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 |
rFlags
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.
Instructions
cbw/cwde/cdqe
These instructions sign extend al into ax, ax into eax, and eax into rax, respectively.
mov
First of all, mov is Turing-complete.
So keep this in mind before stepping into this particular hell.
- The
zxaffix means that themovdoes sign extension
jmp
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.
syscall
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.
leave
The leave instruction is equivalent to:
movq %rsp, %rbp
popq %rbp
Mistakes
If you found something wrong, or a notable omission, I would love to hear it. Please, send me an e-mail.