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snakes.asm
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snakes.asm
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//
// Copyright 1982-1984,2019 by T.Zoerner (tomzo at users.sf.net)
// All rights reserved.
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are met:
//
// 1. Redistributions of source code must retain the above copyright notice, this
// list of conditions and the following disclaimer.
// 2. Redistributions in binary form must reproduce the above copyright notice,
// this list of conditions and the following disclaimer in the documentation
// and/or other materials provided with the distribution.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
// ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
// WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
// DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR
// ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
// (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
// LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
// ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
// SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
//
// P00 header
_start_data = $123d
.word _start_data
.text
* = _start_data
// variables in ZP:
// #$00: skill/level
// #$01-$02: score digits: lowest 99, middle 99
// #$03-$04: player address
// #$05: score digits > 9999 (i.e. highest 99)
// #$06: 0:player not ready
// #$07: 0:player alive
// #$08-$09: snake head address
// #$0a: snake direction (short)
// #$0b: unused
// #$0348-$034a: unknown
// snake data:
// #$0380: pos. while being eaten
// #$0381: unused?
//
// struct size #$25
// #$0382-$0383: pre-head
// #$0384-$0385: head
// #$0386...: middle part
// #$03a4-$03a5: tail
// #$03a6: direction (up:$20,left:$60,down:$a0,right:$e0)
//
// #$03a7-$03a8: pre-head
// #$03a9-$03aa: head
// #$03ab...: middle part
// #$03c9-$03ca: tail
// #$03cb: direction
// ----------------------------------------------------------------------------
// // Display text including color codes
l123d .byt 22,67,12 // Highscore value (digits%100): 126722
.byt $00
l1241 .byt $28,$00 // "SKILL:"
.byt $38,$00
.byt $24,$00
.byt $25,$00
.byt $25,$00
.byt $1b,$00
.byt $20,$01 // white
.byt $20,$01
.byt $20,$01
l1252 .byt $28,$00 // "SCORE:" (black)
.byt $1f,$00
.byt $26,$00
.byt $27,$00
.byt $21,$00
.byt $1b,$00
.byt $2b,$01 // white
.byt $2b,$01
.byt $2b,$01
.byt $2b,$01
.byt $2b,$01
.byt $2b,$01
.byt $20,$01
l126c .byt $23,$02 // "HIGHSCORE:" (red)
.byt $24,$02
.byt $22,$02
.byt $23,$02
.byt $28,$02
.byt $1f,$02
.byt $26,$02
.byt $27,$02
.byt $21,$02
.byt $1b,$02
.byt $20,$02
.byt $20,$02
.byt $20,$02
.byt $20,$02
.byt $20,$02
.byt $20,$02
.byt $20,$02
.byt $20,$06 // blue
.byt $20,$06
.byt $20,$06
l1295 .byt $07,$12,$05,$01,$14 // "Great"
l129a .byt $13,$0d,$01,$0c,$0c,$20,$02,$0f,$0e,$15,$13, // "Small Bonus: 2000 PTS"
.byt $3a,$20,$32,$30,$30,$30,$20,$10,$14,$13
l12af .byt $02,$19,$20,$5a,$0f,$05,$12,$0e,$05,$12 // "by Zoerner"
l12b9 .byt $53,$03,$08,$0c,$01,$0e,$07,$05,$0e // "Schlangen"
l12c2 .byt $39,$3a,$3b,$00,$39,$3c,$3a,$3d,$3e,$3f // {GUT} {GLUECK}
l12cc .byt $22,$37,$29,$20,$22,$25,$37,$21,$1f,$38,$2a // "GUT GLUECK!"
l12d7 .byt $39,$3a,$3b,$39,$3c,$3a,$3d,$3e,$3f // {GUTGLUECK}: bonus letters on playing field
// .text
// * = $12e0
// ----------------------------------------------------------------------------
// // Sub-function: Increase & print score
// // Parameter: X = points to be added
l12e0 inc $01 // increment first/lowest decimal
lda $01
cmp #$64 // first==100?
bne l12f6
inc $02 // increment second decimal
sty $01 // zero first decimal
lda $02
cmp #$64 // second==100?
bne l12f6
inc $05 // increment third decimal
sty $02 // zero second decimal
l12f6 dex // loop for number of points to be added
bne l12e0
l12f9 lda $01 // !! Alternate sub-function entry point
sta $24
lda $02
sta $23
lda $05
sta $22
// // fall-through
// ----------------------------------------------------------------------------
// // Sub-function: Print highscore
// // Parameter: $22-$24: MSB-LSB of score
l1305 ldx #$00
l1307 lda #$0a
sta $25
lda #$00 // init counter for multiples of 10
sta $4b
l130f lda $22,x // load next digit
cmp $25 // >= n*10?
bcc l1320
clc
lda #$0a // increment n by 10
adc $25
sta $25
inc $4b
bne l130f
l1320 lda $4b
adc #$2b // user-defined char '0'
sta $4b
l1327 = * + 1 // instruction operand modified by code
l1328 = * + 2
sta $100f,y // draw higher digit
lda $22,x
adc #$36
sbc $25
iny
l1331 = * + 1 // instruction operand modified by code
l1332 = * + 2
sta $100f,y // draw lower digit
inx
cpx #$03 // loop for next 2 digits, max. 2*3
iny
bcc l1307
ldy #$00
rts
// ----------------------------------------------------------------------------
// // Sub-function: Check player movement
//
l133c lda ($03),y // read char at player address on screen
cmp #$1a // hit "excrement"?
bne l1356
lda $00 // any "skill" left?
beq l1363 // no -> disallow
lda $028e // read prev. status of SHIFT key
bne l1352
lda $911f
and #20
bne l1363 // no SHIFT or joystick-fire -> disallow
l1352 dec $00 // reduce "skill" -1
sec // allow movement
rts
l1356 cmp #$06 // hit snake head?
bcc l135e
cmp #$12 // hit snake tail?
bcc l1363
l135e jmp l13cb // check for other special chars
sec // result: allow (never reached!?)
rts
l1363 lda #$aa // result = do not allow
sta $25
clc
rts
// ----------------------------------------------------------------------------
// // Sub-function: Initialize snake
// // Parameters: A = Direction
// // X = LSB snake address on screen
// // Y = MSB snake address on screen
l1369 sta $0348
stx $0349
lda l1efc
cmp #$05
bcs l13b4 // abort if max. number of snakes already reached
nop
inc l1efc // increment number of snakes
ldx #$00 // search for unused snake status struct
l137c lda $03a6,x
beq l1389 // stop if struct is free (invalid direction value)
txa
clc
adc #$25 // advance address to next struct
tax
clc
bcc l137c
l1389 stx $034a // temp. save struct start offset
txa
clc
adc #$22 // add offset to snake's tail address
tax
l1391 lda $0349 // write given snake address
sta $0382,x
tya
sta $0383,x
dex // advance offset to next snake element in struct
dex
cpx $034a // iterate across all snake elements, up to struct start
bne l1391
lda $0348 // write given direction
sta $03a6,x
txa
clc
adc #$0b
cmp l1efa // new max?
bcc l13b4
sta l1efa
l13b4 ldy #$00
rts
// ----------------------------------------------------------------------------
// // Sub-function: Clear player figure
l13b7 lda $03
sta $22
lda $04
clc
adc #$84
sta $23
lda #$07 // color
sta ($22),y
lda #$1a // place excrement
sta ($03),y
rts
// ----------------------------------------------------------------------------
// // Sub-function: eat target (or don't)
// // Parameter: A = char at player figure
// // Returns status.C: 0:nOK, 1:OK (allow movement)
//
l13cb cmp #$1c // is border?
beq l13d3
cmp #$1d
bne l13d6
l13d3 jmp l1363 // return to level control: do not allow movement
l13d6 cmp #$39 // letter characters?
bcs l13dc
sec // return: allow
rts
l13dc lda $06 // player ready?
beq l13d3 // not ready -> do not allow eating letters
ldx l1efe // get number of eaten bonus chars as index
lda l12c2,x
cmp ($03),y // is next expected bonus char?
bne l13d3 // no -> do not allow
sta $11ea,x // mark letter as done in status bar
inc l1efe // increase number of consumed letters
cpx #$02 // skip blank in bonus letter sequence (i.e. after "GUT")
bne l13f7
inc l1efe
l13f7 inc $00 // increase skill +1
ldx #$fa
jsr l12e0 // increase score by 250
sec // return: allow
rts
// ----------------------------------------------------------------------------
// Character generator data
// - 64 user-defined characters, 8x8 bit each
// - mapped to start at $1400 (register $9005)
// // Parameter: A = direction: up:$20,left:$60,down:$a0,right:$e0
l1400 .byt $00,$58,$58,$5c,$5e,$4a,$7a,$7e // #$00: snake head: upwards & closed
.byt $00,$46,$46,$4e,$4e,$5a,$5a,$7e // upwards & open
.byt $00,$0f,$19,$7f,$7b,$03,$7f,$00 // left & closed
.byt $00,$7f,$79,$1f,$07,$01,$7f,$00 // left & open
.byt $7e,$5e,$52,$7a,$3a,$1a,$1a,$00 // down & closed
.byt $7e,$5a,$5a,$72,$72,$62,$62,$00 // down & open
.byt $00,$f0,$98,$fe,$de,$c0,$fe,$00 // right & closed
.byt $00,$fe,$9e,$f8,$e0,$80,$fe,$00 // right & open
.byt $00,$00,$ff,$ff,$ff,$ff,$00,$00 // #$08: snake horizontal
.byt $3c,$3c,$3c,$3c,$3c,$3c,$3c,$3c // #$09: snake vertical
.byt $3c,$7c,$f8,$f8,$f0,$f0,$00,$00 // #$0a: snake corners
.byt $00,$00,$c0,$f0,$f8,$f8,$7c,$3c
.byt $00,$00,$03,$0f,$1f,$1f,$3e,$3c
.byt $3c,$3e,$1f,$1f,$0f,$03,$00,$00
.byt $3c,$5e,$ef,$f7,$e7,$ff,$7e,$3c // #$0e: snake 180° turn
.byt $3c,$7e,$ff,$e7,$d7,$bf,$7e,$3c
.byt $3c,$7e,$ff,$e7,$ef,$f7,$fa,$3c
.byt $3c,$7e,$fd,$eb,$e7,$ff,$7e,$3c
.byt $00,$00,$f8,$fe,$fe,$f8,$00,$00 // #$12: snake tail
.byt $00,$00,$1f,$7f,$7f,$1f,$00,$00
.byt $00,$18,$18,$3c,$3c,$3c,$3c,$3c
.byt $3c,$3c,$3c,$3c,$3c,$18,$18,$00 // end at bottom
.byt $00,$3c,$42,$5a,$5a,$42,$3c,$00 // #$16: target
.byt $7e,$99,$99,$ff,$bd,$c3,$ff,$7e // #$17: player closed
.byt $7e,$99,$ff,$c3,$81,$c3,$e7,$7e // #$18: player open
.byt $00,$00,$18,$24,$24,$18,$00,$00 // #$19: food (score points)
.byt $00,$00,$00,$00,$08,$00,$00,$00 // #$1a: excrement
.byt $00,$00,$08,$00,$00,$08,$00,$00 // #$1b: ':'
.byt $00,$00,$00,$ff,$ab,$d5,$ab,$ff // #$1c: border horizontal
.byt $ff,$ab,$d5,$ab,$d5,$ab,$d5,$ff // #$1d: border vertical
.byt $18,$24,$42,$7e,$42,$42,$42,$00 // #$1e: 'A'
.byt $1c,$22,$40,$40,$40,$22,$1c,$00 // #$1f: 'C'
.byt $00,$00,$00,$00,$00,$00,$00,$00 // #$20: blank
.byt $7e,$40,$40,$78,$40,$40,$7e,$00 // #$21: 'E'
.byt $1c,$22,$40,$4e,$42,$22,$1c,$00 // #$22: 'G'
.byt $42,$42,$42,$7e,$42,$42,$42,$00 // #$23: 'H'
.byt $1c,$08,$08,$08,$08,$08,$1c,$00 // #$25: 'I'
.byt $40,$40,$40,$40,$40,$40,$7e,$00 // #$26: 'L'
.byt $18,$24,$42,$42,$42,$24,$18,$00 // #$27: 'O'
.byt $7c,$42,$42,$7c,$48,$44,$42,$00 // #$28: 'R'
.byt $3c,$42,$40,$3c,$02,$42,$3c,$00 // #$29: 'S'
.byt $3e,$08,$08,$08,$08,$08,$08,$00 // #$2a: 'T'
.byt $12,$12,$12,$12,$00,$00,$12,$00 // #$2a: fat exclamation mark '!'
.byt $3c,$42,$46,$5a,$62,$42,$3c,$00 // #$2b..$34: digits '0'..'9'
.byt $08,$18,$28,$08,$08,$08,$3e,$00
.byt $3c,$42,$02,$0c,$30,$40,$7e,$00
.byt $3c,$42,$02,$1c,$02,$42,$3c,$00
.byt $04,$0c,$14,$24,$7e,$04,$04,$00
.byt $7e,$40,$78,$04,$02,$44,$38,$00
.byt $1c,$20,$40,$7c,$42,$42,$3c,$00
.byt $7e,$42,$04,$08,$10,$10,$10,$00
.byt $3c,$42,$42,$3c,$42,$42,$3c,$00
.byt $3c,$42,$42,$3e,$02,$04,$38,$00
.byt $70,$a8,$f8,$8e,$7b,$1f,$10,$1f // #$35: symbol: grant level + snake
.byt $06,$0e,$08,$3c,$7e,$7e,$7e,$3c // #$36: apple: score bonus
.byt $42,$42,$42,$42,$42,$42,$3c,$00 // #$37: 'U'
.byt $42,$44,$48,$70,$48,$44,$42,$00 // #$38: 'K'
.byt $7e,$81,$bd,$a1,$ad,$a5,$bd,$7e // #$39: {G}
.byt $7e,$81,$a5,$a5,$a5,$a5,$bd,$7e // #$3a: {U}
.byt $7e,$81,$b9,$91,$91,$91,$91,$7e // #$3b: {T}
.byt $7e,$81,$a1,$a1,$a1,$a1,$bd,$7e // #$3c: {L}
.byt $7e,$81,$bd,$a1,$b9,$a1,$bd,$7e // #$3d: {E}
.byt $7e,$81,$bd,$a1,$a1,$a1,$bd,$7e // #$3e: {C}
.byt $7e,$81,$a5,$a9,$b1,$a9,$a5,$7e // #$3f: {K}
// ----------------------------------------------------------------------------
// // Game entry point:
// // start graphics
l1600 lda #$00
sta $900e
lda #$19 // set screen color: screen:white border:white
sta $900f
lda #$c2 // reset screen & character map addresses
sta $9005
jsr $e55f // clear screen
ldx #$09 // print game title on screen
l1614 lda l12b9-1,x
sta $10f7,x
lda #$02 // color: red
sta $94f7,x
dex
bne l1614
l1622 lda $cb // wait for no key pressed
cmp #$40
bne l1622
lda #$ff // disable SHIFT/Commodore case switching
sta $0291
l162d lda $cb // wait for keypress or joystick
cmp #$40
bne l163a
lda $911f
#if 0
// FIXME does not work in VICE
// cmp #$7e
// beq l162d
#else
nop
clc
bcc l162d
#endif
l163a lda $00 // read level to check if this is first run of game
cmp #$4c // no -> skip author display
beq l1643
jmp l1666
l1643 jsr $e55f // clear screen again
ldx #$0a
l1648 lda l12af-1,x // print author's name on screen
sta $10f7,x
lda #$06 // color: blue
sta $94f7,x
dex
bne l1648
lda #$04 // time delay apx. 2 seconds
l1658 ldy #$fe
l165a ldx #$ff
l165c dex
bne l165c
dey
bne l165a
sbc #$01
bne l1658
// // ---- cold start of game ----
l1666 lda #$02
sta l1efd // init number of lives := 2
lda #$00
sta $fb // clear warm boot flag
// fall-through
l166f lda #$1f // ---- warm-boot / restart of game ----
sta l1327 // patch screen output address for score display sub-function
sta l1331
jsr $e55f // clear screen
lda #$ac // set screen color: screen:pink border:purple
sta $900f
lda #$cd // configure screen & character map addresses
sta $9005 // screen:$1000, color:$9400, charmap:$1400
lda #$42 // loop to draw "food" on screen $1000...
sta $00
lda #$10
sta $01
ldx #$02
l168e ldy #$c5
l1690 lda #$19 // "food" char code
sta ($00),y
lda $00 // calculate color code addr: add $9400-$1000 to address
sta $22
lda $01
clc
adc #$84
sta $23
lda #$07 // color: yellow
sta ($22),y
dey
bne l1690
lda #$07
sta $00
inc $01
dex
bne l168e
ldx #$16 // draw horizontal borders at top & bottom
l16b1 lda #$1c
sta $102b,x
lda #$1d
sta $11cd,x
lda #$00
sta $942b,x
sta $95cd,x
dex
bne l16b1
lda #$42 // draw vertical border at left and right side
sta $00
sta $02
lda #$10
sta $01
sta $05
lda #$57
sta $04
sta $06
lda #$94
sta $03
sta $07
lda #$12 // counter for outer loop = border height
sta $08
l16e2 ldy #$00
lda #$1d // border char code
sta ($00),y
sta ($04),y
lda #$00 // color: black
sta ($02),y
sta ($06),y
ldx #$16 // inner loop: add 22 to all pointers
l16f2 inc $00
inc $02
bne l16fc
inc $01
inc $03
l16fc inc $04
inc $06
bne l1706
inc $05
inc $07
l1706 dex
bne l16f2
dec $08
bne l16e2
ldx #$2b // display highscore
ldy #$57
l1711 lda l123d,y
sta $93fe,x
dey
lda l123d,y
sta $0ffe,x
dey
dex
bne l1711
lda l123d
sta $24
lda l123d+1
sta $23
lda l123d+2
sta $22
jsr l1305 // print highscore
lda #$16 // display "target" char in middle of screen: X/Y=10/12
sta $1112
lda #$0f
sta l1327
sta l1331
ldy #$00 // initialize player status: not ready, alive
sty $06
sty $07
lda #$cc // calc initial player address
sta $03
lda #$11
sta $04
lda #$17 // display player char
sta ($03),y
lda #$03 // initialize skill = 3
sta $00
sty l1efc // initialize snake counter = 0
lda #$01
sta $9512
ldx #$00 // clear all snakes: invalid direction value
l1761 lda #$00
sta $03a6,x
txa
clc
adc #$25
tax
cmp #$d0 // loop for 5 snakes
bcc l1761
sty l1efa // init snake max to 0
nop
lda #$a0 // spawn first snake: dir=down, X/Y=5/8
ldx #$b5
ldy #$10
jsr l1369
lda #$20 // spawn second snake: dir:up, X/Y=16/15
ldx #$6f
ldy #$11
jsr l1369
lda l1efd // display number of lives in bottom-left corner
beq l1793
tax
lda #$17 // char for player, drawn once for each life
l178d sta $11e3,x
dex
bne l178d
l1793 lda $fb // warm boot -> init or restore score respectively
bne l17a4
lda #$26 // delay factor
sta $16
sty $01 // init score to 0
sty $02
sty $05
clc
bcc l17b0
l17a4 lda $fc // restore score
sta $01
lda $fd
sta $02
lda $fe
sta $05
l17b0 lda #$09 // copy letters from ROM into RAM for user-defined chars
sta $fc
l17b4 jsr $e094 // get RAND number
lda $8d
and #$01 // calculate addr on screen (LSB random byte; MSB random either $10 or $11)
clc
adc #$10
sta $8d // FIXME overwrites rand number storage with address
lda ($8c),y // read this screen position
cmp #$19 // food char?
bne l17b4 // no -> try another random position
ldx $fc
lda l12d7-1,x // get next bonus letter from sequence
sta ($8c),y // draw letter char
lda $8d
clc
adc #$84 // calc color address
sta $8d
lda #$02 // color: red
sta ($8c),y
dec $fc // next character in iteration
bne l17b4
ldx #$0b
l17de lda l12cc-1,x // print "good luck" message
sta $11e9,x
dex
bne l17de
sty l1efe // initialize count of bonus letters on screen = 0
//-----------------------------------------------------------------
// // Player control
l17ea lda #$ff
sta $9122
lda $cb // poll for keypress
cmp #$09 // key 'w'?
beq l17fc
lda $911f // poll joystick
and #$04
bne l181c
l17fc sty $25 // handle key 'w': up
jsr l13b7
l1801 ldx #$16 // calc new player addr: -= 22 (via loop)
l1803 dec $03
lda $03
cmp #$ff
bne l180d
dec $04
l180d dex
bne l1803
lda $25
bne l1819
jsr l133c
bcc l182e
l1819 jmp l1893
l181c lda $cb
cmp #$21 // key 'z'?
beq l1829
cmp #$0b // key 'y': equiv. 'Z' for German keyboard
beq l1829
l1822 lda $911f
and #$08
bne l1845
l1829 sty $25 // handle 'z': down
jsr l13b7
l182e ldx #$16
l1830 inc $03
bne l1836
inc $04
l1836 dex
bne l1830
lda $25
bne l1842
jsr l133c
bcc l1801
l1842 jmp l1893
l1845 lda $cb
cmp #$11 // key 'a'?
beq l1852
lda $911f
and #$10
bne l186d
l1852 sty $25 // handle 'a': left
jsr l13b7
l1857 dec $03
lda $03
cmp #$ff
bne l1861
dec $04
l1861 lda $25
bne l186a
jsr l133c
bcc l1884
l186a jmp l1893
l186d lda $cb
cmp #$29 // key 's'?
beq l187f
lda #$7f
sta $9122
lda $9120
and #$80
bne l1893
l187f sty $25 // handle 's': right
jsr l13b7
l1884 inc $03
bne l188a
inc $04
l188a lda $25
bne l1893
jsr l133c
bcc l1857
//-----------------------------------------------------------------
// // Checking / level display
l1893 lda #$0a // calculate skill/10 and skill%10
sta $25
sty $4b
l1899 lda $00
cmp $25
bcc l18aa
clc
lda $25
adc #$0a
sta $25
inc $4b
bne l1899
l18aa lda $4b
adc #$2b // add user-defined char '0'
sta $1006 // print skill/10 digit
lda $00
adc #$36
sbc $25
sta $1007 // print skill%10 digit
lda $06 // player ready?
beq l18c9 // no -> do not score for eating food
l18be lda ($03),y // read char under player figure
cmp #$19 // "food" char?
bne l18c9
ldx #$0a // score +10
jsr l12e0
l18c9 lda $06 // player "ready"?
bne l1904 // yes -> skip obsolete check for home position
lda $03 // player address equal home address?
cmp #$12
bne l1904
lda $04
cmp #$11
bne l1904
lda #$01 // yes -> mark player as ready
sta $06
lda #$21 // print "EAT" in upper right corner
sta $1027
lda #$1e
sta $1028
lda #$29
sta $1029
lda $00
beq l1900 // skill zero? -> to pre-reaction
tax // start loop to add score by skill * 100 points
l18f1 inc $02 // increase score by 100 (loop)
lda $02
cmp #$64 // digit pair >= 100?
bcc l18fd
l18f9 sty $02 // increase third digit pair
inc $05 // zero second digit pair
l18fd dex
bne l18f1
l1900 lda #$04 // initialize skill to 4 ("level II")
sta $00
l1904 sty $4c
lda ($03),y // read char under player
cmp #$08 // snake head?
bcs l1915
l190c lda #$f0 // kill player
sta $07
lda #$d8 // sound effect
sta $900c
l1915 cmp #$12 // hit snake tail? ($12-$15)
bcc l1965
cmp #$16
bcs l1965
lda $06 // yes; player ready?
beq l195d
lda $028e // yes; SHIFT key or joystick FIRE pressed?
bne l192d
lda $911f
and #$20
bne l195d // neither -> skip
l192d lda l1efa // search snake array for one with tail at player address
clc
adc #$17
tax
l1934 lda $0382,x
cmp $03
bne l1942
lda $0383,x
cmp $04
beq l1949
l1942 txa
clc
sbc #$24
tax
bpl l1934
l1949 stx $0380 // found snake index
txa
sec
sbc #$22
sta l1ef9
lda #$60
sta $07
dec l1efc
tya
beq l196b
l195d ldx #$07 // score +7
jsr l12e0
clc
bcc l196b // always true
l1965 cmp #$19 // found food?
bne l196b
sta $4c
l196b lda l1efb // time score
asl
asl
asl
asl
bne l197e // only once per 16 iterations
lda l1efc // any snakes alive?
asl
tax
beq l197e // no -> no score
jsr l12e0 // yes -> score +2 for each living snake
l197e lda $06 // player ready?
beq l19a1 // no -> ignore eating apple or snake symbols
lda ($03),y
cmp #$36 // yes; found apple?
bne l1990
ldx #$fa // yes: grant 250 points
jsr l12e0
clc
bcc l199d // increase skill +2
l1990 cmp #$35 // found snake grant symbol?
bne l19a1
lda #$a0
ldx #$12
ldy #$11
jsr l1369 // spawn new snake
l199d inc $00 // increase skill +2
inc $00
l19a1 lda $cb // poll keyboard
cmp #$07 // key INSERT? (added for emulator)
beq l19a2
cmp #$06 // pound key? (skill demand)
bne l19d0
l19a2 lda $00
cmp #$02 // skill >= 2?
bcs l19d0 // yes -> disallow
lda $02 // middle digit pair of score
cmp #$0a // >= 10?
bcs l19c2 // yes (i.e. score < 1000) -> OK
lda $05 // highest digit pair
cmp #$01 // >= 1? (i.e. score >= 10000)
bcc l19d0 // no -> disallow
dec $05 // yes; decrement and (temporary) add 100 to lower digit pair
lda $02
clc
adc #$64
sta $02
l19c2 lda $02
sec // score -1000 points: subtract 10 from middle digit pair
sbc #$0a
sta $02
inc $00 // increase skill +2
inc $00
jsr l12f9 // display score
l19d0 lda $cb // poll keyboard
cmp #$08 // left arrow key?
beq l19d9
cmp #$27 // key F1? (added for emulator)
beq l19d9
l19d6 jmp l1a80
l19d9 lda l1efe
cmp #$0a // all 10 bonus chars picked-up?
bne l19d6 // no -> disallow
lda l1efc // any snakes left alive?
bne l19e9
lda #$0b // no -> great bonus: 2750
bne l19eb
l19e9 lda #$07 // yes -> small bonus: 1750
l19eb sta $fa
l19ed ldx #$fa // loop to add N*250 as determined above
jsr l12e0
dec $fa
bne l19ed
ldx #$15 // print bonus message
l19f8 lda l129a-1,x
sta $11e3,x
lda #$02
sta $95e3,x
dex
bne l19f8
lda $00 // skill zero?
beq l1a13
l1a0a ldx #$fa // no; loop to score +250 for each skill/level left
jsr l12e0
dec $00
bne l1a0a
l1a13 lda l1efc
bne l1a28 // snakes still alive
lda #$33
sta $11f1
ldx #$05 // print "great"
l1a1f lda l1295-1,x
sta $11e3,x
dex
bne l1a1f
l1a28 sty $22 // split screen
sty $23
sty $24
l1a2e lda $9004 // query video line being scanned for TV-out
cmp #$7e
bcc l1a3a
lda #$c0
clc
bcc l1a3c
l1a3a lda #$cd // re-configure screen & character map addresses
l1a3c sta $9005
inc $22
bne l1a2e
inc $23
bne l1a2e
inc $24
cmp #$10
bcc l1a2e
inc $00
l1a4f ldx #$fa
jsr l12e0
dec $00
bne l1a4f
lda l1efd
cmp #$03
bcs l1a64
inc l1efd
bne l1a69
l1a64 ldx #$ff
jsr l12e0
l1a69 dec $16
dec $16
l1a6d lda #$80 // set flag to indicate warm boot
sta $fb
lda $01 // backup score value
sta $fc
lda $02
sta $fd
lda $05
sta $fe
jmp l166f // to start of game
l1a80 lda $cb // poll keyboard
cmp #$08 // left arrow key?
beq l1a81
cmp #$27 // key F1? (added for emulator)
bne l1aa3
l1a81 lda $00 // skill zero?
beq l1a95 // yes -> allow
lda $1112 // read screen at pos. of target symbol
cmp #$16 // target symbol still present? (else: eaten by snake)
beq l1aa3
lda $06 // target gone; player still non-ready?
bne l1aa3 // no -> disallow
l1a95 lda l1efd // any lifes left?
bne l1a9d
jmp l1b17 // no -> update highscore & enter post-game
l1a9d dec l1efd // yes; remove one life and continue
jmp l1a6d
//-----------------------------------------------------------------
// // Place player: Sound
l1aa3 lda ($03),y // check char at player position
cmp #$35 // grant symbol found?
bcc l1aae