ref: a843f825632bd19e34a81bccf9b8a015a0196586
dir: /lib/sys/syscall+freebsd-x64.s/
.globl sys$syscall sys$syscall: /* hack: We load 6 args regardless of how many we actually have. This may load junk values, but if the syscall doesn't use them, it's going to be harmless. */ movq %rdi,%rax movq %rsi,%rdi movq %rdx,%rsi movq %rcx,%rdx movq %r8,%r10 movq %r9,%r8 movq 8(%rsp),%r9 syscall ret