Rasmus Rendal

Rasmus Rendal

Nested for loops go brrrr

03 Oct 2021

Assembly Reference

I’m pretty good at learning x86_64 assembly. I’ve done it more times than I can count. But now I’ve reached the point where I would like to stop learning assembly, and just have a good reference for assembly. But I couldn’t find one, so I made a reference for assembly.

This reference is a work in progress, and will be continuously updated as I find myself looking up stuff.

General Purpose Registers

The following are the “general purpose” registers for x86_64. The calling convention describes the convention for *nix systems. All registers not denoted “Callee Saved” are caller saved, if required.

Quadword Doubleword Word Byte Calling Convention
rax eax ax al Return Value
rbx ebx bx bl Callee Saved
rcx ecx cx cl Argument 4
rdx edx dx dl Argument 3
rsi esi si sil Argument 2
rdi edi di dil Argument 1
rbp ebp bp bpl Base Pointer (Callee Saved)
rsp esp sp spl Stack Pointer (Callee Saved)
r8 r8d r8w r8b Argument 5
r9 r9d r9w r9b Argument 6
r10 r10d r10w r10b
r11 r11d r11w r11b
r12 r12d r12w r12b Callee Saved
r13 r13d r13w r13b Callee Saved
r14 r14d r14w r14b Callee Saved
r15 r15d r15w r15b Callee Saved


The rFlags register is a special register that contains information about the status of, amongst other things, arithmetic operations. It is used a lot in, for instance, control flow operations. Many of them are “system registers”, which I haven’t had a reason to care about yet, and therefore are not included.

Bit Mnemonic Description
0 CF Carry Flag
2 PF Parity Flag
4 AF Auxilary Carry Flag
6 ZF Zero Flag
7 SF Sign Flag
10 DF Direction Flag
11 OF Overflow Flag

Carry Flag

If the last integer operation has resulted in a carry on the most significant bit, the value of this flag is set to 1. Otherwise, it is set to zero. On subtraction, it is set to one in the case of a borrow.

It is not changed by increment, decrement. Bit shifting shift into the carry flag.

Zero flag

If the last arithmetic operation resulted in zero, this flag is set to one, otherwise it is set to zero. This flag is also set by the test and cmp instructions.



These instructions sign extend al into ax, ax into eax, and eax into rax, respectively.


First of all, mov is Turing-complete. So keep this in mind before stepping into this particular hell.

  • The zx affix means that the mov does sign extension


The jmple, jmpge, etc. are computed based on the information in rFlags. Things like jmple can be computed by combining the information from the carry flag and the zero flag. While jmple and jmpge are mnemonics for “jump less than or equal” and “jump greater than or equal”, there are also jmpbe, jmpae and friends, which treat the operands as unsigned numbers.


Syscalls are how programs interact with the kernel. Syscalls have their own calling convention, separate from procedure calls. Each syscall has its own number, which is put into the rax register. The arguments are put into the registers rdi, rsi, rdx, r10, r8 and r9, and the values of rcx and r11 are destroyed. rax is used as the return value.


The leave instruction is equivalent to:

movq %rsp, %rbp
popq %rbp


If you found something wrong, or a notable omission, I would love to hear it. Please, send me an e-mail.

Sources / Further Information: