x86-64 General-Purpose Registers

x86-64 has 16 general-purpose integer registers, each 64 bits wide. Sub-register aliases provide access to the lower 32, 16, and 8 bits.

Register map

64-bit	32-bit	16-bit	8-bit high	8-bit low	Conventional use
`rax`	`eax`	`ax`	`ah`	`al`	Return value, accumulator
`rbx`	`ebx`	`bx`	`bh`	`bl`	Callee-saved (SysV & MSVC)
`rcx`	`ecx`	`cx`	`ch`	`cl`	4th arg (MSVC) / shift count
`rdx`	`edx`	`dx`	`dh`	`dl`	3rd arg (MSVC) / 2nd return
`rsi`	`esi`	`si`	—	`sil`	2nd arg (SysV) / string src
`rdi`	`edi`	`di`	—	`dil`	1st arg (SysV) / string dst
`rbp`	`ebp`	`bp`	—	`bpl`	Frame pointer (callee-saved)
`rsp`	`esp`	`sp`	—	`spl`	Stack pointer
`r8`	`r8d`	`r8w`	—	`r8b`	5th arg (SysV) / 3rd arg (MSVC)
`r9`	`r9d`	`r9w`	—	`r9b`	6th arg (SysV) / 4th arg (MSVC)
`r10`	`r10d`	`r10w`	—	`r10b`	Temp / syscall arg 4 (Linux)
`r11`	`r11d`	`r11w`	—	`r11b`	Temp / clobbered by SYSCALL
`r12`	`r12d`	`r12w`	—	`r12b`	Callee-saved
`r13`	`r13d`	`r13w`	—	`r13b`	Callee-saved
`r14`	`r14d`	`r14w`	—	`r14b`	Callee-saved
`r15`	`r15d`	`r15w`	—	`r15b`	Callee-saved (often GOT ptr in PIC)

"High" 8-bit aliases (ah, bh, ch, dh) exist only for the original four registers and cannot be used in instructions that also use a REX prefix.

Sub-register write behaviour

Write width	Effect on full 64-bit register
8-bit (`al`)	Merges into `rax` — upper 56 bits unchanged
16-bit (`ax`)	Merges into `rax` — upper 48 bits unchanged
32-bit (`eax`)	Zero-extends into `rax` — upper 32 bits cleared
64-bit (`rax`)	Full register replaced

The 32-bit zero-extension rule is a frequent source of confusion: writing eax implicitly clears the upper half of rax, so xor eax, eax suffices to zero the full rax.

Reverse-engineering notes

Seeing r15 used as a base pointer throughout a function often means the compiler spilled the Global Offset Table pointer there (PIC shared libraries on Linux).
rcx is clobbered by SYSCALL and by the Microsoft ABI's first argument — knowing the target ABI is essential before labelling rcx as loop counter vs. argument.
Decompilers infer variable types partly from which sub-register is used: al suggests char/uint8_t, ax suggests short/uint16_t, eax suggests int/uint32_t.