Groups > comp.sys.apple2.programmer > #2205

Re: most efficient way to save/restore 6502 registers

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NNTP-Posting-Date	Thu, 04 Feb 2016 14:31:05 -0600
From	Scott Hemphill <hemphill@hemphills.net>
Newsgroups	comp.sys.apple2.programmer
Subject	Re: most efficient way to save/restore 6502 registers
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Date	Thu, 04 Feb 2016 15:31:05 -0500
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Mark Lemmert <mark.lemmert@gmail.com> writes:

> I am wondering if anybody knows why the most Apple II assembly
> subroutines save/restore registered via the stack (method 1 below),
> instead of using variables (method 2 below)?
>
> And, what are the advantages and disadvantages are for each method?
>
> Thanks a lot for any insights!

(1) I think you pretty much covered it.  Note that if you do a lot of
saving/restoring, you could (with both schemes) reduce the per instance
memory usage by doing this:

  JSR XYSAVE
  ...
  JSR XYRESTORE

You could even omit the call to XYRESTORE if XYSAVE manipulates the
stack by putting the address to XYRESTORE on it, so that the RET from
your subroutine automatically restores the registers.

(2) If your subroutine calls itself recursively, you need to store the
registers on a stack.

(3) If your subroutine doesn't know which memory banks are switched in
(e.g. when processing an interrupt) then you may not want to store the
registers in memory.

(4) Saving the registers in memory allows you to restore them multiple
times.

Scott
-- 
Scott Hemphill	hemphill@alumni.caltech.edu
"This isn't flying.  This is falling, with style."  -- Buzz Lightyear

Thread

most efficient way to save/restore 6502 registers Mark Lemmert <mark.lemmert@gmail.com> - 2016-02-04 11:41 -0800
  Re: most efficient way to save/restore 6502 registers Scott Hemphill <hemphill@hemphills.net> - 2016-02-04 15:31 -0500
    Re: most efficient way to save/restore 6502 registers Mark Lemmert <mark.lemmert@gmail.com> - 2016-02-04 19:22 -0800
      Re: most efficient way to save/restore 6502 registers Michael Barry <barrym95838@yahoo.com> - 2016-02-04 20:21 -0800
        Re: most efficient way to save/restore 6502 registers wssimms@gmail.com - 2016-02-04 21:20 -0800

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