Routines
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CBCE: Routine at CBCE - road drawing stuff
Used by the routine at main_loop_12.
CBCE LD HL,$EEEC Load two high bytes ($EExx table offset, $ECxx table offset)
CBD1 LD DE,$44ED $ED $44 => NEG instr for CC21 & CC22
CBD4 JR $CBDC
This entry point is used by the routine at main_loop_12.
CBD6 LD HL,$EDE9 Load two high bytes ($ED00 table offset, $E900 table offset)
CBD9 LD DE,$0000 Pair of NOPs for CC21 & CC22
This entry point is used by the routine at CBCE.
CBDC LD ($CC21),DE Write instructions in DE to CC21 & CC22
CBE0 LD A,H Self modify 'LD HL' @ CC70 to load ($ED00 + H)
CBE1 LD ($CC72),A
CBE4 LD A,L Self modify 'LD HL' @ CCA5 to load ($E900 + L)
CBE5 LD ($CCA7),A
CBE8 LD HL,($A240) Load road_buffer_offset into HL
CBEB LD C,(HL) Read a curvature data byte
There's similar code at CD47.
CBEC LD A,($A23F) A = fast_counter & $E0 -- top three bits
CBEF AND $E0
CBF1 LD B,A
Reduces A by 31.25% ... unsure why that figure.
CBF2 RRC B Divide A by 4 and subtract
CBF4 RRC B
CBF6 SUB B
CBF7 RRC B Divide A by 16 and subtract
CBF9 RRC B
CBFB SUB B
CBFC ADD A,$B0 IY = data_e6b0[A]
CBFE LD IYl,A
CC00 LD A,$E6
CC02 ADC A,$00
CC04 LD IYh,A
CC06 CALL multiply Call multiply (A = multiplier, C = multiplicand)
CC09 NEG A = (128 - A) & $FE
CC0B ADD A,$80
CC0D AND $FE
CC0F LD IXl,A IX = $E500 + A -- distance shift table
CC11 LD IXh,$E5
CC14 LD DE,$E320 not a pointer, separate values
CC17 LD B,$16 20 iterations
CC19 EXX Bank
CC1A LD DE,($A26C) Get road position
CC1E PUSH DE
CC1F EXX Unbank
CC20 LD A,(HL) Read road buffer byte
CC21 NOP Self modified above - Set to NOP or NEG
CC22 NOP
CC23 INC L Advance road buffer pointer (wrapping)
CC24 EXX Bank
CC25 ADD A,IXl
CC27 LD IXl,A
CC29 LD HL,$0000
CC2C LD A,(IX+$00) Reads from the E540 table (distance shift table)
Subtract DE from table entry, result in BC.
CC2F SUB E A -= E
CC30 LD C,A C = A
CC31 LD A,(IX+$01) high byte
CC34 SBC A,D A -= D
CC35 LD B,A B = A
Sampled IY = E71E E71F E720 ..
CC36 LD A,(IY+$00) IY seems to point to $E6xx..E7xx
CC39 INC IY
This is a multiplier of HL (distance shift value computed above) by A (value from $E600+).
CC3B ADD A,A shift left
CC3C JR NC,$CC41 top bit not set
CC3E LD H,B HL = BC copy of distance shift value
CC3F LD L,C
CC40 ADD HL,HL shift result left. HL seems to be zeroed above, so this is confusing
CC41 ADD A,A shift topmost bit out
CC42 JR NC,$CC45 jump if top bit not set
CC44 ADD HL,BC otherwise HL += BC
CC45 ADD HL,HL repeat
CC46 ADD A,A
CC47 JR NC,$CC4A
CC49 ADD HL,BC
CC4A ADD HL,HL repeat
CC4B ADD A,A
CC4C JR NC,$CC4F
CC4E ADD HL,BC
CC4F ADD HL,HL repeat
CC50 ADD A,A
CC51 JR NC,$CC54
CC53 ADD HL,BC
CC54 ADD HL,HL repeat
CC55 ADD A,A
CC56 JR NC,$CC59
CC58 ADD HL,BC
CC59 LD A,H A = H
CC5A RL L L <<= 1
CC5C LD H,$00 H = 0
CC5E ADC A,H A += H + carry
CC5F LD L,A L = A
CC60 ADD A,A A <<= 1
CC61 JR NC,$CC64 jump if top bit not set
CC63 DEC H H--
CC64 RRA A >>= 1
CC65 ADD HL,DE
CC66 EX DE,HL
CC67 EXX Unbank
CC68 LD (DE),A *DE++ = A
CC69 INC E
CC6A DJNZ $CC20 Loop back to read buffer byte bit
CC6C POP DE
CC6D LD B,$00
CC6F EXX Bank
CC70 LD HL,$ED00
CC73 CALL $CCA8 subroutine
CC76 LD A,($A23F) A = fast_counter
CC79 AND $E0 take top three bits
CC7B LD B,A same pattern as above
CC7C RRC B
CC7E RRC B
CC80 SUB B
CC81 RRC B
CC83 RRC B
CC85 SUB B
CC86 ADD A,$60
CC88 LD L,A HL = $E700 | A
CC89 LD H,$E7
CC8B JR NC,$CC8E Jump if A+$60 had no carry
CC8D INC H Add carry otherwise
CC8E LD DE,$E320 [points to presumed lanes data in pristine memory map but must be something else]
CC91 LD B,$16 B = 20
CC93 LD A,(DE) *DE += *HL
This reads from $E760+ in sequence.
CC94 ADD A,(HL)
CC95 LD (DE),A }
CC96 INC HL HL++
CC97 INC E E++
CC98 DJNZ $CC93 Loop while B
CC9A LD HL,($A26C) HL = road position - 295
CC9D LD DE,$FED9
CCA0 ADD HL,DE
CCA1 EX DE,HL swap
CCA2 LD B,$00 B = 0
CCA4 EXX Unbank
CCA5 LD HL,$E900 table?
CCA8 LD IY,$E300 table?
CCAC LD B,$15 B = 21
CCAE LD ($CCF5),SP Save SP to restore later (self modify)
CCB2 LD SP,HL Put address in SP (so we can use POP for speed)
CCB3 EXX Bank
CCB4 LD A,B A = B - 2
CCB5 SUB $02
CCB7 ADD A,(IY+$00) A += *IY
CCBA INC IYl IY++
CCBC SUB (IY+$00) A -= *IY
CCBF JP M,$CD0C
CCC2 ADD A,$02 A += 2
CCC4 LD (IY+$4E),A IY[$4E] = A
CCC7 SUB B A -= B
CCC8 LD C,A C = A
CCC9 LD B,A B = A
CCCA LD L,(IY+$1F) L = IY[$1F]
CCCD BIT 7,L test bit 7 of L
CCCF JR Z,$CCDE jump if ?
CCD1 LD A,L L = -L
CCD2 NEG
CCD4 LD L,A
CCD5 LD A,B A = B
CCD6 CP L A < L ?
CCD7 LD A,$1B Opcode for DEC DE
CCD9 JR C,$CCF8 jump if A < L
CCDB JP $CCE3
CCDE CP L A < L ?
CCDF LD A,$13 Opcode for INC DE
CCE1 JR C,$CCF8 jump if A < L
CCE3 LD ($CCED),A Self modify instruction below
CCE6 LD A,B A = B
CCE7 RRA A >>= 1
CCE8 ADD A,L A += L
CCE9 CP C A < C ?
CCEA JR C,$CCEE jump if A < C
CCEC SUB C A -= C
CCED INC DE Self modified: INC DE or DEC DE
CCEE PUSH DE
CCEF DJNZ $CCE8 Loop while B
CCF1 EXX Unbank
CCF2 DJNZ $CCB3 Outer loop?
CCF4 LD SP,$0000 Restore original SP (must be self modified)
CCF7 RET Return
CCF8 LD ($CCFC),A Self modify instruction below
CCFB XOR A A = 0
CCFC INC DE Self modified: INC DE or DEC DE
CCFD ADD A,C A += C
CCFE JR C,$CD03 jump if overflow
CD00 CP L A < L ?
CD01 JR C,$CCFC jump if A < L
CD03 SUB L A -= L
CD04 PUSH DE
CD05 DJNZ $CCFC Loop while B
CD07 EXX Bank
CD08 DJNZ $CCB3 Outer loop?
CD0A JR $CCF4 Exit
CD0C LD (IY+$4E),$01 IY[$4E] = 1
CD10 INC A A++
CD11 JR Z,$CD27
CD13 INC A A++
CD14 LD B,A B = A
CD15 LD A,(IY+$1F) A = IY[$1F]
CD18 LD L,A L = A
CD19 ADD A,A A <<= 1
CD1A SBC A,A A = A - A - carry
CD1B LD H,A H = A
CD1C ADD HL,DE HL += DE
CD1D EX DE,HL swap
CD1E PUSH DE push
CD1F EXX Unbank
CD20 DEC B B--
CD21 JP NZ,$CCB3 Outer loop?
CD24 JP $CCF4 Exit
CD27 LD B,A B = A
CD28 LD A,(IY+$1F) A = IY[$1F]
CD2B LD L,A L = A
CD2C ADD A,A A <<= 1
CD2D SBC A,A A = A - A - carry
CD2E LD H,A H = A
CD2F ADD HL,DE HL += DE
CD30 EX DE,HL swap
CD31 PUSH DE push
CD32 EXX Bank
CD33 DEC B B--
CD34 JP NZ,$CCB3 Outer loop?
CD37 JP $CCF4 Exit
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