4 ms·

This is really interesting, because this is how later machines did it (at least with sum call instructions). I was actually surprised that the Programming Manua

This is really interesting, because this is how later machines did it (at least with sum call instructions). I was actually surprised that the Programming Manual suggested to insert the address directly at the final jump to return. (Regarding recursion, you could emulate a stack by storing the top of stack in a particular address by convention, say 0000, and fix up the 'r' command on the fly to insert PC+2 at this location. Then, every subroutine would look up 0000 for the top of stack and fix up its final jump to return. However, this would seriously mess up the accumulator and you had to store any return values in another conventional address, etc...)

drfuchs 7y ago
Hmmm; you’re closer to correct than I. Actual details are covered in http://ed-thelen.org/comp-hist/lgp-30-man.html#R3.16 http://ed-thelen.org/comp-hist/lgp-30-man.html#R3.16 where it explains that the “R” instruction would store away the address part of what was typically an unconditional branch. So, it was only by convention that this would be kept at $FOO, and that FOO would return by branching to $FOO. A super-smart compiler (they existed) would know the address of the last instruction of each routine, and be able to R into that location directly before calling (actually, just jumping) to $FOO. One other cute thing was that all memory was on a spinning drum, and you could really speed up your program’s execution by placing variables at addresses that were just about to be under the read head when the current operation was being performed. Otherwise, everything had to wait for up to a whole drum rotation. Kind of the original “pipeline stall”.