C stack-oriented ABI calling conventions also started with PDP-11, for which the first portable C compiler was written, and the convention then tagged along to nearly every other architecture as the pcc was progressively ported to other platforms.
If a function return value could fit into %r0 on the PDP-11 (%ax/%eax/%rax on x86), it would be returned in there. %r1-%r4 would be used to pass function parameters in, if they could fit, and/or spill over into the stack.
Heck, even UNIX system call conventions on x86 can be traced back to PDP-11, i.e. the syscall number is passed in %r0 (%eax) followed by a TRAP (INT on x86) instruction (can't remember which TRAP number, though).
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C stack-oriented ABI calling conventions also started with PDP-11, for which the first portable C compiler was written, and the convention then tagged along to nearly every other architecture as the pcc was progressively ported to other platforms.
If a function return value could fit into %r0 on the PDP-11 (%ax/%eax/%rax on x86), it would be returned in there. %r1-%r4 would be used to pass function parameters in, if they could fit, and/or spill over into the stack.
Heck, even UNIX system call conventions on x86 can be traced back to PDP-11, i.e. the syscall number is passed in %r0 (%eax) followed by a TRAP (INT on x86) instruction (can't remember which TRAP number, though).