Routines
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CBCE: Road drawing - curvature stuff?
Used by the routine at layout_road.
This gets hit during road forks.
sub_cbce_fork CBCE LD HL,$EEEC Load two table high-bytes: $EE, $EC
CBD1 LD DE,$44ED $ED $44 => NEG instruction for CC21 & CC22
CBD4 JR sub_cbce_self_modify
This entry point is used by the routine at layout_road.
This is the normal entry point?
sub_cbce_non_fork CBD6 LD HL,$EDE9 Load two table high-bytes: $ED, $E9
CBD9 LD DE,$0000 Pair of NOP instructions for CC21 & CC22
sub_cbce_self_modify CBDC LD ($CC21),DE Write instructions in DE to CC21 & CC22
CBE0 LD A,H Self modify 'LD HL' @ CC70 to load ($<H>00)
CBE1 LD ($CC72),A
CBE4 LD A,L Self modify 'LD HL' @ CCA5 to load ($<L>00)
CBE5 LD ($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 = $E500 + A -- point into inward_bend_table
CC11 LD IXh,$E5
CC14 LD DE,$E320 D = $E3, E = $20
CC17 LD B,$16 20 iterations
CC19 EXX Bank
CC1A LD DE,($A26C) Get road position
CC1E PUSH DE
CC1F EXX Unbank
sub_cbce_fork_0 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 IX.low += A -- point into distance shift table
CC27 LD IXl,A
CC29 LD HL,$0000 HL = 0 -- not self modified
CC2C LD A,(IX+$00) Reads from the inward_bend_table table (inward_bend_table)
Subtract DE from table entry, result in BC.
CC2F SUB E C = A - E -- E is $20 here I think
CC30 LD 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,sub_cbce_fork_1 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
sub_cbce_fork_1 CC41 ADD A,A shift topmost bit out
CC42 JR NC,sub_cbce_fork_2 jump if top bit not set
CC44 ADD HL,BC otherwise HL += BC
sub_cbce_fork_2 CC45 ADD HL,HL repeat
CC46 ADD A,A
CC47 JR NC,sub_cbce_fork_3
CC49 ADD HL,BC
sub_cbce_fork_3 CC4A ADD HL,HL repeat
CC4B ADD A,A
CC4C JR NC,sub_cbce_fork_4
CC4E ADD HL,BC
sub_cbce_fork_4 CC4F ADD HL,HL repeat
CC50 ADD A,A
CC51 JR NC,sub_cbce_fork_5
CC53 ADD HL,BC
sub_cbce_fork_5 CC54 ADD HL,HL repeat
CC55 ADD A,A
CC56 JR NC,sub_cbce_fork_6
CC58 ADD HL,BC
sub_cbce_fork_6 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,sub_cbce_fork_7 jump if top bit not set
CC63 DEC H H--
sub_cbce_fork_7 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 sub_cbce_fork_0 Loop back to read buffer byte bit
CC6C POP DE
CC6D LD B,$00
CC6F EXX Bank
CC70 LD HL,$ED00
CC73 CALL sub_cbce_fork_10 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,sub_cbce_fork_8 Jump if A+$60 had no carry
CC8D INC H Add carry otherwise
Converts the 20 bytes at $E320 to totals.
sub_cbce_fork_8 CC8E LD DE,$E320 [points to presumed lanes data in pristine memory map but must be something else]
CC91 LD B,$16 B = 20
sub_cbce_fork_9 CC93 LD A,(DE) *DE += *HL
This reads from $E760+ in sequence.
CC94 ADD A,(HL)
CC95 LD (DE),A }
CC96 INC HL HL++
CC97 INC E E++
CC98 DJNZ sub_cbce_fork_9 Loop while B > 0
CC9A LD HL,($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?
sub_cbce_fork_10 CCA8 LD IY,$E300 table?
CCAC LD B,$15 B = 21
CCAE LD ($CCF5),SP Save SP to restore on exit (self modify)
CCB2 LD SP,HL Put address in SP (so we can use POP for speed)
sub_cbce_fork_11 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,sub_cbce_fork_20
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,sub_cbce_fork_12 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,sub_cbce_fork_17 jump if A < L
CCDB JP sub_cbce_fork_13
sub_cbce_fork_12 CCDE CP L A < L ?
CCDF LD A,$13 Opcode for INC DE
CCE1 JR C,sub_cbce_fork_17 jump if A < L
sub_cbce_fork_13 CCE3 LD ($CCED),A Self modify instruction below
CCE6 LD A,B A = B
CCE7 RRA A >>= 1
sub_cbce_fork_14 CCE8 ADD A,L A += L
CCE9 CP C A < C ?
CCEA JR C,sub_cbce_fork_15 jump if A < C
CCEC SUB C A -= C
CCED INC DE Self modified: INC DE or DEC DE
sub_cbce_fork_15 CCEE PUSH DE
CCEF DJNZ sub_cbce_fork_14 Loop while B > 0
CCF1 EXX Unbank
CCF2 DJNZ sub_cbce_fork_11 Outer loop?
sub_cbce_fork_16 CCF4 LD SP,$0000 Restore original SP (must be self modified)
CCF7 RET Return
sub_cbce_fork_17 CCF8 LD ($CCFC),A Self modify instruction below
CCFB XOR A A = 0
sub_cbce_fork_18 CCFC INC DE Self modified: INC DE or DEC DE
CCFD ADD A,C A += C
CCFE JR C,sub_cbce_fork_19 jump if overflow
CD00 CP L A < L ?
CD01 JR C,sub_cbce_fork_18 jump if A < L
sub_cbce_fork_19 CD03 SUB L A -= L
CD04 PUSH DE
CD05 DJNZ sub_cbce_fork_18 Loop while B > 0
CD07 EXX Bank
CD08 DJNZ sub_cbce_fork_11 Outer loop?
CD0A JR sub_cbce_fork_16 Exit
sub_cbce_fork_20 CD0C LD (IY+$4E),$01 IY[$4E] = 1
CD10 INC A A++
CD11 JR Z,sub_cbce_fork_21
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,sub_cbce_fork_11 Outer loop?
CD24 JP sub_cbce_fork_16 Exit
sub_cbce_fork_21 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 result?
CD32 EXX Bank
CD33 DEC B B--
CD34 JP NZ,sub_cbce_fork_11 Outer loop?
CD37 JP sub_cbce_fork_16 Exit
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