Chase H.Q.: Routine at BC3E

BC3E: Copies the backbuffer at $F000 to the screen

Used by the routines at load_stage, attract_mode, main_loop, run_pregame_screen, escape_scene and page_in_stage_128k.
draw_screen	BC3E	LD HL,$4811	Point HL at screen pixel (136,64). This is positioned halfway across so we can PUSH to the screen via SP.
	BC41	EXX
	BC42	LD HL,$F001	Point HL' at backbuffer + 1 byte.
	BC45	LD ($BDBE),SP	Self modify SP restore instruction
A sequence that transfers 16 bytes
draw_screen_0	BC49	LD SP,HL	Point SP at the back buffer
	BC4A	POP AF	Pull in 16 bytes from the back buffer (order: AF' DE BC AF' DE' BC' IX IY), incrementing SP
	BC4B	EX AF,AF'
	BC4C	POP DE
	BC4D	POP BC
	BC4E	EXX
	BC4F	POP AF
	BC50	POP DE
	BC51	POP BC
	BC52	POP IX
	BC54	POP IY
	BC56	LD SP,HL	Point SP at screen
	BC57	PUSH IY	Push out 16 bytes to the screen (reversing the order), decrementing SP
	BC59	PUSH IX
	BC5B	PUSH BC
	BC5C	PUSH DE
	BC5D	PUSH AF
	BC5E	INC H	Advance screen pointer
	BC5F	EXX	Continue pushing
	BC60	PUSH BC
	BC61	PUSH DE
	BC62	EX AF,AF'
	BC63	PUSH AF
	BC64	INC H	Advance back buffer pointer
	BC65	LD SP,HL	Transfer another 16 bytes (128 pixels)
	BC66	POP AF
	BC67	EX AF,AF'
	BC68	POP DE
	BC69	POP BC
	BC6A	EXX
	BC6B	POP AF
	BC6C	POP DE
	BC6D	POP BC
	BC6E	POP IX
	BC70	POP IY
	BC72	LD SP,HL
	BC73	PUSH IY
	BC75	PUSH IX
	BC77	PUSH BC
	BC78	PUSH DE
	BC79	PUSH AF
	BC7A	INC H
	BC7B	EXX
	BC7C	PUSH BC
	BC7D	PUSH DE
	BC7E	EX AF,AF'
	BC7F	PUSH AF
	BC80	INC H
	BC81	LD SP,HL	Transfer another 16 bytes (128 pixels)
	BC82	POP AF
	BC83	EX AF,AF'
	BC84	POP DE
	BC85	POP BC
	BC86	EXX
	BC87	POP AF
	BC88	POP DE
	BC89	POP BC
	BC8A	POP IX
	BC8C	POP IY
	BC8E	LD SP,HL
	BC8F	PUSH IY
	BC91	PUSH IX
	BC93	PUSH BC
	BC94	PUSH DE
	BC95	PUSH AF
	BC96	INC H
	BC97	EXX
	BC98	PUSH BC
	BC99	PUSH DE
	BC9A	EX AF,AF'
	BC9B	PUSH AF
	BC9C	INC H
	BC9D	LD SP,HL	Transfer another 16 bytes (128 pixels)
	BC9E	POP AF
	BC9F	EX AF,AF'
	BCA0	POP DE
	BCA1	POP BC
	BCA2	EXX
	BCA3	POP AF
	BCA4	POP DE
	BCA5	POP BC
	BCA6	POP IX
	BCA8	POP IY
	BCAA	LD SP,HL
	BCAB	PUSH IY
	BCAD	PUSH IX
	BCAF	PUSH BC
	BCB0	PUSH DE
	BCB1	PUSH AF
	BCB2	INC H
	BCB3	EXX
	BCB4	PUSH BC
	BCB5	PUSH DE
	BCB6	EX AF,AF'
	BCB7	PUSH AF
	BCB8	INC H
	BCB9	BIT 2,H	is the back buffer row a multiple of 4?
	BCBB	JP NZ,draw_screen_0	loop if so
	BCBE	LD DE,$F810	Advance back buffer pointer (backwards) by a half row?
	BCC1	ADD HL,DE
	BCC2	EXX
	BCC3	LD DE,$F80E	Advance screen pointer (backwards) by half a row?
	BCC6	ADD HL,DE
	BCC7	EXX
This is a similar sequence but only moves 14 bytes (not using IY)
draw_screen_1	BCC8	LD SP,HL	Point SP at the back buffer
	BCC9	POP AF	Pull in 16 bytes from the back buffer (order: AF' DE BC AF' DE' BC' IX), incrementing SP
	BCCA	EX AF,AF'
	BCCB	POP DE
	BCCC	POP BC
	BCCD	EXX
	BCCE	POP AF
	BCCF	POP DE
	BCD0	POP BC
	BCD1	POP IX
	BCD3	LD SP,HL
	BCD4	PUSH IX
	BCD6	PUSH BC
	BCD7	PUSH DE
	BCD8	PUSH AF
	BCD9	INC H
	BCDA	EXX
	BCDB	PUSH BC
	BCDC	PUSH DE
	BCDD	EX AF,AF'
	BCDE	PUSH AF
	BCDF	INC H
	BCE0	LD SP,HL	Transfer another 14 bytes (112 pixels)
	BCE1	POP AF
	BCE2	EX AF,AF'
	BCE3	POP DE
	BCE4	POP BC
	BCE5	EXX
	BCE6	POP AF
	BCE7	POP DE
	BCE8	POP BC
	BCE9	POP IX
	BCEB	LD SP,HL
	BCEC	PUSH IX
	BCEE	PUSH BC
	BCEF	PUSH DE
	BCF0	PUSH AF
	BCF1	INC H
	BCF2	EXX
	BCF3	PUSH BC
	BCF4	PUSH DE
	BCF5	EX AF,AF'
	BCF6	PUSH AF
	BCF7	INC H
	BCF8	LD SP,HL	Transfer another 14 bytes (112 pixels)
	BCF9	POP AF
	BCFA	EX AF,AF'
	BCFB	POP DE
	BCFC	POP BC
	BCFD	EXX
	BCFE	POP AF
	BCFF	POP DE
	BD00	POP BC
	BD01	POP IX
	BD03	LD SP,HL
	BD04	PUSH IX
	BD06	PUSH BC
	BD07	PUSH DE
	BD08	PUSH AF
	BD09	INC H
	BD0A	EXX
	BD0B	PUSH BC
	BD0C	PUSH DE
	BD0D	EX AF,AF'
	BD0E	PUSH AF
	BD0F	INC H
	BD10	LD SP,HL	Transfer another 14 bytes (112 pixels)
	BD11	POP AF
	BD12	EX AF,AF'
	BD13	POP DE
	BD14	POP BC
	BD15	EXX
	BD16	POP AF
	BD17	POP DE
	BD18	POP BC
	BD19	POP IX
	BD1B	LD SP,HL
	BD1C	PUSH IX
	BD1E	PUSH BC
	BD1F	PUSH DE
	BD20	PUSH AF
	BD21	INC H
	BD22	EXX
	BD23	PUSH BC
	BD24	PUSH DE
	BD25	EX AF,AF'
	BD26	PUSH AF
	BD27	INC H
	BD28	BIT 2,H	Is the back buffer row multiple of 4?
	BD2A	JP NZ,draw_screen_1
	BD2D	BIT 3,H	Is the back buffer row multiple of 8?
	BD2F	JR NZ,draw_screen_bd4d
	BD31	LD H,$F0	L -= $F0, H = $F0
	BD33	LD A,L
	BD34	SUB H
	BD35	LD L,A
	BD36	JP NC,draw_screen_attributes	L >= $F0
	BD39	JP PE,draw_screen_bd45	PE = parity even
	BD3C	EXX
	BD3D	LD DE,$F812	Increment the screen address
	BD40	ADD HL,DE
	BD41	EXX
	BD42	JP draw_screen_0
draw_screen_bd45	BD45	EXX
	BD46	LD HL,$5011	Point HL at screen pixel (136,128).
	BD49	EXX
	BD4A	JP draw_screen_0	Copy more
draw_screen_bd4d	BD4D	EXX
	BD4E	LD DE,$F812
	BD51	ADD HL,DE	Which one are we modifying here?
	BD52	EXX
	BD53	LD A,L
	BD54	SUB $10	Ditto, which?
	BD56	LD L,A
	BD57	JP draw_screen_0
draw_screen_attributes	BD5A	LD A,($A220)	Don't update the attributes if the level intro screen is being shown
	BD5D	AND A
	BD5E	JR NZ,draw_screen_exit
	BD60	LD HL,$E34C
	BD63	LD A,(HL)	Seems to be related to the horizon level
	BD64	INC L
	BD65	LD E,(HL)
	BD66	LD (HL),A
	BD67	LD A,E
	BD68	OR A	Jump to exit check if both are zero
	BD69	JR Z,draw_screen_4
	BD6B	ADD A,A	E = A * 4
	BD6C	ADD A,A
	BD6D	LD E,A
	BD6E	SBC A,A	A = $FF if carry set, zero otherwise
	BD6F	LD D,A	D = A
	BD70	LD HL,($A186)	Fetch the address of the first line of ground attributes (+ 31)
	BD73	LD BC,$6868	Set the sky colour screen attributes (always black over bright cyan)
	BD76	JR Z,draw_screen_2	If A was zero then jump (Z => sky, NZ => ground)
	BD78	LD BC,($5CF4)	Load the ground colour screen attributes (varies per level)
	BD7C	ADD HL,DE	Point SP at the screen attributes (DE = $FFE0 = -32)
draw_screen_2	BD7D	LD SP,HL	Point SP at the screen attributes (DE = $FFE0 = -32)
	BD7E	PUSH BC	Fill 30 bytes - length of attribute line minus the two blank edges
	BD7F	PUSH BC
	BD80	PUSH BC
	BD81	PUSH BC
	BD82	PUSH BC
	BD83	PUSH BC
	BD84	PUSH BC
	BD85	PUSH BC
	BD86	PUSH BC
	BD87	PUSH BC
	BD88	PUSH BC
	BD89	PUSH BC
	BD8A	PUSH BC
	BD8B	PUSH BC
	BD8C	PUSH BC
	BD8D	JR NZ,draw_screen_3
	BD8F	ADD HL,DE	Move to next line of attributes
draw_screen_3	BD90	LD ($A186),HL	Save the address of the first line of ground attributes (+ 31)
draw_screen_4	BD93	LD A,($A22E)	Exit if sighted_flag is zero (flashing lights / smash mode)
	BD96	AND A
	BD97	JR Z,draw_screen_exit
	BD99	LD A,($A230)	Jump if perp_caught_stage >= 3 (when car stops)
	BD9C	CP $03
	BD9E	JR NC,draw_screen_exit
Set the smash meter attributes
	BDA0	LD HL,$5962	Screen attribute (2,11)
	BDA3	LD DE,$0020	= 32
	BDA6	LD C,$50	Black over bright red
	BDA8	LD (HL),C	Set two attrs
	BDA9	ADD HL,DE
	BDAA	LD (HL),C
	BDAB	ADD HL,DE
	BDAC	LD C,$58	Black over bright magenta
	BDAE	LD (HL),C	Set two attrs
	BDAF	ADD HL,DE
	BDB0	LD (HL),C
	BDB1	ADD HL,DE
	BDB2	LD C,$60	Black over bright green
	BDB4	LD (HL),C	Set two attrs
	BDB5	ADD HL,DE
	BDB6	LD (HL),C
	BDB7	ADD HL,DE
	BDB8	LD C,$78	Black over bright white
	BDBA	LD (HL),C	Set two attrs
	BDBB	ADD HL,DE
	BDBC	LD (HL),C
draw_screen_exit	BDBD	LD SP,$0000	Restore SP (self modified at start of routine)
	BDC0	RET	Return