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Thomas Jentzsch

Convenient bankswitching using macros

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I suppose everyone who ever had to use standard Atari bankswitching sooner or later developed some kind of pattern how to use it. And there are numerous ways to do it. But most of have to be used very carefully and are often not easy to maintain.


So I came up with the idea of using macros to make things as convenient as possible. The price for that convenience are more CPU cycles and destroyed registers and flags.

So here they are:

BANK_SIZE = $1000

; Use at the start of each bank, required by JMP_LBL macro
  MAC START_BANK ; bank number

; this is not related to bankswitching, but nevertheless useful 

    SEG     code
    ORG     BANK_ORG, $55

    ECHO    "  start of bank", [BANK_NUM]d, ", ORG", BANK_ORG, ", RORG", BANK_RORG

; Insert code for executing the bankswitch,
; should be put at the very end of each bank, right before the hotspots
    RORG BANK_RORG + $ff4 - BANKSWITCH_SIZE, "before bankswitching code"

SwitchBank  = (. & $fff) | $1000
    lda     BANK0,y   ; e.g $fff8

BANKSWITCH_SIZE = ((. & $fff) | $1000) - SwitchBank

; Define a label which can be used by JMP_LBL macro
; Example:
;    DEF_LBL Foo
  MAC DEF_LBL ; address
{1}_BANK    = BANK_NUM

; Jump to a label in other or same bank. The assembler will take care if the
; code has to bankswitch or not.
; Example:
;    JMP_LBL Foo
  MAC JMP_LBL ; address
   IF {1}_BANK != BANK_NUM      ; convenience IF 
    ldy     #{1}_BANK
    lda     #>({1}-1)
    ldx     #<({1}-1)
    jmp     SwitchBank          ; always at the same address in each bank
    jmp     {1}

As of now only JMP is supported, but macros for JSR could be created that way too.

Edited by Thomas Jentzsch
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Thank you, I am in the process of upgrading my 4K project to a 16K one, and this will be very helpful.

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In TIM1T, I used the ARPM bankswitching macros found here and found them very helpful - you might take a look at them for inspiration or comparison. Just note that the original macros have a small bug - see my post in that thread for details.

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Those macros look very interesting. I will check them for sure. Maybe they are even easier to use than mine.

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And for reference, here is how SpiceWare has done it. It's intended to be used after manually setting the ORG rather than automating that process for you. For my case, I will probably just take ideas from these concepts for my own method.

Edited by JeremiahK

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