Syscall.sh (syscall.sh)

Besides the point, but are there any deployed Itanium systems left? They seem to have fizzled out somewhere around the 2010s.

Was it? I'm not saying you're wrong but I don't recall x64 ever referring to Itanium, not even colloquially. "IA-64" was the name that I remember.

nice work.

(hopefully) constructive feedback from me, it would be nice if you can list all the syscall items in one page for ctrl+f reasons. and if you can somehow enumerate and include the parameters they take that'd be really nice, but I know in windows land you don't have documentation of the function signatures, I get why you didn't do that already, it is super tedious.

Indeed, in bytecode golfing, you can actually make use of SYSCALL's use of RCX for its return address: make a dummy syscall, then use RCX-relative addresses to refer to nearby data, since they're significantly shorter than RIP-relative addresses.

(Something I still don't know: On returning from a syscall, is it ever possible for RCX to contain something other than the syscall's return address on Linux, assuming that another process isn't explicitly overwriting it through ptrace or other means? Some people have vaguely hinted so in the case of an interruption by a signal, but I have failed to find any actual example of this.)