Routines
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91BC: A trapdoor, closed (type $18): it opens every 256 passes
On a pass when TICKS' low byte is zero it swaps the picture: XOR the current one off (mode bits forced to XOR), flip bit 0 of the type -- $18 closed to $19 open -- XOR the new one on, restore the mode, draw the colours, and rasp. The open trapdoor's handler, TRAPDOOR, enters the same tail at TRAPDOOR_CLOSED_0 to close again, when the byte RUNNING_SUM happens to be zero. An earlier reading, "what a thing being destroyed looks and sounds like", was wrong.
TRAPDOOR_CLOSED 91BC LD A,($5E12) Only when TICKS' low byte is zero: every 256 passes.
91BF AND A
91C0 JP NZ,DRAW_DOOR
91C3 JR TRAPDOOR_CLOSED_0
TRAPDOOR 91C5 LD A,($5E05)
91C8 AND A
91C9 JP NZ,TRAPDOOR_FALL
TRAPDOOR_CLOSED_0 91CC LD A,(IX+$05) XOR the current picture off.
91CF PUSH AF
91D0 OR $03
91D2 LD (IX+$05),A
91D5 CALL DRAW_RECORD
91D8 LD A,(IX+$00) The other state's picture -- the type's bit 0 flipped -- XORed on.
91DB XOR $01
91DD LD (IX+$00),A
91E0 CALL DRAW_RECORD
91E3 POP AF Put the mode back and draw the colours.
91E4 LD (IX+$05),A
91E7 CALL DRAW_DOOR
91EA JP SOUND_NOISE_BURST And the rasp.
BIG_DOOR 91ED LD BC,$2020
91F0 JR DOORWAY
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