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Register Usage

The x64 architecture provides for 16 general-purpose registers (hereafter referred to as integer registers) as well as 16 XMM/YMM registers available for floating-point use. Volatile registers are scratch registers presumed by the caller to be destroyed across a call. Nonvolatile registers are required to retain their values across a function call and must be saved by the callee if used.

The following table describes how each register is used across function calls:

Register

Status

Use

RAX

Volatile

Return value register

RCX

Volatile

First integer argument

RDX

Volatile

Second integer argument

R8

Volatile

Third integer argument

R9

Volatile

Fourth integer argument

R10:R11

Volatile

Must be preserved as needed by caller; used in syscall/sysret instructions

R12:R15

Nonvolatile

Must be preserved by callee

RDI

Nonvolatile

Must be preserved by callee

RSI

Nonvolatile

Must be preserved by callee

RBX

Nonvolatile

Must be preserved by callee

RBP

Nonvolatile

May be used as a frame pointer; must be preserved by callee

RSP

Nonvolatile

Stack pointer

XMM0, YMM0

Volatile

First FP argument; first vector-type argument when __vectorcall is used

XMM1, YMM1

Volatile

Second FP argument; second vector-type argument when __vectorcall is used

XMM2, YMM2

Volatile

Third FP argument; third vector-type argument when __vectorcall is used

XMM3, YMM3

Volatile

Fourth FP argument; fourth vector-type argument when __vectorcall is used

XMM4, YMM4

Volatile

Must be preserved as needed by caller; fifth vector-type argument when __vectorcall is used

XMM5, YMM5

Volatile

Must be preserved as needed by caller; sixth vector-type argument when __vectorcall is used

XMM6:XMM15, YMM6:YMM15

Nonvolatile (XMM), Volatile (upper half of YMM)

Must be preserved as needed by callee. YMM registers must be preserved as needed by caller.

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