166 lines
2.9 KiB
ArmAsm
166 lines
2.9 KiB
ArmAsm
;================================
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;================================
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; mockingboard interrupt handler
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;================================
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;================================
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; On Apple II/6502 the interrupt handler jumps to address in 0xfffe
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; This is in the ROM, which saves the registers
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; on older IIe it saved A to $45 (which could mess with DISK II)
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; newer IIe doesn't do that.
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; It then calculates if it is a BRK or not (which trashes A)
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; Then it sets up the stack like an interrupt and calls 0x3fe
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; Note: the IIc is much more complicated
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; its firmware tries to decode the proper source
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; based on various things, including screen hole values
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; we bypass that by switching out ROM and replacing the
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; $fffe vector with this, but that does mean we have
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; to be sure status flag and accumulator set properly
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interrupt_handler:
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php ; save status flags
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; we don't use decimal mode so no need to clear it?
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; A is saved in $45 by firmware
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; we are assuming a II/II+/IIe here
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txa
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pha ; save X
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tya
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pha ; save Y
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; inc $0404 ; debug (flashes char onscreen)
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ay3_irq_handler:
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;==========================================
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; clear 6522 interrupt by reading T1C-L ; 4
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bit MOCK_6522_T1CL
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;============================
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; see if still counting down
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lda SONG_COUNTDOWN
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bpl done_update_song
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set_notes_loop:
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;==================
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; load next byte
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ldy SONG_OFFSET
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lda tracker_song,Y
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;==================
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; see if hit end
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cmp #$C0
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bne all_ok
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;====================================
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; if at end, loop back to beginning
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lda #0 ; reset song offset
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sta SONG_OFFSET
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beq set_notes_loop ; bra
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all_ok:
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; see if note
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tay
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and #$C0
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cmp #$C0
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beq handle_timing
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note_only:
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tya
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; CCOONNNN -- c=channel, o=octave, n=note
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; TODO: OONNNNCC instead?
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lsr
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lsr
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lsr
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lsr
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sta OCTAVE ; save octave for later
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lsr
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and #$FE ; fine register value, want in X
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tax
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tya ; get note
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and #$3F
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tay ; lookup in table
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lda frequency_lookup_low,Y
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sta AY_REGS,X ; set proper register value
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; set coarse note A
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; hack: if octave=0 (C2) then coarse=1
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; else coarse=0
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inx ; point to corase register
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lda OCTAVE
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and #$3 ; if 0 then 1
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; if 1,2,3 then 0
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beq invert
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lda #1
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invert:
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eor #$1
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sta AY_REGS,X
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jsr ay3_write_regs ; trashes A/X/Y
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;============================
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; point to next
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inc SONG_OFFSET
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; assume less than 256 bytes
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bne set_notes_loop ; bra
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handle_timing:
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; was timing
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tya
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and #$3f
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sta SONG_COUNTDOWN
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inc SONG_OFFSET
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done_update_song:
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dec SONG_COUNTDOWN
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;=================================
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; Finally done with this interrupt
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;=================================
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done_ay3_irq_handler:
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pla
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tay ; restore Y
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pla
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tax ; restore X
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; on II+/IIe the firmware saves A in $45
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; this won't work on a IIc/IIgs
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lda $45 ; restore A
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plp ; restore flags
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rti ; return from interrupt ; 6
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;============
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; typical
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; ???? cycles
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