4 ms·

> As I recall, some processors stored the return address at the word before the first instruction of the subroutine. Yep, that's what the PDP-8 did. The evolu

> As I recall, some processors stored the return address at the word before the first instruction of the subroutine. Yep, that's what the PDP-8 did. The evolution of the PDP-8 is arguably a journey in hardware support for recursion. Initially the JMS instruction stuck the return address in the first word of the function (as an aside, a lot of time caller would put it's arguments after the JMS instruction, and the callee would read arguments offset of the return instruction, incrementing it with each read argument until the return address pointed to code again). Then it became relatively common to use one of the autoincrement locations (the PDP-8 had 8 memory locations that would increment any time you used them as a pointer) to create a simple stack, and function prologues/epilogues manually managed this stack to allow full recursion. Then later on hardware stacks were added in the microprocessor implementations like the Harris 6120 to make this more performant.

wglb 3y ago
As did the IBM 1800 and IBM 1130 and many machines of that era. Machines such as the Xerox Sigma series had enough registers to avoid this practice.
drfuchs 3y ago
Way back in 1956, Librascope's LGP-30 had an "R" instruction ("store Return address") that stored (already-incremented)PC+1 into the address portion of the instruction at the destination, which was by convention an unconditional branch just in front of the beginning of a subroutine. You'd follow the "R" instruction with a "U" (unconditional branch) to that subroutine. The subroutine would return by branching to the address just in front of it, which conveniently was an unconditional branch back to just after the call location. So, recursion was out of the question unless you used some more advanced calling convention. (And all opcodes in assembly language were a single letter.)

wglb 3y ago

As did the IBM 1800 and IBM 1130 and many machines of that era. Machines such as the Xerox Sigma series had enough registers to avoid this practice.

drfuchs 3y ago

Way back in 1956, Librascope's LGP-30 had an "R" instruction ("store Return address") that stored (already-incremented)PC+1 into the address portion of the instruction at the destination, which was by convention an unconditional branch just in front of the beginning of a subroutine. You'd follow the "R" instruction with a "U" (unconditional branch) to that subroutine. The subroutine would return by branching to the address just in front of it, which conveniently was an unconditional branch back to just after the call location. So, recursion was out of the question unless you used some more advanced calling convention. (And all opcodes in assembly language were a single letter.)