Chase H.Q.: Routine at FB99

Reached from the title screen's "PRESS ENTER FOR OPTIONS" prompt (see the text block at CC50). Boots interrupts and tune 0, then loops: draws the control-select screen text (FC29 via print_string -- "ENTER OPTION" / P1. SINCLAIR JOYSTICK / P2. CURSOR JOYSTICK / P3. KEMPSTON JOYSTICK / P4. KEYBOARD / P5. DEFINE KEYS) and polls the keyboard (port $FE, keys 1-5 read as a 5-bit mask) to dispatch into one of the five listed choices.

Keys are read from half-row $F7FE (keys "1"-"5"), which is exactly the "P1"-"P5" option-select row shown by the FC29 "ENTER OPTION" text. FBB2-$FBB3 inverts and masks to a 5-bit "pressed" mask (bit 0 = key "1" .. bit 4 = key "5"); no key pressed loops back to poll again. FBB7-$FBC1 then shifts that mask right one bit at a time via RRA, testing each bit's carry in turn: key "1" -> run_title_tune_0 (Sinclair joystick, control-key list A), "2" -> run_title_tune_1 (Cursor joystick, list B, shares the copy loop at run_title_tune_2), "3" -> detect_kempston_joystick (Kempston joystick detect), "4" -> run_title_tune_3 (keyboard, handled inline). Key "5" falls through the whole RRA chain unbranched, so it is the default action: redefine_keys_screen, the "DEFINE KEYS" key-redefinition screen.

Used by the routine at C000.

Reached from the title screen's "PRESS ENTER FOR OPTIONS" prompt (see the text block at CC50). Boots interrupts and tune 0, then loops: draws the control-select screen text (FC29 via print_string -- "ENTER OPTION" / P1. SINCLAIR JOYSTICK / P2. CURSOR JOYSTICK / P3. KEMPSTON JOYSTICK / P4. KEYBOARD / P5. DEFINE KEYS) and polls the keyboard (port $FE, keys 1-5 read as a 5-bit mask) to dispatch into one of the five listed choices. Keys are read from half-row $F7FE (keys "1"-"5"), which is exactly the "P1"-"P5" option-select row shown by the FC29 "ENTER OPTION" text. FBB2-$FBB3 inverts and masks to a 5-bit "pressed" mask (bit 0 = key "1" .. bit 4 = key "5"); no key pressed loops back to poll again. FBB7-$FBC1 then shifts that mask right one bit at a time via RRA, testing each bit's carry in turn: key "1" -> run_title_tune_0 (Sinclair joystick, control-key list A), "2" -> run_title_tune_1 (Cursor joystick, list B, shares the copy loop at run_title_tune_2), "3" -> detect_kempston_joystick (Kempston joystick detect), "4" -> run_title_tune_3 (keyboard, handled inline). Key "5" falls through the whole RRA chain unbranched, so it is the default action: redefine_keys_screen, the "DEFINE KEYS" key-redefinition screen. Used by the routine at C000.
options_menu_driver	FB99	CALL setup_im2_interrupt_table
	FB9C	XOR A
	FB9D	CALL titlescr_start_tune
	FBA0	EI
	FBA1	HALT
This entry point is used by the routine at run_title_screen.
omd_redraw_and_poll	FBA2	CALL clear_options_screen
	FBA5	LD HL,$FC29
	FBA8	CALL print_string
This entry point is used by the routine at detect_kempston_joystick.
omd_service_and_read_keys	FBAB	CALL run_title_tune	Service sound/music while polling
	FBAE	LD A,$F7	Read keyboard half-row $F7FE (keys "1"-"5")
	FBB0	IN A,($FE)
	FBB2	CPL
	FBB3	AND $1F
	FBB5	JR Z,omd_service_and_read_keys	No key -> poll again
	FBB7	RRA	Key "1"
	FBB8	JR C,run_title_tune_0
	FBBA	RRA	Key "2"
	FBBB	JR C,run_title_tune_1
	FBBD	RRA	Key "3"
	FBBE	JR C,detect_kempston_joystick
	FBC0	RRA	Key "4"
	FBC1	JR C,run_title_tune_3
	FBC3	CALL redefine_keys_screen	Key "5" (default): "DEFINE KEYS" screen
	FBC6	JR omd_redraw_and_poll