188 lines
4.5 KiB
ArmAsm
188 lines
4.5 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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cld ; clear decimal mode
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pha ; save A ; 3
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; A is saved in $45 by firmware
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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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inc TIMER_COUNT
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pt3_irq_smc1:
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bit MOCK_6522_T1CL ; clear 6522 interrupt by reading T1C-L ; 4
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lda DONE_PLAYING ; 3
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beq ym_play_music ; if song done, don't play music ; 3/2nt
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jmp done_pt3_irq_handler ; 3
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;============
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; 13
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ym_play_music:
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; see if need to pre-load next chunk
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lda BASE_FRAME_L
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cmp #1
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bne no_preload_chunk
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lda BASE_FRAME_H
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and #1
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bne no_preload_chunk
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inc LOAD_NEXT_CHUNK ; defer this until after interrupt
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no_preload_chunk:
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lda BASE_FRAME_L ; reset frame pointer to beginning
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sta CURRENT_FRAME_L
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lda BASE_FRAME_H
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sta CURRENT_FRAME_H
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ldx #0
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frame_loop:
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ldy #0
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lda (CURRENT_FRAME_L),Y
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cmp #$ff
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bne all_good
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cpx #1 ; see if A coarse is $FF
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beq go_next_chunk ; if so, end of song, loop
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all_good:
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jsr update_ay_register ; output to the AY
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clc
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lda CURRENT_FRAME_H ; next register set is 2 pages away
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adc #$2 ; was 4
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sta CURRENT_FRAME_H
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inx
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cpx #12 ; only output 11 regs
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bne frame_loop
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inc BASE_FRAME_L ; increment frame ptr (16bit)
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bne not_oflo
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inc BASE_FRAME_H ; wrap to next page
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lda BASE_FRAME_H ; if too big, go to next chunk
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and #$3
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cmp #$2 ; was D4
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bne not_oflo
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go_next_chunk:
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lda CHUNK_NEXT_PLAY ; toggle $D0/$E8
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eor #$38
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sta CHUNK_NEXT_PLAY
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sta BASE_FRAME_H
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lda #0
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sta BASE_FRAME_L ; in case song ended early
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inc CURRENT_CHUNK ; point to next chunk
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; inc LOAD_NEXT_CHUNK ; defer this until after interrupt
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; jsr load_song_chunk
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not_oflo:
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exit_interrupt:
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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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pla ; restore a ; 4
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; on II+/IIe (but not IIc) we need to do this?
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interrupt_smc:
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lda $45 ; restore A
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plp
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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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;=========================
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; update ay_register
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; reg in X
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; value in A
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update_ay_register:
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pt3_irq_smc2:
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stx MOCK_6522_ORA1 ; put address on PA1 ; 4
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stx MOCK_6522_ORA2 ; put address on PA2 ; 4
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ldy #MOCK_AY_LATCH_ADDR ; latch_address for PB1 ; 2
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pt3_irq_smc3:
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sty MOCK_6522_ORB1 ; latch_address on PB1 ; 4
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sty MOCK_6522_ORB2 ; latch_address on PB2 ; 4
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ldy #MOCK_AY_INACTIVE ; go inactive ; 2
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pt3_irq_smc4:
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sty MOCK_6522_ORB1 ; 4
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sty MOCK_6522_ORB2 ; 4
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; value
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pt3_irq_smc5:
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sta MOCK_6522_ORA1 ; put value on PA1 ; 4
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sta MOCK_6522_ORA2 ; put value on PA2 ; 4
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lda #MOCK_AY_WRITE ; ; 2
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pt3_irq_smc6:
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sta MOCK_6522_ORB1 ; write on PB1 ; 4
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sty MOCK_6522_ORB1 ; 4
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pt3_irq_smc7:
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sta MOCK_6522_ORB2 ; write on PB2 ; 4
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sty MOCK_6522_ORB2 ; 4
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rts
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done_pt3_irq_handler:
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disable_music:
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sei
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ldx #1
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stx DONE_PLAYING
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jsr clear_ay_both
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; mute the sound
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ldx #7
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lda #$ff
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jsr update_ay_register
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rts
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