dos33fsprogs/music/tiny_tracker2/interrupt_handler.s
2022-01-10 19:34:02 -05:00

166 lines
2.9 KiB
ArmAsm

;================================
;================================
; 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
; we don't use decimal mode so no need to clear it?
; A is saved in $45 by firmware
; we are assuming a II/II+/IIe here
txa
pha ; save X
tya
pha ; save Y
; inc $0404 ; debug (flashes char onscreen)
ay3_irq_handler:
;==========================================
; clear 6522 interrupt by reading T1C-L ; 4
bit MOCK_6522_T1CL
;============================
; see if still counting down
lda SONG_COUNTDOWN
bpl done_update_song
set_notes_loop:
;==================
; load next byte
ldy SONG_OFFSET
lda tracker_song,Y
;==================
; see if hit end
cmp #$C0
bne all_ok
;====================================
; if at end, loop back to beginning
lda #0 ; reset song offset
sta SONG_OFFSET
beq set_notes_loop ; bra
all_ok:
; see if note
tay
and #$C0
cmp #$C0
beq handle_timing
note_only:
tya
; CCOONNNN -- c=channel, o=octave, n=note
; TODO: OONNNNCC instead?
lsr
lsr
lsr
lsr
sta OCTAVE ; save octave for later
lsr
and #$FE ; fine register value, want in X
tax
tya ; get note
and #$3F
tay ; lookup in table
lda frequency_lookup_low,Y
sta AY_REGS,X ; set proper register value
; set coarse note A
; hack: if octave=0 (C2) then coarse=1
; else coarse=0
inx ; point to corase register
lda OCTAVE
and #$3 ; if 0 then 1
; if 1,2,3 then 0
beq invert
lda #1
invert:
eor #$1
sta AY_REGS,X
jsr ay3_write_regs ; trashes A/X/Y
;============================
; point to next
inc SONG_OFFSET
; assume less than 256 bytes
bne set_notes_loop ; bra
handle_timing:
; was timing
tya
and #$3f
sta SONG_COUNTDOWN
inc SONG_OFFSET
done_update_song:
dec SONG_COUNTDOWN
;=================================
; Finally done with this interrupt
;=================================
done_ay3_irq_handler:
pla
tay ; restore Y
pla
tax ; restore X
; on II+/IIe the firmware saves A in $45
; this won't work on a IIc/IIgs
lda $45 ; restore A
plp ; restore flags
rti ; return from interrupt ; 6
;============
; typical
; ???? cycles