Routines
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BC3E: Copies the backbuffer at $F000 to the screen (and sets attributes)
Copies 240 x ? pixels from the backbuffer to the screen. This is a thinner than the real screen due to the main gameplay area's left and right black borders. It then sets up the attributes.
Input
Used by the routines at load_stage, attract_mode, main_loop, run_pregame_screen, escape_scene and
#R$F220.
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 ($BDBE),SP Self modify SP restore instruction
A sequence that transfers 16 bytes
ds_loop BC49 LD SP,HL Point SP at the back buffer
BC4A POP AF Pull in 16 bytes from the back buffer (order: AF' DE BC AF' DE' BC' IX IY), incrementing SP
BC4B EX AF,AF'
BC4C POP DE
BC4D POP BC
BC4E EXX
BC4F POP AF
BC50 POP DE
BC51 POP BC
BC52 POP IX
BC54 POP IY
BC56 LD SP,HL Point SP at screen
BC57 PUSH IY Push out 16 bytes to the screen (reversing the order), decrementing SP
BC59 PUSH IX
BC5B PUSH BC
BC5C PUSH DE
BC5D PUSH AF
BC5E INC H Advance screen pointer
BC5F EXX Continue pushing
BC60 PUSH BC
BC61 PUSH DE
BC62 EX AF,AF'
BC63 PUSH AF
BC64 INC H Advance back buffer pointer
BC65 LD SP,HL Transfer another 16 bytes (128 pixels)
BC66 POP AF
BC67 EX AF,AF'
BC68 POP DE
BC69 POP BC
BC6A EXX
BC6B POP AF
BC6C POP DE
BC6D POP BC
BC6E POP IX
BC70 POP IY
BC72 LD SP,HL
BC73 PUSH IY
BC75 PUSH IX
BC77 PUSH BC
BC78 PUSH DE
BC79 PUSH AF
BC7A INC H
BC7B EXX
BC7C PUSH BC
BC7D PUSH DE
BC7E EX AF,AF'
BC7F PUSH AF
BC80 INC H
BC81 LD SP,HL Transfer another 16 bytes (128 pixels)
BC82 POP AF
BC83 EX AF,AF'
BC84 POP DE
BC85 POP BC
BC86 EXX
BC87 POP AF
BC88 POP DE
BC89 POP BC
BC8A POP IX
BC8C POP IY
BC8E LD SP,HL
BC8F PUSH IY
BC91 PUSH IX
BC93 PUSH BC
BC94 PUSH DE
BC95 PUSH AF
BC96 INC H
BC97 EXX
BC98 PUSH BC
BC99 PUSH DE
BC9A EX AF,AF'
BC9B PUSH AF
BC9C INC H
BC9D LD SP,HL Transfer another 16 bytes (128 pixels)
BC9E POP AF
BC9F EX AF,AF'
BCA0 POP DE
BCA1 POP BC
BCA2 EXX
BCA3 POP AF
BCA4 POP DE
BCA5 POP BC
BCA6 POP IX
BCA8 POP IY
BCAA LD SP,HL
BCAB PUSH IY
BCAD PUSH IX
BCAF PUSH BC
BCB0 PUSH DE
BCB1 PUSH AF
BCB2 INC H
BCB3 EXX
BCB4 PUSH BC
BCB5 PUSH DE
BCB6 EX AF,AF'
BCB7 PUSH AF
BCB8 INC H
BCB9 BIT 2,H Is the back buffer row a multiple of 4?
BCBB JP NZ,ds_loop Loop if so
BCBE LD DE,$F810 Advance back buffer pointer (backwards) by a half row?
BCC1 ADD HL,DE
BCC2 EXX
BCC3 LD DE,$F80E Advance screen pointer (backwards) by half a row?
BCC6 ADD HL,DE
BCC7 EXX
This is a similar sequence but only moves 14 bytes (not using IY)
draw_screen_0 BCC8 LD SP,HL Point SP at the back buffer
BCC9 POP AF Pull in 16 bytes from the back buffer (order: AF' DE BC AF' DE' BC' IX), incrementing SP
BCCA EX AF,AF'
BCCB POP DE
BCCC POP BC
BCCD EXX
BCCE POP AF
BCCF POP DE
BCD0 POP BC
BCD1 POP IX
BCD3 LD SP,HL
BCD4 PUSH IX
BCD6 PUSH BC
BCD7 PUSH DE
BCD8 PUSH AF
BCD9 INC H
BCDA EXX
BCDB PUSH BC
BCDC PUSH DE
BCDD EX AF,AF'
BCDE PUSH AF
BCDF INC H
BCE0 LD SP,HL Transfer another 14 bytes (112 pixels)
BCE1 POP AF
BCE2 EX AF,AF'
BCE3 POP DE
BCE4 POP BC
BCE5 EXX
BCE6 POP AF
BCE7 POP DE
BCE8 POP BC
BCE9 POP IX
BCEB LD SP,HL
BCEC PUSH IX
BCEE PUSH BC
BCEF PUSH DE
BCF0 PUSH AF
BCF1 INC H
BCF2 EXX
BCF3 PUSH BC
BCF4 PUSH DE
BCF5 EX AF,AF'
BCF6 PUSH AF
BCF7 INC H
BCF8 LD SP,HL Transfer another 14 bytes (112 pixels)
BCF9 POP AF
BCFA EX AF,AF'
BCFB POP DE
BCFC POP BC
BCFD EXX
BCFE POP AF
BCFF POP DE
BD00 POP BC
BD01 POP IX
BD03 LD SP,HL
BD04 PUSH IX
BD06 PUSH BC
BD07 PUSH DE
BD08 PUSH AF
BD09 INC H
BD0A EXX
BD0B PUSH BC
BD0C PUSH DE
BD0D EX AF,AF'
BD0E PUSH AF
BD0F INC H
BD10 LD SP,HL Transfer another 14 bytes (112 pixels)
BD11 POP AF
BD12 EX AF,AF'
BD13 POP DE
BD14 POP BC
BD15 EXX
BD16 POP AF
BD17 POP DE
BD18 POP BC
BD19 POP IX
BD1B LD SP,HL
BD1C PUSH IX
BD1E PUSH BC
BD1F PUSH DE
BD20 PUSH AF
BD21 INC H
BD22 EXX
BD23 PUSH BC
BD24 PUSH DE
BD25 EX AF,AF'
BD26 PUSH AF
BD27 INC H
BD28 BIT 2,H Is the back buffer row multiple of 4?
BD2A JP NZ,draw_screen_0
BD2D BIT 3,H Is the back buffer row multiple of 8?
BD2F JR NZ,ds_bd4d
BD31 LD H,$F0 L -= $F0, H = $F0
BD33 LD A,L
BD34 SUB H
BD35 LD L,A
BD36 JP NC,ds_attributes L >= $F0
BD39 JP PE,ds_bd45 PE = parity even
BD3C EXX
BD3D LD DE,$F812 Increment the screen address
BD40 ADD HL,DE
BD41 EXX
BD42 JP ds_loop
ds_bd45 BD45 EXX
BD46 LD HL,$5011 Point HL at screen pixel (136,128).
BD49 EXX
BD4A JP ds_loop Copy more
ds_bd4d BD4D EXX
BD4E LD DE,$F812
BD51 ADD HL,DE Which one are we modifying here?
BD52 EXX
BD53 LD A,L
BD54 SUB $10 Ditto, which?
BD56 LD L,A
BD57 JP ds_loop
ds_attributes BD5A LD A,($A220) Don't update the attributes if the level intro screen is being shown
BD5D AND A
BD5E JR NZ,ds_exit
BD60 LD HL,$E34C Seems to be related to the horizon level
BD63 LD A,(HL)
BD64 INC L Read $E34D
BD65 LD E,(HL)
BD66 LD (HL),A Write $E34D
BD67 LD A,E Jump to exit check if E is zero
BD68 OR A
BD69 JR Z,draw_screen_3
BD6B ADD A,A E = A * 4
BD6C ADD A,A
BD6D LD E,A
BD6E SBC A,A A = $FF if carry set, zero otherwise (sign extending?)
BD6F LD D,A D = A
BD70 LD HL,($A186) Load 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_1 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 = $FFE0 = -32)
draw_screen_1 BD7D LD SP,HL
BD7E PUSH BC Fill 30 bytes - length of attribute line minus the two blank edges
BD7F PUSH BC
BD80 PUSH BC
BD81 PUSH BC
BD82 PUSH BC
BD83 PUSH BC
BD84 PUSH BC
BD85 PUSH BC
BD86 PUSH BC
BD87 PUSH BC
BD88 PUSH BC
BD89 PUSH BC
BD8A PUSH BC
BD8B PUSH BC
BD8C PUSH BC
BD8D JR NZ,draw_screen_2
BD8F ADD HL,DE Move to next line of attributes
draw_screen_2 BD90 LD ($A186),HL Save the address of the first line of ground attributes (+ 31)
draw_screen_3 BD93 LD A,($A22E) Exit if sighted_flag is zero (flashing lights / smash mode)
BD96 AND A
BD97 JR Z,ds_exit
BD99 LD A,($A230) Jump if perp_caught_phase >= 3 (when car stops)
BD9C CP $03
BD9E JR NC,ds_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
ds_exit BDBD LD SP,$0000 Restore original SP (self modified at start of routine)
BDC0 RET Return
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