162 lines
2.7 KiB
ArmAsm
162 lines
2.7 KiB
ArmAsm
play_frame:
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;============================
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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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set_notes_loop:
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;==================
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; load next byte
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; ldy SONG_OFFSET ; Y = offset into song
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track_smc:
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lda track4;,Y ; get next byte into A
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;==================
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; see if hit end
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cmp #$ff ; FF means we hit end
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bne not_end
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;====================================
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; if at end, loop back to beginning
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; ldy WHICH_TRACK ; get current track in Y
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; iny ; increment track
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; cpy #5 ; see if off end
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; bne no_wrap
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ldy #1 ; loop to track 1
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no_wrap:
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sty WHICH_TRACK
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lda tracks_l,Y ; self-modify track
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sta track_smc+1
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; lda tracks_h,Y ; enforce in same page
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; sta track_smc+2
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lda bamps_l,Y ; self modify B-amplitude
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sta bamp_smc+1
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; lda bamps_h,Y ; enforce in same page
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; sta bamp_smc+2
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; lda #0 ; reset song offset
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; sta SONG_OFFSET
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jmp set_notes_loop ; bra ; try again in new track
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not_end:
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; NNNNNEEC -- c=channel, e=end, n=note
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pha ; save note
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and #1
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asl
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tax ; put channel offset*2 in X
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pla ; restore note
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pha
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and #$6 ; get note length
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lsr
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tay
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lda lengths,Y ; lookup in table
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sta SONG_COUNTDOWN ;
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pla
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lsr
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lsr
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lsr ; get note in A
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tay ; lookup in table
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lda frequencies_high,Y ; get high frequency
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sta AY_REGS+1,X ; put in AY register
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lda frequencies_low,Y ; get low frequency
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sta AY_REGS,X ; also put in AY register
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;============================
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; point to next
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; assume less than 256 bytes
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inc track_smc+1 ; SONG_OFFSET
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done_update_song:
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;=================================
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; coundown song
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dec SONG_COUNTDOWN ; if length was 0, means there
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; was another note starting at same
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; time, so go back and play that too
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bmi set_notes_loop
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;===============================
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;===============================
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; things that happen every frame
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;===============================
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;===============================
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;=================================
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; rotate through channel A volume
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lda FRAME ; repeating 8-long pattern
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and #$7
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tay
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lda channel_a_volume,Y
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sta AY_REGS+8 ; A volume
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;=================================
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; handle channel B volume
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chanb:
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; lda FRAME
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; and #$7
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tya
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bne bamps_skip
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lda BAMP_COUNTDOWN ; b-amp conutdown
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bne bamps_good
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bamp_smc:
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lda bamps4 ; get new value
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pha
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and #$f ; bottom 4 its is ampllitude
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sta AY_REGS+9 ; B volume
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pla
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lsr ; top 4 bits are length
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lsr
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lsr
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lsr
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sta BAMP_COUNTDOWN
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inc bamp_smc+1 ; increment to next location
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; assumes on same page
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; and less than 256
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bamps_good:
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dec BAMP_COUNTDOWN ; countdown
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bamps_skip:
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;=================================
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; inc frame counter
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inc FRAME
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