Chase H.Q.: Routine at FF0C

Walks the classic 8 half-row ports ($FEFE, $FDFE, ..., $7FFE, rotating the row-select byte in B via RLC). D starts at $FF and is incremented once per row found to have any key held, then overwritten with the key's bit-position within its row (via the SRL/JR NC shift-count at $FF1D-$FF21); E counts down the row number. Returns NZ (an early RET NZ at $FF1A/$FF23) if more than one row has a key held (reject as ambiguous); otherwise always returns with Z set (via the unconditional CP A at $FF2A) and the caller (read_new_key_definition) tells "no key" from "one key" by checking whether D is still $FF.

Used by the routine at read_new_key_definition.

Walks the classic 8 half-row ports ($FEFE, $FDFE, ..., $7FFE, rotating the row-select byte in B via RLC). D starts at $FF and is incremented once per row found to have any key held, then overwritten with the key's bit-position within its row (via the SRL/JR NC shift-count at $FF1D-$FF21); E counts down the row number. Returns NZ (an early RET NZ at $FF1A/$FF23) if more than one row has a key held (reject as ambiguous); otherwise always returns with Z set (via the unconditional CP A at $FF2A) and the caller (read_new_key_definition) tells "no key" from "one key" by checking whether D is still $FF. Used by the routine at read_new_key_definition.
scan_keyboard_matrix	FF0C	LD DE,$FF2F
	FF0F	LD BC,$FEFE
scan_keyboard_matrix_0	FF12	IN A,(C)
	FF14	CPL
	FF15	AND $1F
	FF17	JR Z,scan_keyboard_matrix_2
	FF19	INC D
	FF1A	RET NZ
	FF1B	LD H,A
	FF1C	LD A,E
scan_keyboard_matrix_1	FF1D	SUB $08
	FF1F	SRL H
	FF21	JR NC,scan_keyboard_matrix_1
	FF23	RET NZ
	FF24	LD D,A
scan_keyboard_matrix_2	FF25	DEC E
	FF26	RLC B
	FF28	JR C,scan_keyboard_matrix_0
	FF2A	CP A
	FF2B	RET