Pently uses more persistent zero page than competing engines (particularly FamiTone2), and zero page is often considered more precious than BSS. In addition, disabling features doesn't reduce the amount of zero page used. It should be possible to pack the pointers more tightly and move things that aren't pointers out.
Pently's current 36-byte zero page layout is as described in src/pentlymusic.s:
; +0 +1 +2 +3
; 0 | Sq1 sound effect data ptr Sq1 envelope data ptr
; 4 | Sq2 sound effect data ptr Sq2 envelope data ptr
; 8 | Tri sound effect data ptr Tri envelope data ptr
; 12 | Noise sound effect data ptr Noise envelope data ptr
; 16 | Sq1 music pattern data ptr Play/Pause Attack channel
; 20 | Sq2 music pattern data ptr Tempo
; 24 | Tri music pattern data ptr Tempo counter
; 28 | Noise music pattern data ptr Conductor segno
; 32 | Attack music pattern data ptr Conductor track position
I propose moving music pattern data 2 bytes earlier to compact zero page to 32 bytes when attack is enabled:
; +0 +1 +2 +3
; 0 | Sq1 sound effect data ptr Sq1 music pattern data ptr
; 4 | Sq2 sound effect data ptr Sq2 music pattern data ptr
; 8 | Tri sound effect data ptr Tri music pattern data ptr
; 12 | Noise sound effect data ptr Noise music pattern data ptr
; 16 | Sq1 envelope data ptr Attack music pattern data ptr
; 20 | Sq2 envelope data ptr Conductor track position
; 24 | Tri envelope data ptr Tempo counter
; 28 | Noise envelope data ptr Play/Pause Attack channel
This moves Conductor segno and Tempo to BSS.
When attack envelopes are completely disabled (PENTLY_USE_ATTACK_PHASE = 0), it can be compacted further to 21 bytes:
; +0 +1 +2 +3
; 0 | Sq1 sound effect data ptr Sq1 music pattern data ptr
; 4 | Sq2 sound effect data ptr Sq2 music pattern data ptr
; 8 | Tri sound effect data ptr Tri music pattern data ptr
; 12 | Noise sound effect data ptr Noise music pattern data ptr
; 16 | Conductor track position Tempo counter
; 20 | Play/Pause
Pently uses more persistent zero page than competing engines (particularly FamiTone2), and zero page is often considered more precious than BSS. In addition, disabling features doesn't reduce the amount of zero page used. It should be possible to pack the pointers more tightly and move things that aren't pointers out.
Pently's current 36-byte zero page layout is as described in src/pentlymusic.s:
I propose moving music pattern data 2 bytes earlier to compact zero page to 32 bytes when attack is enabled:
This moves Conductor segno and Tempo to BSS.
When attack envelopes are completely disabled (
PENTLY_USE_ATTACK_PHASE = 0), it can be compacted further to 21 bytes: