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https://github.com/deater/dos33fsprogs.git
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137 lines
2.3 KiB
ArmAsm
137 lines
2.3 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 $0404 ; debug (flashes char onscreen)
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ay3_irq_handler:
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bit MOCK_6522_T1CL ; clear 6522 interrupt by reading T1C-L ; 4
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; see if still counting down
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lda SONG_COUNTDOWN
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bpl done_update_song
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try_again:
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ldy SONG_OFFSET
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set_notes_loop:
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; see if hit end
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lda (SONG_L),Y
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cmp #$FF
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bne all_ok
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; if at end, reset
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lda #0
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sta SONG_OFFSET
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beq try_again
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all_ok:
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; see if note
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tax
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and #$C0
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cmp #$C0
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bne note_only
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; was timing
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txa
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and #$3f
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sta SONG_COUNTDOWN
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iny
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sty SONG_OFFSET
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jmp done_update_song
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note_only:
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txa
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; CCOONNNN -- c=channel, o=octave, n=note
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; FIXME: OONNNNCC instead?
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pha
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lsr
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lsr
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lsr
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lsr
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lsr
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and #$FE
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sta out_smc+1
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pla
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and #$3F
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tax
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lda frequency_lookup,X
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sty y_smc+1
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out_smc:
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ldx #$00
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jsr ay3_write_reg ; trashes A/X/Y
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; set coarse note A (assume 0)
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; could get extra octave by putting 1 here
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y_smc:
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ldy #0
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iny
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jmp set_notes_loop
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done_update_song:
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dec SONG_COUNTDOWN
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;=================================
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; Finally done with this interrupt
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;=================================
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done_ay3_irq_handler:
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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 ; restore flags
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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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