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Newsgroups: comp.sys.apple2.programmer
Date: Fri, 16 Jan 2015 09:41:00 -0800 (PST)
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Subject: Re: Method to address both Legend and Saturn RAM cards
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On Thursday, January 15, 2015 at 10:21:37 PM UTC-6, Anton Treuenfels wrote:
> "D Finnigan" wrote in message 
> 
> > BANKBASE EQU $C084 ; BASE ADDRESS FOR BANK SELECT ADDR
> > RAMTYPE EQU $06 ; TYPE OF RAM CARD INSTALLED
> > *
> > *
> > * SELECT A 16K RAM BANK. DESIRED BANK NUMBER
> > * SHOULD BE PASSED IN THE A-REG.
> > *
> > BANKSEL
> > LDA RAMTYPE ; WHAT TYPE OF RAM CARD?
> > BEQ :LEGEND ; 0 = LEGEND, 1 = SATURN
> > TAX
> > STA BANKBASE,X ; SELECT BANK
> > RTS
> > :LEGEND LDX #0
> > STA BANKBASE,X ; SELECT BANK
> > RTS
> 
> I don't know squat about either card, but:
> 
> - if the bank number is passed in the A-register, doesn't the instruction
> 
> LDA RAMTYPE
> 
> wipe it out first thing?
> 
> - if the value of the X-register is zero, there is no practical difference 
> between
> 
> STA BANKBASE,X
> 
> and
> 
> STA BANKBASE
> 
> unless there's some weird hardware quirk that "notices" the address mode and 
> does something special about it.
> 
> - so how about this:
> 
> BANKSEL
>   LDX RAMTYPE
>   BEQ LEGEND
>   TAX
> LEGEND
>   STA BANKBASE,X
>   RTS
> 
> - Anton Treuenfels


No.  That doesn't quite work either.

You need to be able to store a 0-$F in the Saturns register.

So Maybe something like this

LDY #0-3 ; for Legend
LDY #0-$F; for Saturn

BANKSEL  LDX RAMTYPE        ; WHAT TYPE OF RAM CARD?
         BEQ :LEGEND        ; 0 = LEGEND, 1 = SATURN
         TYA
         STA BANKBASE,X     ; SELECT BANK
         RTS

:LEGEND  CPY #4
         BCS RETURN
         TYA
         TAX
         STA BANKBASE,X     ; SELECT BANK
RETURN   RTS