a newbie guide to assembler programing

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Common instructions A newbie guide to Assembler programing

Common Instructions

1997 by Cruehead / MiB

Here...

we'll describe the instructions that you must know in order to understand whatsgoing on. This is only the basics and if you want to become a serious cracker

(and I hope you want to), you better learn more about the asm instructions.

MOV dest, source

This instuction simply moves a value into a location in the memory (register or

variable).

EG : MOV AX, 1234h ; AX = 1234h

MOV BX, AX ; BX = AX

First of all this would move the value 1234 hex (4660 dec) into the AX register.

Then the value in AX (1234h) would be moved into the BX register. In a high

level language this would be the same as AX:=$1234; BX:=AX; (pascal notation).

ADD dest, value

This simply add's something to the value stored in dest.

EG : MOV AX, 10h ; Ax is now 10h

ADD AX, 10h ; Ax is now 20h

ADD AX, 5h ; Ax is now 25h

SUB dest, value

This substracts something from the value stored in dest.

EG : MOV AX, 10h ; AX is now 10h

SUB AX, 2h ; Ax is now 8h

INC dest

Increments something (register, variable or anything).

EG : MOV AX, 10h ; AX is 10h

INC AX ; Ax is now 11h


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DEC dest

Decrements something (register, variable or anything).


dec AX ; Ax is now Fh

CMP source, dest

Compares source with dest.


MOV BX, 11h ; BX is 11h

CMP AX, BX ; Compares AX with BX

The line after CMP AX,BX will problaby be a conditional jump. If we wanted tojump if AX=BX, we would place a JE (Jump if equal) location (location would be

an offset) after the CMP instruction. If we wanted to jump if AX was NOT equal

to BX, we would place a JNE (Jump if not equal) location after the CMP

instruction. There are alot of conditional jumps - here is a list of them.

JMP location

Jumps to another location in the code.

EG : JMP 200h ; The program would here jump to offset 200h

MOVSB or MOVSW

Moves (well, copies really) either a byte (MOVSB) or a word (MOVSW) from DS:SI

to ES:DI Increments SI.

EG : Lets say that DS:SI points to a byte which holds the value of 5h

MOVSB ; Takes the byte that DS:SI points to and places it in ES:DI

The byte that ES:DI points to now has the value of 5h

These instructions is very common in cracking, when a string is copied to

another location. The instructions are then used together with the REP

instruction.

LODSB or LODSW

Loads either a byte or a word from DS:SI and puts it in AL (LODSB) or AX(LODSW). Increments SI.

EG : Lets say that DS:SI points to a word which holds the value of EBh

LODSW ; Copies the word that DS:SI points to and places it in AX


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AX will now contain the value of EBh

These instructions are often used together with the REP instruction.

STOSB or STOSW

Takes the value in AL (STDSB) or AX (STDSW) and places it in DS:SI. Increments

SI.

EG : Lets say that AX holds the value of EBh

STOSW ; Copies the value in AX and places it in the word that DS:SI points to.

; DS:SI now points to a word containing EBh

These instructions are often used together with the REP instruction.

REP

Repeat an instruction for the number of times specified in the CX register. A

REP infront of a MOVSB,LODSB or STOSB (or infront of the word versions of these

instructions) would cause that instruction to repeat itself.

EG : MOV AL,Bh ; AL now contains bh

MOV CX,5h ; CX now contains 5h

REP STOSB ; This would copy the value of AL (5h) into whatever DS:SI points to 5 times

; and increment SI for every time.

CALL procedure

Calls a procedure, and after the procedure is finnished, returns.

EG : CALL 4020 ; Jumps to the offset 4020 and continues the execution there until it

; reaches a RET. Then it continues on the next line.

This was a near call. When a near call is executed you only jump to a diffrent

offset. There are also far calls. They jump to a complete diffrent segment and

offset.

EG : CALL 013f:2310 ; Jumps to segment 013f, and the offset points to 2310.

Back to Asm tutorial page...

Copyright MiB 1998. All rights reversed.

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