hmmm

papers/export-tech.md

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+# ROM export technical details
+
+## instrument data
+
+TODO
+
+## pattern data
+
+read sequentially.
+
+first byte determines what to read next:
+
+```
+NVI..EEE
+
+N: note
+V: volume
+I: instrument
+
+EEE: effect count (0-7)
+```
+
+if you read 0, end of pattern.
+otherwise read in following order:
+
+1. note
+2. volume
+3. instrument
+4. effect and effect value
+
+then read number of rows until next value, minus 1.

src/asm/6502/pattern.s

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+patPos=$90
+tempByte=$80
+
+nextNote=$a0
+nextIns=$a1
+nextVolume=$b0
+nextRow=$b1
+nextEffect1=$c0
+nextEffect2=$d0
+nextEffect3=$e0
+nextEffect4=$f0
+
+.macro nextByte
+  lda (patPos,x)
+  inc patPos,x
+  bcc :+
+  inc patPos+1,x
+: clc
+.endmacro
+  
+; read next row.
+; load X with channel<<1
+readNext:
+  lda #$ff
+  sta nextNote,x   ; clear last values
+  sta nextIns,x
+  sta nextVolume,x
+  sta nextEffect1,x
+  sta nextEffect2,x
+  sta nextEffect3,x
+  sta nextEffect4,x
+  nextByte
+  sta tempByte ; temporary store
+  beq endOfPat
+  lda #$20     ; start pattern check
+  bit tempByte ; NVI..EEE
+  bpl :+
+  php          ; read note
+  nextByte
+  sta nextNote,x
+  plp
+: bvc :+
+  php          ; read volume
+  nextByte
+  sta nextVolume,x
+  plp
+: bne :+
+  nextByte     ; read instrument
+  sta nextIns,x
+: lda tempByte
+  and #7
+  tay
+  txa
+  pha
+  clv
+readEffectLoop:
+  dey                 ; check if we're done
+  beq readLength
+  nextByte            ; effect
+  sta nextEffect1,x
+  nextByte            ; effect val
+  sta nextEffect1+1,x
+  txa                 ; add 16 to offset
+  adc #$10
+  tax
+  bvc readEffectLoop
+readLength:
+  pla
+  tax
+  nextByte
+  sta nextRow,x
+endOfPat:
+  rts