dos33fsprogs/games/lemm/interrupt_handler.s
Vince Weaver a38436af74 lemm: full song plays
awkward pauses though, takes more than 50Hz/20ms to decompress
2022-03-06 00:15:31 -05:00

162 lines
3.9 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
cld ; clear decimal mode
pha ; save A ; 3
; A is saved in $45 by firmware
txa
pha ; save X
tya
pha ; save Y
; inc $0404 ; debug (flashes char onscreen)
pt3_irq_smc1:
bit MOCK_6522_T1CL ; clear 6522 interrupt by reading T1C-L ; 4
lda DONE_PLAYING ; 3
beq ym_play_music ; if song done, don't play music ; 3/2nt
jmp done_pt3_irq_handler ; 3
;============
; 13
ym_play_music:
lda BASE_FRAME_L
sta CURRENT_FRAME_L
lda BASE_FRAME_H
sta CURRENT_FRAME_H
ldx #0
frame_loop:
ldy #0
lda (CURRENT_FRAME_L),Y
cmp #$ff
bne all_good
cpx #1 ; see if A coarse is $FF
beq go_next_chunk ; if so, end of song, loop
all_good:
jsr update_ay_register
clc
lda CURRENT_FRAME_H
adc #$4
sta CURRENT_FRAME_H
inx
cpx #12
bne frame_loop
inc BASE_FRAME_L
bne not_oflo
inc BASE_FRAME_H
lda BASE_FRAME_H
cmp #$D4
bne not_oflo
go_next_chunk:
inc CURRENT_CHUNK
jsr load_song_chunk
not_oflo:
jmp exit_interrupt
;=================================
; Finally done with this interrupt
;=================================
quiet_exit:
stx DONE_PLAYING
jsr clear_ay_both
; mute the sound
ldx #7
lda #$ff
jsr update_ay_register
exit_interrupt:
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
rti ; return from interrupt ; 6
;============
; typical
; ???? cycles
;=========================
; update ay_register
; reg in X
; value in A
update_ay_register:
pt3_irq_smc2:
stx MOCK_6522_ORA1 ; put address on PA1 ; 4
stx MOCK_6522_ORA2 ; put address on PA2 ; 4
ldy #MOCK_AY_LATCH_ADDR ; latch_address for PB1 ; 2
pt3_irq_smc3:
sty MOCK_6522_ORB1 ; latch_address on PB1 ; 4
sty MOCK_6522_ORB2 ; latch_address on PB2 ; 4
ldy #MOCK_AY_INACTIVE ; go inactive ; 2
pt3_irq_smc4:
sty MOCK_6522_ORB1 ; 4
sty MOCK_6522_ORB2 ; 4
; value
pt3_irq_smc5:
sta MOCK_6522_ORA1 ; put value on PA1 ; 4
sta MOCK_6522_ORA2 ; put value on PA2 ; 4
lda #MOCK_AY_WRITE ; ; 2
pt3_irq_smc6:
sta MOCK_6522_ORB1 ; write on PB1 ; 4
sty MOCK_6522_ORB1 ; 4
pt3_irq_smc7:
sta MOCK_6522_ORB2 ; write on PB2 ; 4
sty MOCK_6522_ORB2 ; 4
rts
done_pt3_irq_handler: