) ;buffer to store data from
    					    ;file one bigger for $
    END

    Function: 3Ch: Create file
    Creates a new empty file on a specified drive with a specified pathname.

    INPUT:
            AH = 3Ch
            CX = file attribute
                    bit 0 = 1 read-only file
                    bit 1 = 1 hidden file
                    bit 2 = 1 system file
                    bit 3 = 1 volume (ignored)
                    bit 4 = 1 reserved (0) - directory
                    bit 5 = 1 archive bit
                    bits 6-15 reserved (0)
        DS:DX = segment:offset of ASCIIZ pathname

    OUTPUT:
        CF = 0 function is successuful
        AX = handle
        CF = 1 an error has occured
                    03h path not found
                    04h no availible handle
                    05h access denied

    Important: If a file of the same name exists then it will be lost. Make sure that there
    is no file of the same name. This can be done with the function below.

    Function 4Eh: find first matching file
    Searches for the first file that matches the filename given.

    INPUT:
        AH = 4Eh
        CX = file attribute mask (bits can be combined)
                    bit 0 = 1 read only
                    bit 1 = 1 hidden
                    bit 2 = 1 system
                    bit 3 = 1 volume label
                    bit 4 = 1 directory
                    bit 5 = 1 archive
                    bit 6-15 reserved
        DS:DX = segment:offset of ASCIIZ pathname

    OUTPUT:
        CF = 0 function is successful
        [DTA] Disk Transfer Area = FindFirst data block

    Example of checking if file exists:
    File    DB "C:\file.txt",0   ;name of file that we want

            LEA dx,File      ;address of filename
            MOV cx,3Fh       ;file mask 3Fh - any file
            MOV ah,4Eh      	;function 4Eh - find first file
            INT 21h          ;call dos service
            JC NoFile
            ;print message saying file exists
    NoFile:
            ;continue with creating file

    This example program creates a file and then writes to it.
    .MODELSMALL
    .STACK
    .CODE
    	mov ax,@data     ;base jaddress of data
    	mov ds,ax        ;segment

    	mov dx,offset StartMessage    ;display the starting
    							;message
         mov ah,09h     ;using function 09h
         int 21h        ;call dos service

         mov dx,offset FileName ;put offset of filename in dx
         xor cx,cx      ;clear cx - make ordinary file
         mov ah,3Ch     ;function 3Ch - create a file
         int 21h        ;call DOS service
         jc Error       ;jump if there is an error

         mov dx,offset FileName ;put offset of filename in dx
         mov al,2       ;access mode -read and write
         mov ah,3Dh     ;function 3Dh - open the file
         int 21h        ;call dos service
         jc Error       ;jump if there is an error
         mov Handle,ax  ;save value of handle

         mov dx,offset WriteMe 	;address of information to
    						;write to the file
         mov bx,Handle  ;file handle for file
         mov cx,38      ;38 bytes to be written
         mov ah,40h     ;function 40h write to file
         int 21h        ;call dos service
         jc error       ;jump if there is an error
         cmp ax,cx      ;was all the data written? Does
    				;ax=cx?
         jne error      ;if it wasn't then there was an error

         mov bx,Handle  ;put file handle in bx
         mov ah,3Eh     ;function 3Eh - close a file
         int 21h        ;call dos service

         mov dx,offset EndMessage ;display the final message
    						;on the screen
         mov ah,09h     ;using function 09h
         int 21h        ;call dos service

    ReturnToDOS:
         mov AX,4C00h   ;terminate program and return to DOS
         int 21h        ;using interrupt 21h function 4CH

    Error:
         mov dx,offset ErrorMessage ;display an error message
    						  ;on the screen
         mov ah,09h      ;using function 09h
         int 21h         ;call dos service
         jmp ReturnToDOS ;lets end this now

    .DATA
    StartMessage    DB "This program creates a file called�,
    			    �NEW.TXT in the C: directory.$"
    EndMessage      DB 0Ah,0Dh,"File create OK, look at�,
    			    �file to be sure.$"
    Handle          DW ? ;variable to store file handle
    ErrorMessage    DB "An error has occurred!$"
    WriteMe         DB "HELLO, THIS IS A TEST, HAS IT�,
    			    �WORKED?",0 ;ASCIIZ
    FileName        DB "C:\new.txt",0
    END

    How to find out the DOS version:

    In many programs it is necessary to find out what the DOS version is. This could be
    because you are using a DOS function that needs the revision to be over a certain
    level.

    Firstly this method simply finds out what the version is.

    	mov	ah,30h 	;function 30h - get MS-DOS version
    	int 	21h		;call DOS function

    This function returns the major version number in AL and the minor version number in
    AH. For example if it was version 4.01, AL would be 4 and AH would be 01. The
    problem is that if on DOS 5 and higher SETVER can change the version that is
    returned. The way to get round this is to use this method.

    	mov	ah,33h    ;function 33h - actual DOS version
     	mov	al,06h	;subfunction 06h
    	int	21h	  	;call interrupt 21h

    This will only work on DOS version 5 and above so you need to check using the
    former method. This will return the actual version of DOS even if SETVER has
    changed the version. This returns the major version in BL and the minor version in BH.

    Fast string print:

    We have been using a DOS service, function 9 of interrupt 21h to print a string on the
    screen. This isn�t too fast nor does it allow us to use different colours or position the
    text. There is another way to print a string to the screen - direct to memory. This is
    harder as you have to set up everything manually but it has a lot of benifits mainly
    speed.

    TextAttribute db 7 	;contains the character attribute
    				;default is grey on black
    ......
    FastTextPrint PROC
    .286    ;need this for shift instructions. Take out if
    less
            ;than 286
    ;========================================================
    ;INPUT: AH - Row
    ;       AL - Column
    ;       CX - Length of string
    ;       DS:DX - The string
    ;       TextAttribute - the colour of the text
    ;OUTPUT: none
    ;========================================================
            push ax bx cx dx bp si di es ;save registers
            mov bl,ah     	;move row into bl
            xor bh,bh        ;clear bh
            shl bx,5         ;shift bx 5 places to the left
            mov si,bx        ;move bx into si
            shl bx,2         ;shift bx 2 places to the left
            add bx,si        ;add si to bx
            xor ah,ah        ;clear ah
            shl ax,1         ;shift ax 1 place to the left
            add bx,ax        ;add ax onto bx
            mov ax,0b800h    ;ax contains text video memory
            mov es,ax        ;move ax into es
            mov si,dx        ;mov dx into si
    FastTextPrintLoop:
           mov ah,ds:[si]	;put the char at ds[si] into ah
           mov es:[bx],ah    ;move the char in ah to es[bx]
           inc si           	;increment si (si+1)
           inc bx            ;increment bx (bx+1)
           mov ah,TextAttribute ;put the attribute into ah
           mov es:[bx],ah    ;put ah into es position at bx
           inc bx            ;increment bx (bx+1)
           loop FastTextPrintLoop 	;loop CX times
           pop es di si bp dx cx bx ax ;restore registers
           ret			;return
    FastTextPrint ENDP

    Explanation  of new terms in this procedure:

    In this procedure there was several things that you have not come across before. Firsly
    the lines:

    	push ax bx cx dx bp si di es 	;save registers
    	pop es di si bp dx cx bx ax 	;restore registers

    This is just an easier way of pushing and popping more than one register. When TASM
    (or A86) compiles these lines it translates it into separate pushes an pops. This way
    just saves you time typing and makes it easier to understand.

    Note: To make these lines compile in A86 you need to put commas (,) in between the
    registers.

    This line might cause difficulty to you at first but they are quite easy to understand.

    	mov ah,ds:[si] 	;put the char at ds[si] into ah

    What this does is to move the number stored in DS at the location stored in SI into
    AH. It is easier to think of DS being like an array in this command. It is the same as
    this line in C.

    	ah = ds[si];

    Shifts:

    There are four different ways of shifting numbers either left or right one binary
    position.

    SHL Unsigned multiple by two
    SHR Unsigned devide by two
    SAR Signed devide by two
    SAL same as SHL

    The syntax for all four is the same.

    Syntax:
    SHL operand1,operand2

    Note: The 8086 cannot have the value of opperand2 other than 1. 286/386 cannot
    have operand2 higher than 31.

    Using shifts is a lot faster than using MUL�s and DIV�s.

    Loops:

    Using Loop is a better way of making a loop then using JMP�s. You place the amount
    of times you want it to loop in the CX register and every time it reackes the loop
    statement it decrements CX (CX-1) and then does a short jump to the label indicated.
    A short jum means that it can only 128 bytes before or 127 bytes after the LOOP
    instuction.

    Syntax:
    Loop Label

    Using graphics in mode 13h:

    Mode 13h is only availible on VGA, MCGA cards and above. The reason that I am
    talking about this card is that it is very easy to use for graphics because of how the
    memory is arranged.

    First check that mode 13h is possible:

    It would be polite to tell the user if his computer cannot support mode 13h instead of
    just crashing his computer without warning. This is how it is done.

    CheckMode13h:
    ;Returns: DX=0 Not supported, DX=1 supported
    	mov ax,1A00h      	;Request video info for VGA
    	int 10h        	;Get Display Combination Code
    	cmp al,1Ah    		;Is VGA or MCGA present?
    	je Mode13hSupported ;mode 13h is supported
    	xor dx,dx			;mode 13h isn�t supported dx=0
    Mode13hSupported:
    	mov dx,1			;return mode13h supported

    Just use this to check if mode 13h is supported at the beginning of your program to
    make sure that you can go into that mode.

    Note: I have not tested this on a computer that doesn�t hav VGA as I don�t have any.
    In theory this should work but you should test this on computers that don�t have VGA
    and see if it works this out.

    Setting the Video Mode:

    It is very simple to set the mode. This is how it is done.

     	mov  ax,13h		;set mode 13h
     	int  10h			;call bios service

    Once we are in mode 13h and have finished what we are doing we need to we need to
    set it to the video mode that it was in previously. This is done in two stages. Firstly we
    need to save the video mode and then reset it to that mode.

    VideoMode db ?
    ....
    	mov ah,0Fh		;function 0Fh - get current mode
    	int 10h			;Bios video service call
    	mov VideoMode,al	;save current mode

    	;program code here

    	mov al,VideoMode	;set previous video mode
    	xor ah,ah			;clear ah - set mode
    	int 10h			;call bios service
    	mov ax,4C00h		;exit to dos
    	int 21h			;call dos function

    Now that we can get into mode 13h lets do something. Firstly lets put some pixels on
    the screen.

    Function 0Ch - Write Graphics Pixel

    Makes a color dot on the screen at the specified graphics coordinates.

    INPUT:
    	AH = 0Ch
    	AL = Color of the dot
    	CX = Screen column (x coordinate)
    	DX = Screen row (y coordinate)

    OUTPUT:
    	Nothing except pixel on screen.

    Note: This function performes exclusive OR (XOR) with the new color value and the
    current context of the pixel of bit 7 of AL is set.

    	mov ah,0Ch	;function 0Ch
    	mov al,7		;color 7
    	mov cx,160	;x position -160
    	mov dx,100	;y position -100
    	int 10h		;call bios service

    This example puts a pixel into the middle of the screen in a the color grey. The
    problem with this method is that calling interrupts is really slow and should be avoided
    in speed critical areas. With pixel plotting if you wanted to display a picture the size of
    the screen you would have to call this procedure 64,000 times (320 x 200).

    Some optimizations:

    This method isn�t too fast and we could make it a lot faster. How? By writing direct to
    video memory. This is done quite easily.

    The VGA memory starts at 0A000h. To work out where each pixel goes you use this
    simple formula:

    	Location = 0A000h + Xposition + (Yposition x 320)

    Location is the memory location which we want to put the pixel.

    This procedure is quite a fast way to put a pixel onto the screen. Thanks go to Denthor
    of Asphyxia as I based this on his code.

    PutPixel PROC
    .286		;enable 286 instructions for shifts remove if
    		;you have less than an 286.
    ;========================================================
    ;INPUT: 	BX=X postion
    ;		DX=Y position
    ;		CL=colour
    ;OUTPUT:  None
    ;========================================================
    ;this can be optimized by not pushing ax if you don�t
    ;need to save it. For A86 change push ds es ax to push
    ;ds,es,ax and do the same thing with pop.
    	push ds es ax		;save ds,es and ax
    	mov ax,0A000h		;ax contains address of video
    	mov es,ax			;es contains address of video
    	mov di,bx			;move x position into di
    	mov bx,dx			;mov y postion into bx
    	shl dx,8			;shift dx 8 places to the left
    	shl bx,6			;shift bx 6 places to the left
    	add dx,bx			;add dx and bx together
    	add di,bx			;add di and bx together
    	mov al,cl			;put colour in al
    	stosb			;transfer to video memory
    	pop ax es ds		;restore ds,es and ax
    	ret
    PutPixel ENDP

    Thank you for reading. I hope that you have learnt something from this. If you
    need any more help then email me.

    Gavin Estey 19/2/95

    Gavin�s Introduction to Assembly		Page 1

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