Retro Isle - Atari VCS/2600 Articles

; Disassembly of defender.bin
; Disassembled Mon Feb 10 21:05:22 1997
; Using DiStella v2.0
;
; Command Line: D:\STELLA\VCS\DISTELLA.EXE -paf defender.bin
;
; Comments by Nick Bensema
; Last updated Feb 24, 1997
;

processor 6502

;GAMEVAR = $D2
;BADGUYS = $E9
;PLRNUM = $A2
CITYGFX = $D4
JOYSTICK = $B3

VSYNC = $00
VBLANK = $01
WSYNC = $02
NUSIZ0 = $04
NUSIZ1 = $05
COLUP0 = $06
COLUP1 = $07
COLUPF = $08
COLUBK = $09
CTRLPF = $0A
REFP0 = $0B
PF0 = $0D
PF1 = $0E
PF2 = $0F
RESP0 = $10
RESP1 = $11
AUDC0 = $15
AUDF0 = $17
AUDV0 = $19
GRP0 = $1B
GRP1 = $1C
ENAM0 = $1D
ENAM1 = $1E
ENABL = $1F
HMP0 = $20
VDELP0 = $25
VDEL01 = $26
;VDELP1 = $26 ;Duhhh...
HMOVE = $2A
HMCLR = $2B
CXCLR = $2C
CXM0P = $30
CXPPMM = $37
INPT4 = $3C
SWCHA = $0280
SWCHB = $0282
INTIM = $0284
TIM64T = $0296

ORG $F000

START:
SEI
CLD
; Clear all memory
LDX #$FF
TXS
LDA #$00
LF007: STA WSYNC,X
DEX
BNE LF007
LDX #$FF
STX $98 ;Load $98 and $9B
STX $9B ;with #$FF
JSR LFD3B
JSR LFD1C
STA $A2
JSR LFD17
LF01D: JSR LF08C
LDA $CB
BNE LF037
LDA $B4
BNE LF02E
JSR LF0F0
JSR LF1D6
LF02E: JSR LF227
JSR LF2E8
JSR LF33C
LF037: JSR LF3EA
JSR LF551
JSR LF5FF
JSR LF628
JSR LF66E
JSR LF45B
JSR LF4A2
JMP LF6C7
LF04F: LDA $CB
BEQ LF062
INY
STY $E6
DEC $CB
LDA $CB
BNE LF084
JSR LFDC3
JSR LFCE3
LF062: JSR LF8AB
JSR LF969
JSR LF9A2
JSR LF9FB
JSR LFA4F
LDA $91
BNE LF07B
JSR LFADD ;These two executed only
JSR LFB37 ;if $91 equals zero
LF07B: JSR LFB90
JSR LFC0C
JSR LFC43
LF084: LDA INTIM
BNE LF084
JMP LF01D
; Do vertical blanking
LF08C: LDA #$FF
STA WSYNC
STA VSYNC
STA VBLANK
STA WSYNC
STA WSYNC
STA WSYNC
STA WSYNC
; Set timer
LDA #$36
STA TIM64T
LDY #$00
STY PF0
STY VSYNC
LDX #$7F
STX VBLANK
LDY $A2
LDA.wy $00C2,Y
STA $C0 ; Display value for $C2
LDA.wy $00C4,Y
STA $C1 ; Display value for $C4
; I keep seeing stuff like this.
; It smells of random number generation.
LDA $98
ASL
EOR $98
ASL
ASL
ROL $98
LDA $CB
BEQ LF0CD
LDA #$00
STA $BD
STA $BF
JSR LFCAC
; This little routine grabs the appropriate
; joystick readings and shifts them to a common
; location so that it needn't be considered further.
LF0CD: LDA SWCHA
LDY $A2 ;A2 seems to be player number.
AND.wy PLYRJOY,Y ;Y= 0: 0xF0 1: 0x0F
STA JOYSTICK ;AND mask for joytick
CPY #$01
BNE LF0E1
ASL
ASL
ASL
ASL
STA JOYSTICK
LF0E1: LDX $8D
BNE LF0E7
STX $91
LF0E7: CMP #$F0
BEQ LF0EF
LDA #$78
STA $8D
LF0EF: RTS

LF0F0: JSR LFC7B
LDA $CD
BMI LF13A ;Skip if less than zero.
;ON $CD goto LF16E, LF149, LF158
CMP #$01
BEQ LF16E
CMP #$02
BEQ LF149
CMP #$03
BEQ LF158
LDX $BB
; if ( $80 [ $BB ] & 0xF0 )== 0
LDA $80,X
AND #$F0
BNE LRTS1
; and ( $80 [ $BB ] & 0x0F )== 4
LDA $80,X
AND #$0F
CMP #$04
BNE LRTS1
; and $AA & 0x80 == 0
LDA $AA
BMI LRTS1
; and $CE & 0x80 == 0
LDA $CE
BMI LRTS1
; and $A8 == $A9
LDA $A8
CMP $A9
BNE LRTS1
; then $AF = $C6 = $CD = 0xFF
LDA #$FF
STA $CD
STA $C6
STA $AF
; $C7 = ($AC - 1) * 16
LDA $AC
SEC
SBC #$01
ASL
ASL
ASL
ASL
STA $C7
; $CA = $CE
;and if I'm right, $B9 = $BB.
LDA $CE
STA $CA
STX $B9
LF13A: LDA $A3
AND $93
BNE LRTS1
LDA $C7
CMP #$17
BCC LF166
DEC $C7
LRTS1: RTS

LF149: CMP $AF ;Branched here if $CD = 2
BCS LF1B8 ;Branch if $AC > $AF
LDA $A3
AND #$03
BNE LRTS1 ;RTS if $A3 has bits 0 or 1.
DEC $AF
DEC $C7
RTS

LF158: LDA $AD ; Branched here if $CD = 3
CMP #$1E
BNE LRTS1 ;RTS if $AD != #$1E.
STA $8A ;$8A = $AD
LDA #$05
STA $99
BNE LF1C9 ; Always branch
LF166: INC $AF ;Branched here if $C7 < 0x17
LDA $AF
CMP #$0E
BCC LRTS1 ;Branch if $AF < $0E
LF16E: LDA $C7 ; Branched here if $CD = 1
LDY #$01
STY $CD ;
CMP #$87
BCS LF188
LDA $A3
AND $93
BNE LRTS1
INC $C7
LDA $C7
SEC
SBC #$0C
STA $AF
RTS

;Branch here is $C7 == 0x87
LF188: LDX $B9
LDA #$05
STA $80,X
LF18E: LDY $CA
LDA $91
BNE LF1C9
LDX $A2
LDA #$31
STA $8A
LDA $CF,X
ORA LFF12,Y
STA $CF,X
CMP #$FF
BNE LF1C9
STA $CB
LDX #$07
LF1A9: LDA $80,X
AND #$0F
CMP #$04
BNE LF1B3
INC $80,X
LF1B3: DEX
BPL LF1A9
BMI LF1C9
LF1B8: LDA $BA
CMP #$32
BEQ LF18E
JSR LFD2F
LDA #$05
STA $9A
LDA #$02
STA $99
LF1C9: LDA #$64
STA $CA
STA $B9
LDA #$00
STA $CD
STA $AF
RTS

LF1D6: JSR LFC7B
LDY $B8
LDX LFF06,Y
LDA $A3
AND $93
BNE LF208
LDA $89
BNE LF208
CPY #$05
BEQ LF1F0
CPX #$00
BNE LF208
LF1F0: INC $E3
;if $AD < $B0
LDA $B0
CMP $AD
BCC LF1FC
;then
DEC $E3
DEC $E3
LF1FC: INC $E0,X
LDA $A5
CMP $A6
BCS LF208
DEC $E0,X
DEC $E0,X
LF208: INC $E4
LDA $A3
AND #$07
BNE LRTS2
DEC $C8
DEC $AE
DEC $E2
LDA $8B
BPL LF21E
INC $C8
INC $C8
LF21E: LDA $98
BMI LRTS2
INC $E2
INC $E2
LRTS2: RTS

LF227: LDA $91
BEQ LF230
LDA #$01
STA $CC
RTS

LF230: LDX #$08
LDA $B4
BNE LRTS2
LF236: LDA LFE8E,X
CMP JOYSTICK
BEQ LF240
DEX
BNE LF236
LF240: LDA LFE97,X
STA $93
LDA LFEA0,X
TAX
CMP $CC
BEQ LF25F
LDA $A3
AND #$0F
BNE LF261
LDA #$FF
STA $B5
CPX #$00
BEQ LF25F
LDA #$00
STA $E6
LF25F: STX $CC
LF261: LDA $CD
CMP #$03
BNE LF26F
LDA #$82
CMP $AD
BCS LF26F
STA $AD
LF26F: LDA $AD
CMP #$19
BNE LF27B
LDA $A3
AND #$1F
BNE LF2BB
LF27B: LDY #$00
LDX $A2
LDA LFEE6,X
AND SWCHB
BEQ LF288
INY
LF288: STY $94
; if $A3 shares bits with $94
LDA $A3
AND $94 ;Could have used BIT here too.
BNE LF297
; then $AD += $93
LDA $AD
CLC
ADC $93
STA $AD
LF297: LDA $AD
CMP #$AA
BCC LF2B5
LDA #$AA
STA $AD
LDA #$00
STA $B1
STA $E6
LDA $88
BEQ LF2B5
LDA #$3B
STA $CB
LDA $98
STA $C8
STA $AE
LF2B5: LDA $AD
BNE LF2BB
INC $AD
; if $CD == 3
LF2BB: LDA $CD
CMP #$03
BNE LF2E7
; and $CA == $CE
LDA $CA
CMP $CE
BNE LF2E7
; then $C7 = $AD - 0x0F
; $AF = $C7 = $AD - 0x0f
LDA $AD
SEC
SBC #$0F
STA $C7
STA $AF
;If $8B = 5 then X=1 else X=5
LDX #$05
CPX $8B
BEQ LF2D8
LDX #$01
; $93 = X
LF2D8: STX $93
; $A4 = $A5 + $93
LDA $A5
CLC
ADC $93
STA $A4
; $C6 = $A4 / 2 + 0x58
LSR
CLC
ADC #$58
STA $C6
LF2E7: RTS

LF2E8: LDA $CC
STA $94
CMP #$01
BNE LF2F4
LDX #$05
STX $8B
LF2F4: CMP #$FF
BNE LF2FC
LDA #$FB
STA $8B
LF2FC: LDA $8B
LDX #$00
; If $89 == 5
STX $89
CMP #$05
BNE LF321
; then go until RTS.
STX REFP0
LDA $B4
BNE LRTS3
LDA $A5
CMP #$1F
BCC LRTS3
LDA $A3
AND #$01
BNE LF31A
DEC $A5
LF31A: LDA #$01
STA $89
STA $94
RTS

;
; Steer the ship, I think.
;
LF321: LDY #$FF
STY REFP0 ;Yup, that'll rvs the ship.
; if $B4 == 0
LDA $B4
BNE LRTS3
; and $A5 < 0x82
LDA $A5
CMP #$82
BCS LRTS3
; and $A3 & 1
LDA $A3
AND #$01
BNE LF337
; then...
INC $A5
LF337: STY $89
STY $94
LRTS3: RTS

LF33C: LDA $B4
BNE LRTS3
LDA $8B
STA $95
LDA $89
BEQ LF362
LDX #$05
CPX $95
BNE LF350
LDX #$FF
LF350: STX $95
LDA #$00
CMP $B6
BEQ LF362
CMP $CC
BEQ LF370
CMP $B5
BEQ LF362
STA $94
LF362: LDA $94
BEQ LF370
LDA $B6
CMP #$30
BEQ LF38C
INC $B6
BNE LF38C
LF370: LDA $B6
BEQ LRTS6
DEC $B6
LDA $B6
BNE LF37C
STA $B5
LF37C: LDA $94
BNE LF38C
LDX #$01
LDA $95
CMP #$05
BEQ LF38A
LDX #$FF
LF38A: STX $94
LF38C: LDA $B6
BEQ LRTS6
SED
CLC
ADC #$0D
LSR
LSR
LSR
LSR
CLD
TAX
LDA $A3
AND LFEB5,X
BNE LRTS6
LDA $94
BMI LF3C9
DEC $C9
DEC $C9
DEC $C8
LDA $CD
BMI LF3B1
DEC $C6
LF3B1: LDX #$03
LF3B3: LSR CITYGFX+8,X ; 0->[ DC ] -.
ROL CITYGFX+4,X ; ,- [ D8 ]<-'
ROR CITYGFX,X ; `->[D4]---.
LDA CITYGFX,X ; ,---------'
AND #$08 ; |
BEQ LF3C5 ; |
LDA CITYGFX+8,X ; |
ORA #$80 ; `->[ DC ] -.
STA CITYGFX+8,X
LF3C5: DEX
BPL LF3B3
LRTS6: RTS

LF3C9: INC $C9
INC $C9
INC $C8
LDA $CD
BMI LF3D5
INC $C6
LF3D5: LDX #$03
; This routine performs a 20-bit rotate
LF3D7: CLC
ROL CITYGFX,X ; ,- [D4] <- 0
ROR CITYGFX+4,X ; `->[D8 ] -.
ROL CITYGFX+8,X ; ,- [DC ]<-'
BCC LF3E6 ; `-----------.
LDA CITYGFX,X ; |
ORA #$10 ; Start at bit 4 |
STA CITYGFX,X ; [D4] <---'
LF3E6: DEX ;Repeat for all four lines of city gfx.
BPL LF3D7
RTS

LF3EA: LDA $91
BEQ LF3F6
LDA $8D
BEQ LF3F6
LDA INPT4
BPL LF3FC
; if RESET pressed
LF3F6: LDA SWCHB
ROR
BCS LF40C
; then....
LF3FC: LDA $9B
BPL LF404
INC $9B
INC $9B
LF404: JSR LFD3B
JSR LFD1C ; $8d = 0x78, $91 = 0x80
BEQ LF444
; if SELECT not pressed
LF40C: ROR
BCC LF415
; then...
LDX #$01
STX $E7
BNE LF444
LF415: JSR LFD17 ; $91 = 0xFF
JSR LFDE3
DEC $E7
BPL LF444
LDA #$2D
STA $E7
LDA $9B
BPL LF429
INC $9B
LF429: LDA $9B
SED
CLC
ADC #$01
STA $9B
LDX #$78
STX $8D
LDX #$00
STX $A3
STX $A2
CMP #$21
BCC LF443
LDA #$01
STA $9B
LF443: CLD
LF444: LDA #$10
STA NUSIZ0 ; Single-width players,
STA NUSIZ1 ; quad-width missiles.
LDX $E8
STX COLUBK
LDX #$05 ; Ball 1 clock wide
LDA $A3
AND #$0F
BNE LF458
LDX #$25 ; Ball 4 clocks wide
LF458: STX CTRLPF ; Reflect PLF, over players
RTS

LF45B: LDA $8D
BNE LF463
LDA #$FF
STA $91 ;could have done JSR LFD17 ???
LF463: LDA $A3
BNE LF474
INC $8C
LDA $91
BNE LF474
DEC $8D
BPL LF474
JSR LFD17 ;$91 = 0xFF
LF474: LDA SWCHB
LDY #$F7
; If B&W, X=0x0F. If Color, X=0xFF.
LDX #$0F
AND #$08 ;COLOR/B&W
BEQ LF481
LDX #$FF
LF481: LDA $91
BMI LF487
LDY #$FF
LF487: AND $8C
STA $95
STX $96
STY $97
LDX #$06
LF491: LDA LFFEF,X
EOR $95
AND $96
AND $97
STA $E8,X
DEX
BPL LF491
STA CXCLR
RTS

LF4A2: LDX #$00
LDY #$00
STX AUDV0
LDA $91
BNE LF503
LDA $CB
CMP #$97
BEQ LF4F8
BCC LF4C5
; for Y = 7 to 0 step -1
LDY #$07
; X = $98 & 7
LF4B6: LDA $98
AND #$07
TAX
; $E8 [Y] = $E8 [X]
LDA $E8,X
STA.wy $00E8,Y
; next Y
DEY
BPL LF4B6
; goto LF4FA
BMI LF4FA
LF4C5: LDA $B4
BNE LF4FC
LDA $CB
CMP #$3C
BCS LF508
LDA $CD
BPL LF4E6
LDA $AF
CMP #$04
BCS LF4E6
LDX #$05
LDA $98
AND #$37
TAY
LDA #$0E
STA AUDV0
BNE LF503
LF4E6: LDA $8A
BEQ LF520
DEC $8A
CMP #$20
BCS LF4FA
CMP #$1F
BNE LF517
STX $8A
BEQ LF503
LF4F8: STX $CB
; End skipped code
LF4FA: LDA $98
LF4FC: AND #$3F
TAY
LF4FF: LDX #$08
LF501: STX AUDV0
LF503: STX AUDC0
STY AUDF0
RTS

LF508: LDA $A3
AND #$2F
TAY
LDX #$01
LDA #$0A
STA AUDV0
LDA $D1
BEQ LF503
LF517: LDA $A3
AND #$22
TAY
LDX #$0E
BNE LF501
LF520: LDA $AD
CMP #$AA
BNE LF52A
LDA $CB
BNE LF4FA
LF52A: LDA $E5
BNE LF4FA
LDA $CB
BNE LF540
LDA JOYSTICK
CMP #$F0
BEQ LF540
LDY #$30
LDX #$08
LDA #$03
STA AUDV0
LF540: LDA $E6
BEQ LF549
ROL
ROL
TAY
BNE LF4FF
LF549: LDA $B2
CMP #$38
BEQ LF4FA
BNE LF503
LF551: LDA $C0
BEQ LF55D
LDA $8D
BEQ LF55D
LDA $91
BNE LF5C8
LF55D: LDY $A2
LDA $9A
BEQ LF582
TAX
LF564: LDA $9D
CLC
ADC LFFDB,Y
STA $9D
DEX
BNE LF564
STX $9A
AND LFFD9,Y
CMP LFFD5,Y
BNE LF582
INC $99
LDA $9D
AND.wy PLYRJOY,Y
STA $9D
LF582: LDA $99
BEQ LRTS9 ;was LF5FE, assembler hated it.
DEC $99
LDX #$02
LF58A: LDA $9C,X
CLC
ADC LFFDB,Y
STA $9C,X
AND LFFD9,Y
CMP LFFD7,Y
BNE LF5A1
LDA $9C,X
AND LFFDF,Y
STA $9C,X
LF5A1: AND LFFD9,Y
CMP LFFD5,Y
BNE LF5FE
LDA $9C,X
AND PLYRJOY,Y
STA $9C,X
INX
CPX #$04
BNE LF5B8
JSR LFD25
LF5B8: CPX #$06
BNE LF58A
DEX
LF5BD: LDA $9C,X
AND LFFE1,Y
STA $9C,X
DEX
BPL LF5BD
LRTS9: RTS

LF5C8: LDX #$0A
STX $9D
STX $9C
LDY #$04
LF5D0: INX
STX $9C,Y
DEY
BPL LF5D0
LDA $9B
BMI LF5FE
LDA $C0
BEQ LF5FE
JSR LFE09
STA $9D
LDA $9B
LDX #$01
CMP #$11
BCC LF5EC
INX
LF5EC: STX $9C
AND #$0F
STA $A0
; IF $9B & 0xF0
LDA $9B
AND #$F0
BEQ LF5FE
; then $A1 = $9B >> 4
LSR
LSR
LSR
LSR
STA $A1
LF5FE: RTS

LF5FF: LDA $A5
STA $95
LDA $B1
CMP #$60
BNE LF610
;$A5 -= $8B
LDA $A5
SEC
SBC $8B
STA $A5
LF610: LDA $A3
AND #$1F
BEQ LRTS7
LDA $E6
BEQ LRTS7
STA $A5
JSR LFD34
LDA #$6B
STA $B1
LDA #$1F ;player quad width. missile 8 pixels
STA NUSIZ0
LRTS7: RTS

LF628: LDA $D1
BEQ LRTS4
LDA $CB
CMP #$01
BEQ LRTS4
LDA #$64
STA $B0
LDA #$5D
STA $AD
LDX #$46
LDA $A3
AND #$01
BNE LF644
LDX #$4E
LF644: STX $A5
STX $A6
LDA $EC
STA $8E
LDA $E8
STA $8F
LDA #$52
STA $B1
LDA $D2
CPX #$4E
BEQ LF665
LSR
LSR
LSR
LSR
BNE LF665
LDA #$85
STA $B2
RTS
LF665: AND #$0F
TAX
LDA DIGIND,X
STA $B2
LRTS4: RTS

; if $B0 > 128
LF66E: LDA #$80
CMP $B0
BCS LF676
; then $B0 = 128
STA $B0
LF676: LDX #$04
LDA $A4
BEQ LF68A
STA $A7
; if $CA==$CE then $AF=$AB
LDY #$FF
LDA $CA
CMP $CE
BNE LF688
LDY $AF
LF688: STY $AB
;
; The divide-by-15 routine uses a simple technique:
; subtract 15 until you run out of 15s. This is
; such a time-consuming task that if the number
; strays from a certain range, the routine could
; take a variable number of scanlines, causing the
; entire picture beneath it to shift as objects
; move. The following lines of code circumvent this
; problem by subtracting the first 75 all at once.
;
LF68A: LDY #$00
LDA $A5,X ;$A5 is an array of X positions
; if A > 82 then A-=75 : Y=5
CMP #$52
BCC LF696
SBC #$4B
LDY #$05
LF696: CPX #$02 ;a CLC would have sufficed.
ADC #$02
; Do (Y++; A-=15) while A>14
LF69A: INY ; +2 }
SBC #$0F ; +2 } * up to 10, -1
BCS LF69A ; +3 }
; A = -A - 8
EOR #$FF ;
SBC #$06 ;
; Move low bits to where HMove will see it.
ASL ; Get a head-start
STA WSYNC ;BEGIN COUNT
ASL ; (0) +2
ASL ; (2) +2
ASL ; (4) +2
STA HMP0,X ; (6) +4 Fine-tuning register
; DEY-BPL to get exact distance.
LF6AB: DEY ; }(10)
BPL LF6AB ; } + Y*5 +4
STA RESP0,X ; *14+5Y* +4
DEX
BPL LF68A ;Do players 1 and 0
; Now to fine-tune the players.
STA WSYNC ;COUNT
STA HMOVE ; (0) +3
LDY #$05 ; (3) +2
LF6B9: DEY ; }(5)
BPL LF6B9 ; } +29
STA HMCLR ; *34* Clear horizontal motion
STA WSYNC
STA HMOVE ;I don't know why a seocnd hmove?
LDA $95
STA $A5
RTS

; Take this time to convert BCD score into
; 0xFF5F+(digit*8)
LF6C7: LDY #$00
LDX #$00
LF6CB: LDA $9C,X
STX $93
LDX $A2
BEQ LF6D7
LSR
LSR
LSR
LSR
LF6D7: AND #$0F
LDX $93
STA $94
ASL
ASL
ASL
SEC
SBC $94
CLC
ADC #$5F
STA.wy $00EF,Y
LDA #$FF
STA.wy $00F0,Y
INY
INY
INX
CPX #$06
BNE LF6CB
; Wait for timer to wink out
LF6F5: LDA INTIM
BNE LF6F5
STA WSYNC
; Begin vertical blank.
STA VBLANK
INC $A3
LDA $90
STA COLUP1
; X = $D2 AND 3
LDA $D2
AND #$03
TAX
LDA $EB,X
STA WSYNC
STA COLUPF
LDA #$FF
STA PF2
LDA #$01
STA PF1
LDY #$0B
LF719: LDX #$00
STA WSYNC
LDA LFEA9,Y
STA PF2
CPY $AA
BNE LF727
DEX
LF727: STX ENABL
STA WSYNC
LDX #$00
CPY $AC
BNE LF732
DEX
LF732: STX ENAM1
DEY
BPL LF719
STA WSYNC
INY
STY ENAM0
STY ENABL
DEY
STY PF0
STY PF1
STY PF2
LDY #$0F
LDX $A2
LDA $CF,X
LDX $E9
CMP #$FF
BNE LF755
LDY #$00
LDX $E8
LF755: STY $97
STX $96
LDA $8F
STA COLUP0
LDA $8E
STA COLUP1
LDA #$F0
STA CTRLPF
LDY #$84
STA WSYNC
; Clear playfield
LDA #$00
STA PF0
STA PF1
STA PF2
; Beginning of display loop.
; So that you see how tight code has to be,
; I am showing worst possible cycle counts.
LF771: LDA #$00 ;(45)+2 (Retroactive branch)
CPY $B0 ;(47)+3
BCS LF780 ;(50)+2
LDX $B2 ;(52)+3
LDA GFXDATA,X ;(55)+4
BEQ LF780 ;(59)+2
DEC $B2 ;(61)+5
LF780: CPY $AD ;(66)+3
LDX $B1 ;(70)+3 == *73* CYCLES!
STA WSYNC ;BEGIN COUNT (longest-case)
STA GRP1 ;*0* +3
BCS LF795 ;(3) +2 If no branches taken
LDA SHIPGFX,X ;(5) +4
CMP #$F0 :(9) +2
BEQ LF795 ;(11)+2
STA GRP0 ;(13)+3
DEC $B1 ;(16)+5
LF795: LDX #$01 ;(21)+2
TYA ;(23)+2
SBC $AB ;(25)+3
AND $92 ;(28)+3
BNE LF79F ;(31)+2
INX ;(33)+2
LF79F: STX ENAM0 ;*35*+3
DEY ;(37)+2
CPY $97 ;(39)+3
BNE LF771 ;(42)+3 (Must take branch to continue)
LDA $96
STA COLUPF
LDX #$00
STX GRP0 ; Clear ship graphics.
; This display loop does not include the ship, which is
; why the ship is not visible in the cityscape, which we're
; about to draw now.

; X = Y/4
LF7AE: TYA
LSR
LSR
STA WSYNC
TAX
; Copy appropriate graphics into playfield.
LDA CITYGFX,X
STA PF0
LDA CITYGFX+4,X
STA PF1
LDA CITYGFX+8,X
STA PF2
; $93 = Y/4
STX $93
LDX #$01
TYA
SBC $AB
AND $92
BNE LF7CC
INX
LF7CC: STX ENAM0
LDX $93
DEY
BPL LF7AE
STA WSYNC
LDA #$00
STY PF0
STY PF1
STY PF2
LDX #$01
STX CTRLPF
LDY $EE
STA WSYNC
STY COLUBK
STA ENAM0 ;Disable missile 0.
STA GRP1
STY COLUP1
STA PF0
STA PF1
STA PF2
STA REFP0
; Set both players to triple
LDA #$03
STA NUSIZ0
STA NUSIZ1
LDY #$06
STX VDELP0
STX VDEL01
STY $93
STY WSYNC
LF805: DEY
BPL LF805
NOP
STA RESP0
STA RESP1
LDA #$F0
STA HMP0
STA WSYNC
STA HMOVE
LDY #$FE
LDA $E8
STA COLUPF
STA WSYNC
STY PF2
LDX $EC
STX COLUP0
STX COLUP1
LF825: LDY $93 ; (61) +3
LDA ($F9),Y ; p0 p1 (64) +5
STA GRP0 ; F9 (69) +3
STA WSYNC ; Cycle count begins
LDA ($F7),Y ; (0) +5
STA GRP1 ; F7 (5) +3
LDA ($F5),Y ; (8) +5
STA GRP0 ; F5 (13) +3
LDA ($F3),Y ; (16) +5
STA $94 ; (21) +3
LDA ($F1),Y ; (24) +5
TAX ; (29) +2
LDA ($EF),Y ; (31) +5
TAY ; (36) +2
LDA $94 ; (38) +3
STA GRP1 ; F3 (41) +3
STX GRP0 ; F1 (44) +3
STY GRP1 ; EF (47) +3
STA GRP0 ; F3 (50) +3
DEC $93 ; (53) +5
BPL LF825 ; Branch taken, so (58) +3
; End six-digit loop
LDA #$00
STA VDELP0 ;Clear vertical delays
STA VDEL01
STA GRP0 ;Clear players
STA GRP1
STA HMP0 ;Clear missiles.
STA WSYNC ; Cycle count:
STA PF2 ;Clear PF2 [0]
LDA $EA ; [3]
STA COLUP0 ; [6]
STA COLUP1 ; [9]
LDY #$08 ; [12]
STA RESP0 ; *14*
LF867: DEY ;When 8 (17), when 0 (57)
BNE LF867 ;At end of loop, (59)
STA RESP1 ; (61)
; End result: player 0 is at 0, player 1 is at 120.
INY ; Y = 1
STA WSYNC
; The following code amounts to this:
; for Y= 1 to 0 step -1
; X=$C0 [Y]
; Value for repeat register is indexed from LFEDE
; If number is 0 then do not display that icon.
; next Y
LF86F: LDX #$03
LDA.wy $C0,Y
CMP #$03
BCS LF879
TAX ;X = $00C0,Y
LF879: LDA LFEDE,X
STA.wy NUSIZ0,Y ;NUSIZ registar for player Y
LDA LFEE2,X
STA.wy $96,Y ;AND mask, to be or not to be.
DEY
BPL LF86F ; Repeat for both players.
;
LDX #$0B ;X is separate so that the programmer
;can be a lazy girly-man and index from
;SHIPGFX instead of SHIPGFX+6.
LDY #$05
LF88C: STA WSYNC
LDA SHIPGFX,X ;Get ship graphics
AND $96 ; Only display if $C0 > 0
STA GRP0
LDA BOMBGFX,Y ;Get bomb graphics
AND $97 ; Only display if $C1 > 0
STA GRP1
DEX
DEY
BPL LF88C ; Go until Y runs out.
; Set timer
LDA #$26
STA TIM64T
; Reset stack pointer (is it ever used?)
LDX #$FF
TXS
JMP LF04F
LF8AB: INC $BB
LDA $BB
AND #$07
STA $BB
TAX
JSR LFCA0 ; Y = A = $80 [X] AND 0x0F
STA $93
INY
LDA $80,X
AND #$F0
BEQ LF8C2
LDY #$00
LF8C2: LDA $00E8,Y
STA $90
LDY $93
LDA $CD
BEQ LF8D6
CPX $B9
BNE LF8D6
LDA $C6
JMP LF8D9
LF8D6: JSR LFCB5
LF8D9: LSR
LSR
CLC
ADC #$2F
STA $A8
LDA $CD
BEQ LF8ED
CPX $B9
BNE LF8ED
LDA $C7
JMP LF8F0
LF8ED: JSR LFCCA
LF8F0: LSR
LSR
LSR
LSR
CLC
ADC #$02
STA $AC
LDY $A2
LDA #$FF
STA $AA
CMP.wy $00CF,Y
BEQ LF949
INC $CE
LDA $CE
CMP #$05
BCC LF910
LDA #$00
STA $CE
LF910: TAX
LDA.wy $00CF,Y
AND LFF12,X
BNE LF949
LDA $C9
CLC
ADC TIMES40,X
LSR
LSR
CLC
ADC #$30
STA $A9
LDA #$01
STA $AA
LDA $CD
BEQ LF949
LDA $CA
CMP $CE
BNE LF949
LDA $CD
BMI LF949
LDA $C7
LSR
LSR
LSR
LSR
STA $AA
LDA $C6
LSR
LSR
CLC
ADC #$30
STA $A9
LF949: LDA $A3
AND #$0F
BNE LF968
LDA #$FF
STA $CE
LDA $AD
LSR
LSR
LSR
LSR
STA $AA
LDA $A5
LSR
CLC
ADC #$58
LSR
LSR
CLC
ADC #$2F
STA $A9
LF968: RTS

LF969: LDA #$00
STA $A4
LDY $A2
LDA $00CF,Y
CMP #$FF
BEQ LF9A1
AND LFF12,X
BNE LF9A1
LDA $C9
CLC
ADC TIMES40,X
CPX $CA
BNE LF98D
LDY $CD
BPL LF98B
STA $C6
LF98B: LDA $C6
LF98D: CMP #$59
BCC LF9A1
CMP #$A8
BCS LF9A1
SEC
SBC #$58
STA $94
CLC
ADC $94
ADC #$02
STA $A4
LF9A1: RTS

LF9A2: LDY #$07
STY $94
LDA $A6
STA $95
LDA $B7
STA $96
LDA $B0
STA $97
LF9B2: INC $B7
LDA $B7
AND #$07
STA $B7
TAX
LDA $80,X
BMI LF9D3
AND #$0F
TAY
JSR LFCB5
CPX $B9
BNE LF9CB
LDA $C6
LF9CB: CMP #$59
BCC LF9D3
CMP #$A5
BCC LF9E0
LF9D3: DEC $94
BPL LF9B2
LDA #$FF
STA $B7
LDA #$00
STA $A6
RTS

LF9E0: JSR LFCCA
CPX $B9
BNE LF9E9
LDA $C7
LF9E9: STA $B0
JSR LFCB5
CPX $B9
BNE LF9F4
LDA $C6
LF9F4: SEC
SBC #$58
ASL
STA $A6
RTS

LF9FB: JSR LFDEE
LDX $B7
BMI LFA4E
STY $B8
INY
LDA $00E8,Y
STA $8E
DEY
LDA TIMES8,Y
STA $B2
LDA $CD
CMP #$03
BNE LFA24
LDA #$25
CMP $D2
BNE LFA24
CMP $AD
BCC LFA24
LDA #$AE
STA $B2
LFA24: LDA $80,X
AND #$10
BEQ LFA4E
LDA #$38
STA $B2
CPX $B9
BNE LFA34
STA $B9
LFA34: JSR LFCA0
LDA #$80
STA $80,X
CPY #$02
BNE LFA4E
LDA $E5
BNE LFA4E
INY
STY $80,X
STA $B2
LDA $80
BPL LFA4E
STY $80
LFA4E: RTS

LFA4F: LDA $91
BNE LFABF
LDA $B4
BNE LFA90
STA $93
LDA $E6
BNE LFA4E
LDA CXM0P
AND #$40 ; M0 - P0
BEQ LFA7E
LDX $92
CPX #$FC
BNE LFA7C
STA $B9
LDX $CD
CPX #$02
BNE LFA7C
INC $CD
LDX #$05
STX $99
JSR LFD2F
LDA #$00 ;Clear collision
LFA7C: STA $93
LFA7E: LDA CXPPMM
BPL LFA84 ;Branch if players don't collide
STA $93 ;Mark collision
LFA84: LDA $93
BEQ LFA90
LDA #$00
STA $B6
LDA #$1E
STA $B4
LFA90: LDY $B4
BEQ LRTS5
LDA $A3
AND #$03
BNE LFA9C
DEC $B4
LFA9C: JSR LFD34
CPY #$05
BCS LRTS5
LDA LFE89,Y
STA $B1
LDA $B4
BNE LRTS5
JSR LF1C9
LDA #$32
STA $CB
JSR LFCAC
LDX $A2
DEC $C2,X
BNE LFABF
JSR LFCEB
LFABF: LDA $C2 ;Any lives left, player 1?
BNE LRTS5
LDA $9B
CMP #$11
BCC LFAD9
LDA $C3 ;Any lives left, player 2?
BNE LRTS5
LDA $A3
AND #$1F
BNE LFAD9
LDA $A2
EOR #$01
STA $A2
LFAD9: JSR LFD17
LRTS5: RTS

;
; Notice that LDA is used here in order to test bit 7.
; Combat, which predates Defender by four years, uses
; BIT.
;
; This routine is only executed when $91 equals zero.
;
LFADD: LDX $96
LDA $B2
CMP #$38
BEQ LFB36
LDA $80,X
BMI LFAED
LDA CXPPMM ;Collision between players & missiles
BMI LFB02
LFAED: LDA $E5
BEQ LFB36
LDA $99
BNE LFB36
LDX $B7
BPL LFB02
LDX $A2
DEC $C4,X
LDA #$00
STA $E5
RTS

LFB02: JSR LFCA0
LDA #$00
STA $E6
LDA LFEFA,Y
STA $9A
LDA LFF00,Y
STA $99
; Set bit 4 across $80 field.
LDA $80,X
ORA #$10
STA $80,X
CPX $B9
BNE LFB36
LDA $CD
BPL LFB28
LDA $AF
BPL LFB28
INC $CD
RTS

LFB28: LDA #$02
STA $CD
STA $BA
LDA #$32
CMP $C7
BCS LFB36
STA $BA
LFB36: RTS

;
; This routine is called only when $91 equals zero.
;
LFB37: LDA $B4
BNE LFB8F
LDX $A2
LDA INPT4,X
BPL LFB4A
LDA #$00
STA $88
LFB45: LDA $E6
BNE LFB69
RTS

LFB4A: LDA $88
BNE LFB45
JSR LFD1C
DEC $88
LDA #$14
CMP $AD
BCS LFB83
LDA $A5
SEC
SBC #$02
STA $E6
LDX $8B
BPL LFB69
SEC
SBC #$17
STA $E6
LFB69: LDX #$07
LDA $8B
BPL LFB71
LDX #$F9
LFB71: TXA
CLC
ADC $E6
TAX
CMP #$82
BCS LFB7E
CMP #$00
BCS LFB80
LFB7E: LDX #$00
LFB80: STX $E6
RTS

; if ($C1 != 0 && $B7 != 0xFF)
; $E5 = 0xFF
;
LFB83: LDA $C1
BEQ LFB8F
LDA #$FF
CMP $B7
BEQ LFB8F
STA $E5
LFB8F: RTS

LFB90: LDX #$A0
LDY #$00
LDA #$FC
STA $92
LDA $A4
BNE LFC07
LDA $BF
BNE LFBD2
LDA $B7
BMI LFC07
LDY $B8
CPY #$02
BEQ LFC07
LDX #$01
LDA $B0
STA $BC
CMP $AD
BCC LFBB6
LDX #$FF
LFBB6: STX $BE
LDA $8A
BNE LFBBE
INC $8A ;$8A = 1
LFBBE: LDA $A6
STA $BD
CMP $A5
BCC LFBC8
LDX #$FE
LFBC8: STX $BF
CPY #$00
BNE LFBD2
LDA #$F0
STA $BF
; Branched here if $BF is zero.
LFBD2: LDX $BC
LDY $BD
CMP #$F0
BNE LFBE2
LDA $A3
AND #$5A
BEQ LFBFD
BNE LFC03
LFBE2: TXA
CLC
ADC $BE
STA $BC
CMP #$0F
BCC LFBFD
TAX
LDA $BD
CLC
ADC $BF
STA $BD
TAY
LDA $B4
BNE LFBFD
CPY #$A0
BCC LFC03
LFBFD: LDY #$00
STY $BF
LDX #$A0
LFC03: LDA #$FE
STA $92
;Branched here if $A4 or $B7 is zero.
LFC07: STX $AB
STY $A7
RTS

; for X=7 to 0 step -1
LFC0C: LDX #$07
LFC0E: LDA $80,X
BPL LFC42
DEX
BPL LFC0E
LDA #$00
TAY
CLC
ADC $9A
ADC $99
ADC $B4
BNE LFC42
LDX $A2
LDA $CF,X
LDX #$07
LFC27: ROR
BCS LFC2B
INY
LFC2B: DEX
BPL LFC27
STY $99
LDA #$96
STA $CB
STA $D1
STA $B9
LDA $E5
BEQ LFC42
LDX $A2
DEC $C4,X
INC $E5
LFC42: RTS

LFC43: LDA $D3
BEQ LFC7A
LDA $98
AND #$07
TAX
CPX $B9
BEQ LFC7A
LDA $80,X
BPL LFC7A
LDY $A2
LDA $00CF,Y
CMP #$FF
BNE LFC61
LDY #$05
BNE LFC6B
LFC61: LDY #$04
LDA $A3
AND #$1F
BNE LFC6B
LDY #$01
LFC6B: JSR LFCB5
CMP #$59
BCC LFC76
CMP #$A8
BCC LFC7A
LFC76: STY $80,X
DEC $D3
LFC7A: RTS

LFC7B: JSR LFCA6
LDA LFEC4,X
STA $93
CPX #$00
BEQ LFC9F
LDX $D2
STY $97
LDY #$01
CPX #$07
BCC LFC92
DEY
LFC92: DEX
BEQ LFC9D
CPY $93
BEQ LFC9D
LSR $93
BNE LFC92
LFC9D: LDY $97
LFC9F: RTS

LFCA0: LDA $80,X
AND #$0F
TAY
RTS

LFCA6: LDA $9B
AND #$0F
TAX
RTS

;
; Clear $AA, $AB, and $AC with #$A0.
;
LFCAC: LDA #$A0
STA $AB
STA $AC
STA $AA
RTS

;
; IF LFF06[Y] = 3
;
LFCB5: STX $93
; X= LFF06 [Y]
LDX LFF06,Y
LDA #$00
;if X <> 3
CPX #$03
BEQ LFCC2
; then A=$E0 [X]
LDA $E0,X
; A += $C8 + $93 * 40
LFCC2: LDX $93
ADC $C8
ADC TIMES40,X
RTS

;
;If LFF0C[Y] = 3
; A = ( $AE + $93 *40 ) / 2 + 24
;Else
; A = ( $E3[X] + $AE + $93 *40 ) / 2 + 24
;
LFCCA: STX $93
LDX LFF0C,Y
LDA #$00
CPX #$03
BEQ LFCD7
LDA $E3,X
LFCD7: LDX $93
ADC $AE
ADC TIMES40,X
LSR
CLC
ADC #$18
LFCE2: RTS

LFCE3: LDA $D1
BEQ LFCE2
LDA #$00
STA $D1
LFCEB: LDA $9B
CMP #$11
BCC LFD09
LDA $A2
EOR #$01
TAX
LDA $C2,X
BEQ LFD09
STX $A2
CPX #$01
BNE LFD09
; Decrease $D2 by one, in decimal.
SED
LDA $D2
SEC
SBC #$01
STA $D2
CLD
LFD09: JMP LFD73

; Executed when $D2 is a multiple of 5
; (in decimal mode). $9B is initialized
; to $FF on power-up.
LFD0C: JSR LFCA6 ; X = $9B AND 0x0F
LDA LFECD,X
STA $CF
STA $D0
RTS

LFD17: LDA #$FF
STA $91
RTS

; Result of this is stored in $A2.
LFD1C: LDA #$78
STA $8D
LDA #$00
STA $91
RTS

; I have a hunch that this is the "extra man"
; routine. If so, $C4 and $C2 represent a
; player's extra lives and smart bomb supply.
LFD25: STX $93
LDX $A2
INC $C4,X
INC $C2,X
LDX $93
LFD2F: LDA #$1E ;Called when $BA != 0x32
STA $8A
RTS

; $8F = $E8 [$BB]
LFD34: LDX $BB
LDA $E8,X
STA $8F
RTS

; This is the first subroutine ever called in this
; program.
;
; The fact that certain locations are cleared
; here implies that this is called more than once.
LFD3B: LDA #$00
STA $A2
STA $D1
STA $CD
STA $A4
STA $9A
STA $99
STA $B4
STA REFP0 ;Aim ship to the right.
LDA #$96
STA $CB
JSR LFDE3
JSR LFE09
JSR LFD0C
LDA LFEE8,X
STA $D2
; Copy in city graphics.
; I don't know why it uses four-byte chunks.
; It could just copy all twelve bytes in a row.
LDX #$03
LFD61: LDA ROMCITY,X
STA CITYGFX,X
LDA ROMCITY+4,X
STA CITYGFX+4,X
LDA ROMCITY+8,X
STA CITYGFX+8,X
DEX
BPL LFD61
LFD73: LDA $98 ;This byte is set to $FF on powerup.
STA $C9
STA $E3
STA $E4
STA $AE
; This seems to set up indexes.
LDA #$00
STA $B6
STA $BD
STA $BF
STA $CD
LDA #$FE
STA $92
STA $B9
STA $CA
LDA $D2
AND #$0F
BEQ LFD99
CMP #$05
BNE LFD9C
LFD99: JSR LFD0C ;Executed if low nybble
;is 0 or 5.
; Add 1 in decimal.
LFD9C: SED
LDA $D2
CLC
ADC #$01
STA $D2
CLD
LDA #$0F
STA $D3
LDX #$07
LFDAB: LDA LFF18,X
STA $80,X
LDY $A2 ;Y is initialized during
;each iteration, needlessly.
;I expected better.
LDA $00CF,Y
CMP #$FF
BNE LFDBD
LDA #$05
STA $80,X
LFDBD: DEX
BPL LFDAB
JSR LFDEE
LFDC3: LDA #$32
STA $AD
LDA #$1E
STA $A5
JSR LFCAC ;Load $AA, $AB, and $AC with 0xA0
STA $C8
STA $AB
LDA #$05
STA $8B
LDA #$00 ;Again with LDA #$00....
;I'm sure they could have
;consolidated this into an earlier
;section of code.
STA $E0
STA $E1
STA $E2
STA $CC
STA $E6
RTS

;Clear $C2 through $C5 with $03
LFDE3: LDA #$03
STA $C2
STA $C3
STA $C4
STA $C5
RTS

; Some relatively simple function whose purpose
; I shall discover shortly.

;if $B4 == 0
;and ( $A3 & 7 ) == 0
;then $B1 = 0x60; $8F = $EE
;else $B1 = 0x10; $8F = $EA
LFDEE: LDA #$10
STA $B1
LDA $EA
STA $8F
LDA $B4
BNE LFE08
LDA $A3
AND #$07
BNE LFE08
LDA #$60
STA $B1
LDA $EE
STA $8F
LFE08: RTS

; I presume this routine hides the score
; display in order to select a level.
;
; $9C = $9D = 0
; $9E = $9F = $A0 = $A1 = #$AA
LFE09: LDA #$00
STA $9C
STA $9D
LDA #$AA
STA $9E
STA $9F
STA $A0
STA $A1
RTS

LFFEF: .byte $00,$88,$8F,$FF,$1D,$1A,$37,$00
.byte $00,$00,$00,$00,$00
.word START
.byte $00,$F0 ;Interrupt address
; Disassembly of defender.bin

Defender Disassembly