X86 Assembly/NASM Syntax

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x86 Assembly

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[edit] NASM Syntax

Wikipedia has related information at NASM

NASM syntax looks like:

mov ax, 9

This loads the number 9 into register ax. Notice that the instruction format is "dest, src". This follows the Intel style x86 instruction formatting, as opposed to the AT&T style used by the GNU Assembler. Note for people using gdb with nasm, you can set gdb to use Intel-style disassembly by issuing the command:

set disassembly-flavor intel

[edit] NASM Comments

A single semi-colon is used for comments, and can be used like a double slash in C/C++.

[edit] Example I/O (Linux)

To pass the kernel a simple input command on Linux, you would pass values to the following registers and then send the kernel an interrupt signal. To read in a single character from standard input (such as from a user at their keyboard), do the following:

; read a byte from stdin
mov    eax, 3		; 3 is recognized by the system as meaning "read"
mov    edx, 1		; input length (one byte)
mov    ecx, variable	; address to pass to
mov    ebx, 1		; read from standard input
int 0x80                ; call the kernel

Outputting follows a similar convention:

mov    eax, 4           ; the system interprets 4 as "write"
mov    ecx, variable    ; pointer to the value being passed
mov    ebx, 1           ; standard output (print to terminal)
mov    edx, 4           ; length of output (in bytes)
int 0x80

Passing values to the registers in different orders won't affect the execution when the kernel is called, but deciding on a methodology can make it much easier to read.