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55 lines
1.7 KiB
Plaintext
55 lines
1.7 KiB
Plaintext
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;license:MIT
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; By Andrew Roughan
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; in the style of 4am for Total Replay
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;
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; Mockingboard support functions
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;
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;------------------------------------------------------------------------------
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; GetMockingboardSlot
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; detect Mockingboard card by searching for 6522 timers across all slots 7->1
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; access 6522 timers with deterministic cycle counts
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;
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; based on prior art in Mockingboard Developers Toolkit
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; with optimisation from deater/french touch
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; also takes into account FastChip //e clock difference
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;
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; in: none
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; accelerators should be off
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; out:
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; if card was found, X = #$Cn where n is the slot number of the card, otherwise #$00
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; flags clobbered
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; zp $80-$82 clobbered
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; A/Y clobbered
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;------------------------------------------------------------------------------
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GetMockingboardSlot
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lda #$00
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sta $80
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ldx #$C7
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@slotLoop
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stx $81
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ldy #$04 ; 6522 #1 $Cx04
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jsr timercheck
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bne @nextSlot
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ldy #$84 ; 6522 #2 $Cx84
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jsr timercheck
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bne @nextSlot
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rts ; found
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@nextSlot
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dex
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cpx #$C0
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bne @slotLoop
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ldx #$00 ; not found
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rts
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timercheck
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lda ($80),y ; read 6522 timer low byte
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sta $82
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lda ($80),y ; second time
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sec
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sbc $82
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cmp #$F8 ; looking for (-)8 cycles between reads
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beq +
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cmp #$F7 ; FastChip //e clock is different
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+ rts
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