Intel IA-32 Assembly Tutorial - A Guide to the Basics of x86 Assembly - Page 03

Instructions

A line of code, or instruction, may have a label, must have an instruction mnemonic, usually have an operand and optionally, a comment. The following is an example with all four.

	.code
	lblLoop: mov ebx, my_variable ;moving my_variable to ebx

lblLoop: is a code label, mov is an instruction mnemonic, ebx and my_variable are both operands, and the comment begins with a semicolon. A code label (loop:) is used to jmp (jump, unconditionally) to:

	.code
lblLoop:
	mov ebx, eax
	jmp lblLoop

This is an infinite loop.

Data labels define variables in the data (.data) area of a program.

	.data
	my_first_variable DWORD ;creates an unsigned 32-bit variable
	my_second_variable SWORD ;creates a signed 16-bit variable
	my_third_variable REAL4 ;creates a 4 byte single precision real variable
	my_fourth_variable REAL8 ;creates an 8 byte double precision real variable
	my_fifth_variable REAL10 ;creates an 10 byte double precision extended precision real variable
	my_first_array BYTE 10 DUP(?) ;creates ten byte array uninitialized
	my_second_array BYTE 20 DUP(0) ;creates twenty byte array initialized to all zeros
	.code
mov eax,10

my_first_variable is at offset 0 and my_second_variable is at offset 4, etc. Also, the size of a variable can easily be determined using the current location counter which is the $ sign.

	.data
	my_first_variable DWORD ;creates an unsigned 32-bit variable
	my_first_variable_size = ($ - my_first_variable)

	my_second_variable SWORD ;creates a signed 16-bit variable

	my_third_variable REAL4 ;creates a 4 byte single precision real variable

	my_fourth_variable REAL8 ;creates an 8 byte double precision real variable
	my_fourth_variable_size = ($ - my_fourth_variable);

	my_fifth_variable REAL10 ;creates an 10 byte double precision extended precision real variable

	my_first_array BYTE 10 DUP(?) ;creates ten byte array uninitialized
	my_first_array_size = ($ - my_first_array)

	my_second_array BYTE 20 DUP(0) ;creates twenty byte array initialized to all zeros
	my_second_array_size = ($ - my_second_array)
	.code
mov eax,10

Notice that the current location counter ($) must be used immediately after the variable is defined.

Instruction mnemonics examples:

	mov
	add
	sub  ;subtract
	mul  ;multiply
	div  ;divide
	jmp
	call ;call a PROCedure
	stc  ;set the Carry flag
	inc  ;increment by one
	dec  ;decrement by one

The instruction mnemonic can have between zero and three operands.

Comments can be single line ones using the semicolon or a block using the COMMENT directive.

	COMMENT @
		Comment line 1
		Comment line 2
		Comment line 3
	@

More on Directives

The .386 directive identifies the minimum hardware the program will run one. A modern machine would use .686 . The .model flat,stdcall tells the assembler to generate code for a CPU protected mode program using a flat 32-bit memory model (no 16-bit and 24-bit pointers to think about). STDCALL specifies that each procedure should clean the memory stack up after they are done with it. .stack 8192 specifies how large the stack should be.

Little Endian Order

Intel processors store the least significant byte of a multi-byte variable at the lowest address. This table demonstrates how Intel processors store the 32-bit value 12345678h.

To reverse this order to "Big Endian Order" use the instructions BSWAP (Byte-SWAP) for DWORD values or XCHG (eXCHanGe) for WORD values.

	BSWAP eax   ; reverse the order of bytes in eax
	XCHG al, ah ; exchange the order of bytes in ax

Comment