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Merge pull request #810 from corob-msft/cr-regflag-feedback
Update flags register usage requirement
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‎docs/build/register-usage.md

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---
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title: "Register Usage | Microsoft Docs"
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title: "Register usage | Microsoft Docs"
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ms.custom: ""
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ms.date: "11/04/2016"
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ms.technology: ["cpp-tools"]
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ms.author: "corob"
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ms.workload: ["cplusplus"]
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---
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# Register Usage
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The [!INCLUDE[vcprx64](../assembler/inline/includes/vcprx64_md.md)] 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.
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The following table describes how each register is used across function calls:
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||||
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|-|-|-|
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|Register|Status|Use|
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|RAX|Volatile|Return value register|
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|RCX|Volatile|First integer argument|
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|RDX|Volatile|Second integer argument|
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|R8|Volatile|Third integer argument|
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|R9|Volatile|Fourth integer argument|
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|R10:R11|Volatile|Must be preserved as needed by caller; used in syscall/sysret instructions|
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|R12:R15|Nonvolatile|Must be preserved by callee|
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|RDI|Nonvolatile|Must be preserved by callee|
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|RSI|Nonvolatile|Must be preserved by callee|
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|RBX|Nonvolatile|Must be preserved by callee|
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|RBP|Nonvolatile|May be used as a frame pointer; must be preserved by callee|
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|RSP|Nonvolatile|Stack pointer|
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|XMM0, YMM0|Volatile|First FP argument; first vector-type argument when `__vectorcall` is used|
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|XMM1, YMM1|Volatile|Second FP argument; second vector-type argument when `__vectorcall` is used|
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|XMM2, YMM2|Volatile|Third FP argument; third vector-type argument when `__vectorcall` is used|
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|XMM3, YMM3|Volatile|Fourth FP argument; fourth vector-type argument when `__vectorcall` is used|
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|XMM4, YMM4|Volatile|Must be preserved as needed by caller; fifth vector-type argument when `__vectorcall` is used|
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|XMM5, YMM5|Volatile|Must be preserved as needed by caller; sixth vector-type argument when `__vectorcall` is used|
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|XMM6:XMM15, YMM6:YMM15|Nonvolatile (XMM), Volatile (upper half of YMM)|Must be preserved by callee. YMM registers must be preserved as needed by caller.|
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## See Also
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[x64 Software Conventions](../build/x64-software-conventions.md)
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[__vectorcall](../cpp/vectorcall.md)
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# Register usage
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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.
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## Register volatility and preservation
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The following table describes how each register is used across function calls:
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|-|-|-|
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|Register|Status|Use|
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|RAX|Volatile|Return value register|
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|RCX|Volatile|First integer argument|
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|RDX|Volatile|Second integer argument|
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|R8|Volatile|Third integer argument|
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|R9|Volatile|Fourth integer argument|
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|R10:R11|Volatile|Must be preserved as needed by caller; used in syscall/sysret instructions|
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|R12:R15|Nonvolatile|Must be preserved by callee|
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|RDI|Nonvolatile|Must be preserved by callee|
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|RSI|Nonvolatile|Must be preserved by callee|
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|RBX|Nonvolatile|Must be preserved by callee|
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|RBP|Nonvolatile|May be used as a frame pointer; must be preserved by callee|
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|RSP|Nonvolatile|Stack pointer|
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|XMM0, YMM0|Volatile|First FP argument; first vector-type argument when `__vectorcall` is used|
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|XMM1, YMM1|Volatile|Second FP argument; second vector-type argument when `__vectorcall` is used|
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|XMM2, YMM2|Volatile|Third FP argument; third vector-type argument when `__vectorcall` is used|
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|XMM3, YMM3|Volatile|Fourth FP argument; fourth vector-type argument when `__vectorcall` is used|
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|XMM4, YMM4|Volatile|Must be preserved as needed by caller; fifth vector-type argument when `__vectorcall` is used|
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|XMM5, YMM5|Volatile|Must be preserved as needed by caller; sixth vector-type argument when `__vectorcall` is used|
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|XMM6:XMM15, YMM6:YMM15|Nonvolatile (XMM), Volatile (upper half of YMM)|Must be preserved by callee. YMM registers must be preserved as needed by caller.|
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On function exit and on function entry to C Runtime Library calls and Windows system calls, the direction flag in the CPU flags register is expected to be cleared.
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## See also
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- [x64 Software Conventions](../build/x64-software-conventions.md)
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- [__vectorcall](../cpp/vectorcall.md)
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- [Direction flag](../c-runtime-library/direction-flag.md)

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