Alien (BUGFIX 1.7): Routine at 37363

Per-frame driver of the message line's teletype effect. A message's life: dequeue -> RenderMessage draws it over black attributes -> a bright-red cursor sweeps left-to-right, turning each column black-on-cyan (the reveal) -> the message holds (timer 50, or just 2 when more messages wait) -> the cursor sweeps back right-to-left blacking it out -> the next message is fetched. MsgDrawnCol encodes the phase: 0 = line free, bit 7 set = revealing, bit 7 clear = erasing. Called twice per frame, so the sweeps run at ~100 cells/s.

Used by the routine at 36457.

Per-frame driver of the message line's teletype effect. A message's life: dequeue -> RenderMessage draws it over black attributes -> a bright-red cursor sweeps left-to-right, turning each column black-on-cyan (the reveal) -> the message holds (timer 50, or just 2 when more messages wait) -> the cursor sweeps back right-to-left blacking it out -> the next message is fetched. MsgDrawnCol encodes the phase: 0 = line free, bit 7 set = revealing, bit 7 clear = erasing. Called twice per frame, so the sweeps run at ~100 cells/s. Used by the routine at 36457.
37363	LD A,(33709)	message currently holding?
37366	AND A
37367	JR Z,37383
37369	DEC A	yes: run down the hold time
37370	LD (33709),A
37373	RET NZ
37374	LD A,(33710)	hold over: start the erase sweep —
37377	AND 31	MsgDrawnCol = current column,
37379	LD (33708),A	bit 7 clear
37382	RET
37383	LD A,(33708)	a sweep in progress?
37386	AND A
37387	JR NZ,37432
37389	LD A,(33741)	line free: anything queued?
37392	LD HL,33714
37395	CP L
37396	JR NZ,37399
37398	RET	no: idle
37399	LD A,(HL)	pop the head entry:
37400	INC HL	A = msg#,
37401	LD E,(HL)	E/D = actor/target params
37402	INC HL
37403	LD D,(HL)
37404	LD (33712),DE
37408	LD BC,21	compact the queue down
37411	LD DE,33714	by one 3-byte entry
37414	LD HL,33717
37417	LDIR
37419	LD HL,(33741)
37422	DEC HL
37423	DEC HL
37424	DEC HL
37425	LD (33741),HL
37428	CALL 37221	draw it (still invisible)
37431	RET
37432	BIT 7,A	which sweep phase?
37434	JR Z,37477	bit 7 clear: erasing
37436	LD HL,(33710)	revealing: the column behind turns
37439	LD (HL),40	black-on-cyan (readable),
37441	INC HL	the cursor block (bright red)
37442	LD (HL),82	moves one right
37444	LD (33710),HL
37447	DEC A	reached the end of the text?
37448	CP 128
37450	JR Z,37456
37452	LD (33708),A	not yet: keep sweeping
37455	RET
37456	XOR A	reveal done: line phase = idle,
37457	LD (33708),A
37460	LD A,(33741)	hold the message for 50 ticks —
37463	LD HL,33714	only 2 when more messages
37466	CP L	are waiting
37467	LD A,50
37469	JR Z,37473
37471	LD A,2
37473	LD (33709),A
37476	RET
37477	LD HL,(33710)	erasing: black out this column,
37480	LD (HL),0	the cursor block moves one left
37482	DEC HL
37483	LD (HL),82
37485	LD (33710),HL
37488	DEC A	A = columns left (0 frees the line)
37489	DEFB 50	opcode of "LD (msg-head var),A": its operands are the XOR H / ADD A,E at 37490, so falling through stores A and hits that entry's RET