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

Question

I decided to take a crack at assembly the other day, and I've been playing around with really basic things like printing stuff from argv to stdout. I found this great list of linux syscall numbers with arguments and everything, and I'm curious why r10 is used for arguments before r8 and r9. I've found all kinds of weird conventions about what can be used what for what and when, like how loop counters go in rcx. Is there a particular reason why r10 was moved up? Was it more convenient?

I should probably also mention I'm interested in this out of curiosity, not because it's causing me problems.

Edit: I found this question which gets close, referencing the x64 ABI documentation on page 124, where it notes that user level applications use rdi, rsi, rdx, rcx, r8, r9. The kernel on the other hand uses r10 instead of rcx, and destroys rcx and r11. That might explain how r10 ended up there, but then why was it swapped in?

Answer 1

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.