Linux x64: why does r10 come before r8 and r9 in syscalls?

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RCX, along with R11, is used by the syscall instruction, being immediately destroyed by it. Thus these registers are not only not saved after syscall, but they can’t even be used for parameter passing. Thus R10 was chosen to replace unusable RCX to pass fourth parameter.

See also this answer for a bit more information on how syscall uses these registers.

Reference: Intel’s Instruction Set Reference, look for SYSCALL.

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