x64 NASM Cheat Sheet

linux-x64-nasm-cheatsheet.md

x64 NASM cheat sheet

Registers

	64 bit	32 bit	16 bit	8 bit
A (accumulator)	RAX	EAX	AX	AL
B (base, addressing)	RBX	EBX	BX	BL
C (counter, iterations)	RCX	ECX	CX	CL
D (data)	RDX	EDX	DX	DL
	RDI	EDI	DI	DIL
	RSI	ESI	SI	SIL
Numbered (n=8..15)	Rn	RnD	RnW	RnB
Stack pointer	RSP	ESP	SP	SPL
Frame pointer	RBP	EBP	BP	BPL

As well as XMM0 .. XMM15 for 128 bit floating point numbers.

Calling C

Put function arguments (first to last) in the following registers (64 bit representations): RDI, RSI, RDX, RCX, R8, R9, then push to stack (in reverse, has to be cleaned up by the caller!) XMM0 - XMM7 for floats

Return values are stored in RAX (int) or XMM0 (float)

RBP, RBX, R12, R13, R14, R15 will not be changed by the called function, all others may be

Align stack pointer (RSP) to 16 byte, calling pushes 8 bytes!

Keep in mind that strings (in C) are 0-terminated

Like in a normal C program, the label that is (de facto) called first is main, with the args argc (argcount) in RDI, and the char** argv in RSI (the commandline arguments as in C's main function).

Data

Definition size	Definition instruction
8 bit	db
16 bit	dw
32 bit	dd
64 bit	ddq/do
float	dd
double	dq
extended precision	dt

Conditionals

cmp op1, op2 -> mimics sub op1, op2 but only changes the zero and carry flag for comparing.

Prefixes:

• j~ x -> jump to x if ~
• cmov~ x, y -> conditional mov x, y if ~
• setc~ x -> set x to 1 if ~, x is 8 bit reg

Many suffixes, including:

• a (above, >)
• ae (above or equal, >=)
• b (below, <)
• be (below or equal, <=)
• e (equal, =)
• ne (not equal, !=)

Program structure

• global <entry> -> exposes entry point
• extern <function> -> declares a function in another linked .o file (e.g. C function, other asm file)
• section <sectiontype> -> sets section, usually:
  • .text -> program code
  • .data -> data

The program entry point of a standalone program is the label _start. When compiled with gcc, C provides _start, which inits and then jumps to main, which should then be implemented by the program.

Syscalls

• put syscall number in EAX (e.g. on Linux: 60 for exit, 1 for write to stdout)
• put arguments in the registers (see above) like when calling a C function
• execute the syscall instruction

Assemble

• Assemble: nasm -felf64 -o <object> <filename>
• Link with ld: ld -o <output> <object>
• Link with gcc: gcc -o <output> <object>

Sources

• Forked from: mpdrescher
• Main: NASM Tutorial
• Registers: Assembly registers
• Conditionals: Jumps and loops (de)

The 8-bit Registers Are Like:

Register Purpose	16 bit	8 bit
A (accumulator)	AX	AL (lower) AH (higher)
B (base, addressing)	BX	BL (lower) DH (higher)
C (counter, iterations)	CX	CL (lower) CH (higher)
D (data)	DX	DL (lower) DH (higher)