Freeing the stack pointer

By: Paul A. Clayton (paaronclayton.delete@this.gmail.com), June 29, 2019 6:32 am
Room: Moderated Discussions
jv (jvlietinck.delete@this.gmail.com) on June 29, 2019 3:54 am wrote:
[snip]
> When short on register for a particular routine, you can still use the stack pointer as a GPR.
> Usually R13 was used as SP, but saving it to memory allowed you using
> it as a GPR. In total 15 registers were always availalbe R0-R14.

Did SP get saved to a fixed address that could be generated by code (e.g., using an offset from zero)? Such seems problematic for early systems that did not have address translation. Another possibility would be a fixed position in a data structure with a retrievable reference; e.g., element zero or negative one of an array that is being processed.

Although not significant for such an optimization, saving the stack pointer would seem likely to confuse standard debugging tools.

Even in 1985 having a dedicated special cased stack pointer (actually more likely an address register with add and subtract) does not seem problematic. The tradeoffs of limited functionality and instruction encoding would have to be considered carefully, but almost all code would use it as a stack pointer. If one avoided or could bypass microarchitectural stack pointer optimizations, much of the code that would save the stack pointer could still use the specialized address register; however, it is not clear that one should be concerned about the performance compatibility of such code.
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