# AMD64 NASM Cheatsheet

## Sizes
| Size (bits) | Size (bytes) | Name   | NASM Mnemonics / Directives        |
|-------------|--------------|--------|----------------------------------------|
| 8           | 1            | byte   | DB, RESB                               |
| 16          | 2            | word   | DW, RESW                               |
| 32          | 4            | dword  | DD, RESD                               |
| 64          | 8            | qword  | DQ, RESQ                               |
| 80          | 10           | tword  | DT, REST                               |
| 128         | 16           | oword  | DO, RESO, DDQ, RESDQ                   |
| 256         | 32           | yword  | DY, RESY                               |
| 512         | 64           | zword  | DZ, RESZ                               |

## Registers

| Register         | Usage                                                                                      | Preserved Across Function Calls |
|------------------|-------------------------------------------------------------------------------------------|-------------------------------|
| %rax             | Temporary register; with variable arguments passes information about the number of vector registers used; 1st return register | No                            |
| %rbx             | Callee-saved register; optionally used as base pointer                                    | Yes                           |
| %rcx             | Used to pass 4th integer argument to functions                                            | No                            |
| %rdx             | Used to pass 3rd argument to functions; 2nd return register                               | No                            |
| %rsp             | Stack pointer                                                                             | Yes                           |
| %rbp             | Callee-saved register; optionally used as frame pointer                                   | Yes                           |
| %rsi             | Used to pass 2nd argument to functions                                                    | No                            |
| %rdi             | Used to pass 1st argument to functions                                                    | No                            |
| %r8              | Used to pass 5th argument to functions                                                    | No                            |
| %r9              | Used to pass 6th argument to functions                                                    | No                            |
| %r10             | Temporary register, used for passing a function’s static chain pointer                    | No                            |
| %r11             | Temporary register                                                                        | No                            |
| %r12–%r15        | Callee-saved registers                                                                    | Yes                           |
| %xmm0–%xmm1      | Used to pass and return floating point arguments                                          | No                            |
| %xmm2–%xmm7      | Used to pass floating point arguments                                                     | No                            |
| %xmm8–%xmm15     | Temporary registers                                                                       | No                            |
| %mmx0–%mmx7      | Temporary registers                                                                       | No                            |
| %st0, %st1       | Temporary registers; used to return long double arguments                                 | No                            |
| %st2–%st7        | Temporary registers                                                                       | No                            |
| %fs              | Reserved for system (as thread specific data register)                                    | No                            |
| mxcsr            | SSE2 control and status word                                                              | Partial                       |
| x87 SW           | x87 status word                                                                           | No                            |
| x87 CW           | x87 control word                                                                          | Yes                           |


### additional resources  
[Linux syscalls](https://blog.rchapman.org/posts/Linux_System_Call_Table_for_x86_64/)  
[System V AMD64 ABI](https://refspecs.linuxbase.org/elf/x86_64-abi-0.99.pdf)  
[Getting arguments](https://github.com/tonyOreglia/argument-counter/wiki/x86-64-Linux-Assembly-Part-1:-Printing-Command-Line-Arguments)  
[x86 and amd64 instruction reference](https://www.felixcloutier.com/x86/)