There's this sense that they're not for the consumption of mere mortals.

That being said, you can still at least always call syscall(2).

One problem is that not all system calls take the same kinds of arguments on all platforms. Example: SYS_mmap takes a pointer to a struct containing all the arguments on s390. Even better, on Alpha, glibc's syscall cannot call system calls with 6 or more arguments (although maybe that was fixed in the past 8 years?).

> Even better, on Alpha, glibc's syscall cannot call system calls with 6 or more arguments

That's just generic kernel ABI: syscalls can have at most 6 arguments: https://elixir.bootlin.com/linux/latest/source/include/asm-g...

Skip the "or more" part, then, but that doesn't make it less true: it wasn't possible to make a 6-argument system call with syscall on alpha 8 years ago. I don't know whether that's been fixed or not.

The syscall(5) manpage documents the Alpha syscall abi as passing arguments in a0,a1,a2,a3,a4,a5 which would suggest so. (MIPS o32 is the only listed one which is a bit oddball: you can only pass 4 args in registers and then use stack for 5 and 6.)