dos33fsprogs/interlace_demo/interrupt_handler.s

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2019-06-18 19:49:35 +00:00
;================================
;================================
; 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
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)
bit $C404 ; clear 6522 interrupt by reading T1C-L ; 4
lda DONE_PLAYING ; 3
beq pt3_play_music ; if song done, don't play music ; 3/2nt
jmp exit_interrupt ; 3
pt3_play_music:
; decode a frame of music
jsr pt3_make_frame
; handle song over condition
lda DONE_SONG
beq mb_write_frame ; if not done, continue
lda LOOP ; see if looping
beq move_to_next
pt3_loop_smc:
lda #$d1 ; looping, move to loop location
; non-zero to avoid the temptation
; to merge with following lda #$0
sta current_pattern_smc+1
lda #$0
sta current_line_smc+1
sta current_subframe_smc+1
sta DONE_SONG ; undo the next song
beq done_interrupt ; branch always
move_to_next:
; same as "press right"
ldx #$20
jmp quiet_exit
;======================================
; Write frames to Mockingboard
;======================================
; for speed could merge this into
; the decode code
mb_write_frame:
tax ; set up reg count ; 2
;============
; 2
;==================================
; loop through the 14 registers
; reading the value, then write out
;==================================
mb_write_loop:
lda AY_REGISTERS,X ; load register value ; 4
; special case R13. If it is 0xff, then don't update
; otherwise might spuriously reset the envelope settings
cpx #13 ; 2
bne mb_not_13 ; 3/2nt
cmp #$ff ; 2
beq mb_skip_13 ; 3/2nt
;============
; typ 5
mb_not_13:
; address
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
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
sty MOCK_6522_ORB1 ; 4
sty MOCK_6522_ORB2 ; 4
; value
sta MOCK_6522_ORA1 ; put value on PA1 ; 4
sta MOCK_6522_ORA2 ; put value on PA2 ; 4
lda #MOCK_AY_WRITE ; ; 2
sta MOCK_6522_ORB1 ; write on PB1 ; 4
sta MOCK_6522_ORB2 ; write on PB2 ; 4
sty MOCK_6522_ORB1 ; 4
sty MOCK_6522_ORB2 ; 4
;===========
; 56
mb_no_write:
inx ; point to next register ; 2
cpx #14 ; if 14 we're done ; 2
bmi mb_write_loop ; otherwise, loop ; 3/2nt
;============
; 7
mb_skip_13:
;=================================
; Finally done with this interrupt
;=================================
done_interrupt:
exit_interrupt:
pla
tay ; restore Y
pla
tax ; restore X
pla ; restore a ; 4
; this is needed on II+/IIe not not IIc
interrupt_smc:
lda $45 ; restore A
plp
rti ; return from interrupt ; 6
;============
; typical
; ???? cycles
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; stop playing for now
; quiet down the Mockingboard
; (otherwise will be stuck on last note)
quiet_exit:
stx DONE_PLAYING
jsr clear_ay_both
;ldx #$ff ; also mute the channel
stx AY_REGISTERS+7 ; just in case
jmp done_interrupt