Sherlock: Routine at C403

C403: Set The Printer Motor Speed Via Port FB

Used by the routine at C3E4. The game's copy of the ROM COPY_LINE code: writes 02 to port FB when the count in B is below three (slowing the printer motor for the last two pixel lines), and 00 otherwise, then waits until a read from that port comes back negative.
SetPrinterMotorSpeed	C403	LD A,B	Compare B with 03 and leave FF in A when the count is below.
	C404	CP $03
	C406	SBC A,A
	C407	AND %00000010	Keep just bit 1, giving 02 (slow the printer motor) when the count is below three and 00 otherwise.
	C409	OUT ($FB),A	Send that strobe to the ZX Printer on port FB.
	C40B	LD D,A	Keep the motor value just sent in D.
SetPrinterMotorSpeed_0	C40C	IN A,($FB)	Read port FB and return if the ZX Printer is busy.
	C40E	ADD A,A
	C40F	RET M
	C410	JR NC,SetPrinterMotorSpeed_0	Go back to SetPrinterMotorSpeed_0 while the ZX Printer is not ready.
	C412	JP $0F12	Jump to COPY_LINE at 0F12.