Krakout: Front end: title screen, input-device menu, and attract cycle

BCB9: Front end: title screen, input-device menu, and attract cycle

Runs the game's front end, and returns only when the player starts a game. It leaves the chosen input device set.
kstate_save	BCB9	DEFB $00,$00,$00,$00,$00,$00,$00,$00	Saved copy of the ROM keyboard state
frames_save	BCC1	DEFB $00,$00,$00	Saved copy of the ROM FRAMES counter: its low word first, then its high byte
menu_attr_ptr	BCC4	DEFB $0A,$59	The attribute-cell address of the input-device line the menu highlights, low byte first
Setup: black border, cleared bitmap, bright white attributes, logo and menu drawn, credits scroller primed, title tune cued. Falls into the menu loop.
title_screen	BCC6	XOR A	Black border
	BCC7	OUT ($FE),A	Black border
	BCC9	CALL clear_screen
	BCCC	LD A,$47	Paint every attribute cell bright white on black.
	BCCE	CALL fill_attrs	Paint every attribute cell bright white on black.
	BCD1	CALL draw_title_logo
	BCD4	CALL draw_title_menu
	BCD7	CALL credits_scroll_init
	BCDA	LD HL,title_tune	Point the tune pointer at the start of the title tune.
	BCDD	LD (tune_ptr),HL	Point the tune pointer at the start of the title tune.
One pass per loop. Poll the 1–4 device keys and pulse the selected menu line. Check the O key, play one tune note, then run one attract/frame step. An option key does nothing when its device is already current, so a second press of the active option neither beeps nor redraws. There is no option 5 on the menu, so its key is ignored.
title_menu_loop	BCE0	LD A,(input_device)	Stash the current input device in A' for the re-select check below.
	BCE3	EX AF,AF'
	BCE4	LD BC,KBD_ROW_1_5	Read the 1/2/3/4/5 keyboard half-row and keep the five key bits.
	BCE7	IN A,(C)
	BCE9	AND $1F
	BCEB	BIT 0,A	Key 1 down? If not, fall through to the key-2 test.
	BCED	JR NZ,title_check_key2	Key 1 down? If not, fall through to the key-2 test.
	BCEF	EX AF,AF'	Already keyboard? Then just keep the pulse – no reselect.
	BCF0	OR A
	BCF1	JR Z,title_pulse_menu
	BCF3	LD HL,ATTR_BASE+$10A	Otherwise arm the keyboard and select the KEYBOARD line's attributes.
	BCF6	XOR A
	BCF7	JR title_set_device
title_check_key2	BCF9	BIT 1,A	Key 2 down? If not, fall through to the key-3 test.
	BCFB	JR NZ,title_check_key3	Key 2 down? If not, fall through to the key-3 test.
	BCFD	EX AF,AF'	Already Kempston? Then just keep the pulse.
	BCFE	CP $01
	BD00	JR Z,title_pulse_menu
	BD02	LD HL,ATTR_BASE+$14A	Otherwise arm Kempston and select the KEMPSTON line's attributes.
	BD05	LD A,$01
	BD07	JR title_set_device
title_check_key3	BD09	BIT 2,A	Key 3 down? If not, fall through to the key-4 test.
	BD0B	JR NZ,title_check_key4	Key 3 down? If not, fall through to the key-4 test.
	BD0D	EX AF,AF'	Already the cursor keys? Then just keep the pulse.
	BD0E	CP $02
	BD10	JR Z,title_pulse_menu
	BD12	LD HL,ATTR_BASE+$18A	Otherwise arm the cursor keys and select the CURSOR line's attributes.
	BD15	LD A,$02
	BD17	JR title_set_device
title_check_key4	BD19	BIT 3,A	Key 4 down? If not, nothing is selected – go pulse the current line.
	BD1B	JR NZ,title_pulse_menu
	BD1D	EX AF,AF'	Already the Sinclair joystick? Then just keep the pulse.
	BD1E	CP $03
	BD20	JR Z,title_pulse_menu
	BD22	LD HL,ATTR_BASE+$1CA	Otherwise arm the Sinclair/Interface II joystick and select that line's attributes; falls into the setter.
	BD25	LD A,$03
Commit the newly chosen option. Remember the line's attribute address for the pulse and store the device. Redraw the menu to clear the old highlight, and beep.
title_set_device	BD27	LD (menu_attr_ptr),HL	Save the selected line's attribute address (in the buffer just below this routine) for the pulse animation.
	BD2A	LD (input_device),A	Store the chosen device.
	BD2D	CALL draw_title_menu
	BD30	CALL sfx_key_blip
Pulse the selected option. Both character rows of the line step through the bright inks magenta, green, cyan, yellow and white once per frame. From white they wrap back to magenta. The line comes from the choice made by the option keys, the KEYBOARD line by default.
title_pulse_menu	BD33	LD HL,(menu_attr_ptr)	Load the selected line's attribute address; keep a copy for the second row.
	BD36	PUSH HL
	BD37	LD B,$14	Pulse every cell of the line's first character row.
title_pulse_loop1	BD39	LD A,(HL)	Step this cell's attribute, and wrap at the end of the run.
	BD3A	INC A
	BD3B	CP $48
	BD3D	JR NC,title_pulse_wrap1
title_pulse_set1	BD3F	LD (HL),A	Store the stepped attribute, advance to the next cell, and repeat for all 20.
	BD40	INC L
	BD41	DJNZ title_pulse_loop1
	BD43	JR title_pulse_line2
title_pulse_wrap1	BD45	LD A,$43	Past bright white – restart the cell at bright magenta.
	BD47	JR title_pulse_set1	Past bright white – restart the cell at bright magenta.
title_pulse_line2	BD49	POP HL	Drop to the menu line's second character row and pulse it the same way.
	BD4A	CALL advance_buffer_row
	BD4D	LD B,$14
title_pulse_loop2	BD4F	LD A,(HL)	Step each cell of the second row the same way.
	BD50	INC A
	BD51	CP $48
	BD53	JR NC,title_pulse_wrap2
title_pulse_set2	BD55	LD (HL),A	Write the stepped ink back to this cell.
	BD56	INC L
	BD57	DJNZ title_pulse_loop2
	BD59	JR title_check_options
title_pulse_wrap2	BD5B	LD A,$43	Past bright white – restart the cell at bright magenta.
	BD5D	JR title_pulse_set2	Past bright white – restart the cell at bright magenta.
Check the O key (options), then advance the title tune by one note. At the terminator the tune loops.
title_check_options	BD5F	LD BC,KBD_ROW_Y_P	Read the Y/U/I/O/P half-row and keep the five key bits.
	BD62	IN A,(C)
	BD64	AND $1F
	BD66	BIT 1,A	O pressed? Enter the options screen.
	BD68	JP Z,options_menu	O pressed? Enter the options screen.
	BD6B	CALL play_tune_note	Play the next title-tune note. Has the tune reached its terminator?
	BD6E	CP $0D
	BD70	JR NZ,title_attract_loop	Not the end – skip the rewind.
	BD72	LD HL,title_tune	Tune finished – rewind the tune pointer to loop it.
	BD75	LD (tune_ptr),HL	Tune finished – rewind the tune pointer to loop it.
Per-frame step of the menu loop. It busy-waits to set the pace, then waits for one frame, the front end's only interrupt sync. The ROM keyboard state and frame counter are saved and restored around it, because they lie inside the sprite bank. Then scroll the credits two pixels, read the player's controls, and test the start inputs.
title_attract_loop	BD78	LD BC,$05DC	Pace the loop with a busy-wait.
	BD7B	CALL delay	Pace the loop with a busy-wait.
	BD7E	LD IY,$0000	Park IY at zero before the frame sync, because the interrupt's ROM handler writes through it. The keyboard scan sets a flag bit in the byte after IY on each new key. The frame tick increments a byte at a fixed offset from IY each time FRAMES wraps. Parked at zero, both writes land in ROM and do nothing. After a game IY still points at a game record, where they would land in game memory.
	BD82	LD HL,slot_visor_1	Save the ROM keyboard state (KSTATE) into the buffer below this routine. Those bytes are also the head of the sprite bank, and the interrupt that ends the HALT below rewrites them. The same interrupt also writes the byte after them, which nothing restores – see Defects.
	BD85	LD DE,kstate_save
	BD88	LD BC,$0008
	BD8B	LDIR
	BD8D	LD HL,(frames_low)	Save the ROM frame counter (FRAMES).
	BD90	LD (frames_save),HL
	BD93	LD A,(frames_high)
	BD96	LD (frames_save+$2),A
	BD99	EI	Wait for the next frame interrupt.
	BD9A	HALT
	BD9B	DI
	BD9C	LD HL,kstate_save	Restore KSTATE so the interrupt's keyboard scan does not perturb it.
	BD9F	LD DE,slot_visor_1
	BDA2	LD BC,$0008
	BDA5	LDIR
	BDA7	LD HL,(frames_save)	Restore the ROM frame counter.
	BDAA	LD (frames_low),HL
	BDAD	LD A,(frames_save+$2)
	BDB0	LD (frames_high),A
	BDB3	CALL credits_scroll_step	Scroll the credits two pixels this frame (two steps).
	BDB6	CALL credits_scroll_step	Scroll the credits two pixels this frame (two steps).
	BDB9	CP $FF	Credits exhausted? Switch to the high-score display.
	BDBB	JP Z,title_attract_check	Credits exhausted? Switch to the high-score display.
	BDBE	CALL read_ctrl
	BDC1	LD HL,controls	Fire held? Return to start the game.
	BDC4	BIT 4,(HL)
	BDC6	RET NZ
	BDC7	LD BC,KBD_ROW_6_0	Read the 0/9/8/7/6 half-row and keep the five key bits.
	BDCA	IN A,(C)
	BDCC	AND $1F
	BDCE	BIT 0,A	Key 0 up? Loop the menu. Key 0 (START) pressed falls through to RET and starts the game.
	BDD0	JP NZ,title_menu_loop
	BDD3	RET
Credits finished: show the high-score table and play the title tune once. Any key, the end of the tune, or fire restarts the front end. The game cannot be started from this screen – fire here only loops back to the menu.
title_attract_check	BDD4	CALL draw_highscore_table
	BDD7	LD HL,title_tune	Rewind the tune pointer for the high-score wait.
	BDDA	LD (tune_ptr),HL	Rewind the tune pointer for the high-score wait.
title_attract_cont	BDDD	LD BC,$0BB8	Pace the high-score wait.
	BDE0	CALL delay	Pace the high-score wait.
	BDE3	CALL play_tune_note	Play the next tune note. Has the tune reached its terminator?
	BDE6	CP $0D
	BDE8	JP Z,title_screen	Restart the whole front end.
	BDEB	LD BC,$00FE	Read all key half-rows at once and keep the five key bits.
	BDEE	IN A,(C)
	BDF0	AND $1F
	BDF2	XOR $1F
	BDF4	JR NZ,title_attract_end	Any key down? End the wait and restart the front end.
	BDF6	CALL read_ctrl	Any key down? End the wait and restart the front end.
	BDF9	LD HL,controls	Loop until fire is pressed, then restart.
	BDFC	BIT 4,(HL)
	BDFE	JR Z,title_attract_cont
title_attract_end	BE00	JP title_screen	Fire or any key: back to the top of the front end.

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Routines