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cbmeeks
PostPosted: Fri Oct 19, 2018 1:30 pm 
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Posts: 1215
Location: Soddy-Daisy, TN USA
So, my 6502 is a little rusty. Been a while since I was active.

I have a delay routine where I want to preserve A, X and Y. Using 65C02, here is what I have:

Code:
;------------------------------------------------------------------------------
;       Name:           DELAY1
;       Desc:           Slight delay (TODO, calculate and re-name)
;       Destroys:       Nothing
;------------------------------------------------------------------------------
DELAY1:
        PHA                     ; Push A to stack
        TXS                     ; Push X to stack
        TYA                     ; Transfer Y to A (why no PUSH Y to stack??)
        PHA                     ; Push A to stack (previous value of Y)

        LDX     #$00
        LDY     #$10
_DELAY1:
        DEX
        BNE     _DELAY1
        DEY
        BNE     _DELAY1

        PLA
        TAY                     ; Restore Y
        TSX                     ; Restore X
        PLA                     ; Restore A

        RTS


I'm not at my 6502 computer at the moment...but does this seem to be correct?

Any shorter way to do this?

Thanks!

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BigEd
PostPosted: Fri Oct 19, 2018 1:41 pm 
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Oops: TXS is not pushing X to stack! It is copying X to the stack pointer, which is to say resetting the stack. On 6502 you need PHA, TXA, PHA, TYA, PHA or similar. On 65C02 I think you can use PHA, PHX, PHY.
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cbmeeks
PostPosted: Fri Oct 19, 2018 1:44 pm 
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Ah thanks!

I totally read that wrong. I thought the 65C02 had better stack support but I guess my searching skills are failing me.

Thanks again.

Now my routine is:

Code:
;------------------------------------------------------------------------------
;       Name:           DELAY1
;       Desc:           Slight delay (TODO, calculate and re-name)
;       Destroys:       Nothing
;------------------------------------------------------------------------------
DELAY1:
        PHA                     ; Push A to stack
        PHX                     ; Push X to stack
        PHY                     ; Push Y to stack

        LDX     #$00
        LDY     #$10
_DELAY1:
        DEX
        BNE     _DELAY1
        DEY
        BNE     _DELAY1

        PLY                     ; Restore Y
        PLX                     ; Restore X
        PLA                     ; Restore A

        RTS

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Chromatix
PostPosted: Fri Oct 19, 2018 1:58 pm 
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Posts: 1462
That would work, but since you don't clobber the accumulator you can delete the PHA/PLA instructions.

What you do clobber is the status register. You can save and restore that with PHP/PLP, if it happens to matter. I advise saving it first and restoring it last.
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cbmeeks
PostPosted: Fri Oct 19, 2018 2:02 pm 
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Ah, that's true. I was still thinking that I needed A to hold Y so that's why I was pushing it to the stack.

Code:
;------------------------------------------------------------------------------
;       Name:           DELAY1
;       Desc:           Slight delay (TODO, calculate and re-name)
;       Destroys:       Nothing
;------------------------------------------------------------------------------
DELAY1:
        PHP                     ; Push status register
        PHX                     ; Push X to stack
        PHY                     ; Push Y to stack

        LDX     #$00
        LDY     #$10
_DELAY1:
        DEX
        BNE     _DELAY1
        DEY
        BNE     _DELAY1

        PLY                     ; Restore Y
        PLX                     ; Restore X
        PLP                     ; Restore status register
       
        RTS

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barrym95838
PostPosted: Fri Oct 19, 2018 3:19 pm 
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Joined: Sun Jun 30, 2013 10:26 pm
Posts: 1938
Location: Sacramento, CA, USA
Here's one, from the impressive mind of dclxvi:
Code:
; Delay 9*(256*_a+_y)+26 cycles (+12 for jsr/rts)
; Assumes that the BCS does not cross a page boundary
; Credit to dclxvi for the inner mechanics
delay:
      pha
      phy
      lda   #_a
      ldy   #_y
dloop:
      cpy   #1
      dey
      sbc   #0
      bcs   dloop
      ply
      pla
      rts


You can fine-tune your delay anywhere from 26 to 589841 cycles, with a granularity of 9.

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Last edited by barrym95838 on Sat Oct 20, 2018 12:03 am, edited 6 times in total.

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cbmeeks
PostPosted: Fri Oct 19, 2018 3:24 pm 
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Location: Soddy-Daisy, TN USA
Oh wow. That's really clever!
Thanks!

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