dos33fsprogs/music/tiny_music/interrupt_handler.s

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;================================
;================================
; mockingboard interrupt handler
;================================
;================================
; On Apple II/6502 the interrupt handler jumps to address in 0xfffe
; This is in the ROM, which saves the registers
; on older IIe it saved A to $45 (which could mess with DISK II)
; newer IIe doesn't do that.
; It then calculates if it is a BRK or not (which trashes A)
; Then it sets up the stack like an interrupt and calls 0x3fe
; Note: the IIc is much more complicated
; its firmware tries to decode the proper source
; based on various things, including screen hole values
; we bypass that by switching out ROM and replacing the
; $fffe vector with this, but that does mean we have
; to be sure status flag and accumulator set properly
interrupt_handler:
php ; save status flags
cld ; clear decimal mode
pha ; save A ; 3
; A is saved in $45 by firmware
txa
pha ; save X
tya
pha ; save Y
inc $0404 ; debug (flashes char onscreen)
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ay3_irq_handler:
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bit MOCK_6522_T1CL ; clear 6522 interrupt by reading T1C-L ; 4
; see if still counting down
lda SONG_COUNTDOWN
bpl done_update_song
try_again:
ldy SONG_OFFSET
set_notes_loop:
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; see if hit end
lda (SONG_L),Y
cmp #$FF
bne all_ok
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; if at end, reset
lda #0
sta SONG_OFFSET
beq try_again
all_ok:
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; see if note
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tax
and #$C0
cmp #$C0
bne note_only
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; was timing
txa
and #$3f
sta SONG_COUNTDOWN
iny
sty SONG_OFFSET
jmp done_update_song
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note_only:
txa
; CCOONNNN -- c=channel, o=octave, n=note
; FIXME: OONNNNCC instead?
pha
lsr
lsr
lsr
lsr
lsr
and #$FE
sta out_smc+1
pla
and #$3F
tax
lda frequency_lookup,X
sty y_smc+1
out_smc:
ldx #$00
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jsr ay3_write_reg ; trashes A/X/Y
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; set coarse note A (assume 0)
; could get extra octave by putting 1 here
y_smc:
ldy #0
iny
jmp set_notes_loop
done_update_song:
dec SONG_COUNTDOWN
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;=================================
; Finally done with this interrupt
;=================================
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done_ay3_irq_handler:
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pla
tay ; restore Y
pla
tax ; restore X
pla ; restore a ; 4
; on II+/IIe (but not IIc) we need to do this?
interrupt_smc:
lda $45 ; restore A
plp ; restore flags
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rti ; return from interrupt ; 6
;============
; typical
; ???? cycles
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