X86 Assembly/NASM Syntax

The Netwide Assembler is an assembler that uses Intel syntax; it is often used on Linux.

NASM Syntax

The Netwide Assembler (NASM) uses a syntax "designed to be simple and easy to understand, similar to Intel's but less complex". This means that the operand order is dest then src, as opposed to the AT&T style used by the GNU Assembler. For example,

mov ax, 9

loads the number 9 into register ax.

For those using gdb with nasm, you can set gdb to use Intel-style disassembly by issuing the command:

set disassembly-flavor intel

Comments

A single semi-colon is used for comments, and functions the same as double slash in C++: the compiler ignores from the semicolon to the next newline.

Macros

NASM has powerful macro functions, similar to C's preprocessor. For example,

%define newline 0xA
%define func(a, b) ((a) * (b) + 2)

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 ebx, 0           ; read from standard input
mov ecx, variable    ; address to pass to
mov edx, 1           ; input length (one byte)
int 0x80             ; call the kernel

Outputting follows a similar convention:

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