Libasm 2s

64-bit	32-bit	16-bit	8-bit	Function-Call Role	Syscall Role	Save Convention	Additional Notes
RAX	EAX	AX	AL	- Return value register (integers/pointers) - Accumulator for arithmetic	- Syscall number (input) - Return value (output)	Caller-saved (volatile)	Often used for multiplication/division (high part with RDX). Overwritten by syscalls.
RBX	EBX	BX	BL	- General-purpose usage in functions	- Not used by default for syscall arguments	Callee-saved (non-volatile)	Must be restored by callee if used.
RCX	ECX	CX	CL	- 4th argument (func calls) - Often used for loops/shifts	- Not used for direct syscall args (the 4th arg is in R10 on Linux)	Caller-saved (volatile)	Overwritten by most calls; store if you need its value preserved.
RDX	EDX	DX	DL	- 3rd argument (func calls)	- 3rd syscall argument	Caller-saved (volatile)	High half of results in 128-bit multiply/div instructions with RAX.
RSI	ESI	SI	SIL	- 2nd argument (func calls)	- 2nd syscall argument	Caller-saved (volatile)	Source pointer for string ops (MOVS, LODS).
RDI	EDI	DI	DIL	- 1st argument (func calls)	- 1st syscall argument	Caller-saved (volatile)	Destination pointer for string ops (MOVS, STOS).
RBP	EBP	BP	BPL	- Frame pointer (if used)	- Not used for syscall args by default	Callee-saved (non-volatile)	Often used to maintain stack frames; must be restored if used as GP register.
RSP	ESP	SP	SPL	- Stack pointer (not used for register-based args)	- Stack pointer is the same concept in syscalls	(Special role)	Must always point to valid stack location. Not freely used for general-purpose.
R8	R8D	R8W	R8B	- 5th argument (func calls)	- 5th syscall argument	Caller-saved (volatile)	Additional scratch register.
R9	R9D	R9W	R9B	- 6th argument (func calls)	- 6th syscall argument	Caller-saved (volatile)	Additional scratch register.
R10	R10D	R10W	R10B	- Scratch register in function calls	- 4th syscall argument (on Linux)	Caller-saved (volatile)	Often used for preserving RCX across syscalls (since RCX is used for the return address in syscall instruction).
R11	R11D	R11W	R11B	- Scratch register in function calls	- Overwritten by syscall to store flags in some mechanisms	Caller-saved (volatile)	Caller must save it if it wants to keep its value around a syscall. Also used for saving flags on SYSENTER/SYSCALL.
R12	R12D	R12W	R12B	- General-purpose usage	- Not used for syscall args by default	Callee-saved (non-volatile)	Must be preserved if used.
R13	R13D	R13W	R13B	- General-purpose usage	- Not used for syscall args by default	Callee-saved (non-volatile)	Must be preserved if used.
R14	R14D	R14W	R14B	- General-purpose usage	- Not used for syscall args by default	Callee-saved (non-volatile)	Must be preserved if used.
R15	R15D	R15W	R15B	- General-purpose usage	- Not used for syscall args by default	Callee-saved (non-volatile)	Must be preserved if used.

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Quick Reference

• Function Call (System V AMD64) Integer/Pointer Arguments

  1. RDI
  2. RSI
  3. RDX
  4. RCX
  5. R8
  6. R9

• Syscall (Linux x86-64) Arguments

  1. RDI
  2. RSI
  3. RDX
  4. R10
  5. R8
  6. R9

• Syscall Number: RAX

• Syscall Return Value: RAX

• Caller-Saved (Volatile) Registers:
  RAX, RCX, RDX, RSI, RDI, R8, R9, R10, R11

• Callee-Saved (Non-Volatile) Registers:
  RBX, RBP, R12, R13, R14, R15

• RSP: Special stack pointer register; must remain valid at all times.

• Return Value (function or syscall): RAX