PAGE &0E
--------
00:BRK  \player x coordinate
01:BRK \player y coordinate 
02:BRK  \number of lives

04:EQUW &1707  \player sprite data

10:BRK  \player bullet x coordinate
11:BRK  \player bullet y coordinate
14:EQUW 0  \player bullet sprite addr
19:EQUW 0  \player bullet old scr addr


&E20 - &FFF
-----------
30 16-byte blocks of bird data. Attributes at beginning of game are as
follows:

Offset 
00:EQUB 16  \x coordinate (16 for 1st bird) 
01:EQUB 16  \y coordinate (16 for 1st bird) 
02:EQUB 1   \presence flag, 1=present, 0=absent 
03: 
04:EQUW &1526  \sprite addr                                              
06:EQUW &80xx  \old scr addr 
08:EQUW &3540  \new scr addr (&3540 for 1st bird)
0A
0B
0C:BRK  \homing byte


&1000-&107F
-----------
8 16-byte bird bullet blocks


&1080-&11EF
-----------
16 16-byte egg/explosion blocks


PAGE &11 (Continued)
--------------------
E0:EQUB -98  \INKEY code for left key
E1:EQUB -67  \INKEY code for right key
E2:EQUB 47  \ASCII code for fire key
E3:
E4:BRK  \silence flag, sound on=0, sound off=1
E5:EQUW 0  \copy of score at end of game

PAGE &15
--------
\Each sprite comprises [columns,rows] in decimal
\then (column x rows) of data in hex

\bird up sprite data
26:EQUB 6,8
28:EQUB 04,00,00,00,00,08 \020000000020
   EQUB 04,08,05,0A,04,08 \022003300220
   EQUB 00,0C,0D,0E,0C,00 \002223322200
   EQUB 00,04,0C,0C,08,00 \000222222000
   EQUB 00,00,0C,0C,00,00 \000022220000
   EQUB 00,00,26,19,00,00 \000052250000
   EQUB 00,00,22,11,00,00 \000050050000
   EQUB 00,00,22,11,00,00 \000050050000

\bird down sprite data
88:EQUB 6,8
8A:EQUB 00,00,00,00,00,00 \000000000000
   EQUB 00,00,05,0A,00,00 \000003300000
   EQUB 00,04,0D,0E,08,00 \000223322000
   EQUB 00,0C,0C,0C,0C,00 \002222222200
   EQUB 04,08,0C,0C,04,08 \022022220220
   EQUB 04,00,0C,19,00,08 \020022250020
   EQUB 00,00,22,11,00,00 \000050050000
   EQUB 00,00,22,11,00,00 \000050050000


PAGE &16
--------
\egg sprite data
4C:EQUB 3,8
4E:EQUB 00,2A,00 \007000
   EQUB 15,3F,00 \077700
   EQUB 15,3F,00 \077700
   EQUB 3F,3F,2A \777770
   EQUB 3F,3F,2A \777770
   EQUB 3F,3F,2A \777770
   EQUB 3F,3F,2A \777770
   EQUB 15,3F,00 \077700

\'20' bonus sprite data
8A:EQUB 3,5
8C:EQUB F0,50,F0 \CC0CCC
   EQUB 50,50,50 \0C0C0C
   EQUB F0,50,50 \CC0C0C
   EQUB A0,50,50 \C00C0C
   EQUB F0,50,F0 \CC0CCC

\player bullet sprite data
F3:EQUB 1,8
F5:EQUB 2A \70
   EQUB 2A \70
   EQUB 2A \70
   EQUB 2A \70
   EQUB 2A \70
   EQUB 2A \70
   EQUB 2A \70
   EQUB 2A \70

FD:EQUB 1,8
FE:EQUB 28 \60
   EQUB 28 \60
   EQUB 28 \60
   EQUB 28 \60
   EQUB 28 \60
   EQUB 28 \60
   EQUB 28 \60
   EQUB 28 \60


PAGE &17
--------
\player ship sprite data
07:EQUB 4,12
09:EQUB 00,05,00,00 \00030000
   EQUB 00,05,00,00 \00030000
   EQUB 00,05,00,00 \00030000
   EQUB 00,0F,0A,00 \00333000
   EQUB 00,0F,0A,00 \00333000
   EQUB 40,0F,4A,00 \08333800
   EQUB 01,85,81,00 \01838100
   EQUB 03,85,81,02 \11838110
   EQUB 03,07,03,02 \11131110
   EQUB 03,07,03,02 \11131110
   EQUB 03,02,03,02 \11101110
   EQUB 03,02,03,02 \11101110

\bird/egg explosion sprite data 
39:EQUB 4,12
3B:EQUB 00,CD,45,00 \00AB0B00
   EQUB 45,40,88,00 \0B08A000
   EQUB CE,41,41,88 \BA0909A0
   EQUB 40,82,00,00 \08900000
   EQUB 00,88,C2,88 \00A098A0
   EQUB CE,C2,41,45 \BA98090B
   EQUB 00,00,80,88 \000080A0
   EQUB 44,41,40,45 \0A09080B
   EQUB 00,8A,82,8A \00B090B0
   EQUB 45,44,44,88 \0B0A0AA0
   EQUB 00,45,C4,00 \000B8A00
   EQUB 00,00,00,00 \00000000

\player ship explosion sprite data
6B:EQUB 9,16
6D:EQUB 00,00,00,00,00,00,00,00,00 \000000000000000000
   EQUB 00,00,00,00,00,00,00,00,00 \000000000000000000
   EQUB 00,00,00,8A,8A,8A,8A,00,00 \000000B0B0B0B00000
   EQUB 00,00,8A,00,00,00,00,00,00 \0000B0000000000000
   EQUB 00,00,45,00,8A,45,00,8A,00 \00000B00B00B00B000
   EQUB 00,45,00,00,88,88,00,00,00 \000B0000A0A0000000
   EQUB 00,00,00,8A,00,00,45,00,00 \000000B000000B0000
   EQUB 45,00,8A,00,8A,8A,88,00,8A \0B00B000B0B0A000B0
   EQUB 00,45,00,8A,CD,88,00,45,00 \000B00B0ABA0000B00
   EQUB 00,00,45,88,82,00,8A,88,45 \00000BA09000B0A00B
   EQUB 45,00,00,41,41,C2,CD,00,8A \0B0000090998AB00B0
   EQUB 00,8A,8A,C2,00,41,00,00,00 \00B0B0980009000000
   EQUB 00,44,41,00,82,00,C6,CE,00 \000A090090009ABA00
   EQUB 00,00,44,C1,C1,40,00,00,8A \00000A8989080000B0
   EQUB 00,00,CE,00,C2,82,C2,45,00 \0000BA009890980B00
   EQUB 45,CE,41,C1,C1,C2,00,8A,45 \0BBA0989899800B00B


PAGE &18
--------
00:JMP &1806  \EXECUTE
03:JMP &24C9  \zeroise score, define envelopes etc
06:LDA &2800:CMP #33:BNE &1810  \?&2800 is always 33
   JMP &2801  \so we always go to &2801

\ the top level layout of the game is this:
\ &1800:JMP &1806
\ &1806:JMP &2801
\ &2801:JMP &280C
\ &280C:REPEAT set up keys, draw mode 7 screen, play game until all lives
\ lost, check score against hi scores, enter name if required UNTIL FALSE

\ if you change ?&2800 to any number <> 33 the following will happen:
10:LDA #0:STA &2490:STA &2491  \zeroise hi score
   JMP &2492  \and jump to alternate title page

\ *** sprite routine ***
\ entry: &82/3 points to parameter block:
\ 0    x coordinate of sprite
\ 1    y coordinate of sprite
\ 4-5  new sprite data addr
\ 6-7  old sprite data addr
\ 9-10 old screen addr. 
\ Offset 9 usually set to &80 by routines to give &80xx, an address in ROM, 
\ when plotting sprite on scr for 1st time.
  
\exit: &82/3 points to updated parameter block, extra details as thus:
\ 8  y coordinate AND 7
\ 11 x coordinate AND 1

\calculate screen address
\in Basic this would be &3000 + (x AND &FE)*8 + (x AND 7) +
\ (y AND &F8)*80 + (y AND 7) 
1B:LDY #0:LDA (&82),Y  \get x coordinate
   AND #1:STA &70  \odd or even coordinate
   LDA (&82),Y:AND #&FE:STA &72  \remainder
   STY &73:STY &75  \zeroise &73/75
   ASLA:ROL &73:ASLA:ROL &73:STA &72  \now &72/3 = (x AND &FE)*8
   INY:LDA (&82),Y  \get y coordinate
   AND #7:STA &71  \&71 = row within char row
   LDA (&82),Y:AND #&F8  \remainder
   ASLA:ROL &75:ASLA:ROL &75
   ASLA:ROL &75:ASLA:ROL &75:STA &74  \&74/5 = (y AND &F8)*16
   LDA &75:STA &77
   LDA &74:ASLA:ROL &77
   ASLA:ROL &77:STA &76  \&76/7 = (y AND &F8)*64 
   LDA &72:CLC:ADC &71:STA &72
   LDA &73:ADC #0:STA &73  \(x AND &FE)*8 + (x AND 7)
   LDA &72:CLC:ADC &74:STA &72
   LDA &73:ADC &75:STA &73  \ + (y AND &F8)*16
   LDA &72:CLC:ADC &76:STA &72
   LDA &73:ADC &77:STA &73  \ + (y AND &F8)*64
   NOP:CLC:ADC &72:STA &72
   LDA #&30:NOP:CLC:ADC &73:STA &73  \ + &3000

93:LDY #4:LDA (&82),Y:STA &74  \
   INY:LDA (&82),Y:STA &75  \Y=5, &74/5 = new sprite data address
   INY:LDA (&82),Y:STA &76  \Y=6
   INY:LDA (&82),Y:STA &77  \Y=7, &76/7 = old sprite address
   INY:LDA (&82),Y:STA &7C  \Y=8, &7C = (old y AND 7)
   INY:LDA (&82),Y:STA &7A  \Y=9 
   INY:LDA (&82),Y:STA &7B  \Y=10 ,&7A/B old screen address
   INY:LDA (&82),Y:STA &7D  \Y=11. &7D = (oldx AND 1)
   LDY #0:LDA (&74),Y:STA &78  \&78 = rows
   INY:LDA (&74),Y:STA &79  \Y=1, &79 = columns 
   LDY #6:LDA &74:STA (&82),Y  \
   INY:LDA &75:STA (&82),Y  \Y=7, sprite data address
   INY:LDA &71:STA (&82),Y  \Y=8 (new y AND 7)
   INY:LDA &72:STA (&82),Y  \Y=9
   INY:LDA &73:STA (&82),Y  \Y=10 new screen address
   INY:LDA &70:STA (&82),Y  \Y=11 (new x AND 1)
 
   DEC &79
   LDA &74:CLC:ADC #2:STA &74  \was pointing to rows+columns data
   LDA &75:ADC #0:STA &75  \now point to sprite data itself
   LDA &76:CLC:ADC #2:STA &76  \
   LDA &77:ADC #0:STA &77  \

PAGE &19
--------
\ remember
\ &71   = y AND 7, counter indicating position in char row
\ &72/3 = new scr addr
\ &74/5 = new sprite addr
\ &76/7 = old sprite addr
\ &78   = rows
\ &79   = columns
\ &7A/B = old scr addr

02:LDY #0:JSR &190B  
   DEC &79:BNE &1902  \dec columns until all done
0B:LDX &78  \load rows count
   LDA &72:PHA:LDA &73:PHA  \preserve new scr
   LDA &7A:PHA:LDA &7B:PHA  \preserve old scr
\plot byte at new scr addr
19:LDA &70:BEQ &1938  \even newx so don't split byte
   LDA (&74),Y:BEQ &1940  \zero byte (you don't really need to do this)
   LSRA:AND #85  \mask right pixel
   EOR (&72),Y:STA (&72),Y  \store right pixel in this column
   LDA (&74),Y:ASLA:AND #170  \mask left pixel
   LDY #8  \point to next column
   EOR (&72),Y:STA (&72),Y
   LDY #0:JMP &1940  \point to last column and jump back
\store pixels straight in memory without splitting
38:LDA (&74),Y:BEQ &1940
   EOR (&72),Y:STA (&72),Y
\plot byte at old scr addr
40:LDA &7D:BEQ &195F  \even oldx so don't split pixels
   LDA (&76),Y:BEQ &1967
   LSRA:AND #85  \mask right pixel
   EOR (&7A),Y:STA (&7A),Y  \store right pixel in this column
   LDA (&76),Y:ASLA:AND #170  \mask left pixel
   LDY #8  \point to next column
   EOR (&7A),Y:STA (&7A),Y
   LDY #0:JMP &1967  \point to last column and jump
\store pixels in memory without splitting
5F:LDA (&76),Y:BEQ &1967
   EOR (&7A),Y:STA (&7A),Y
67:LDA &72:CLC:ADC #8:STA &72  \point to next column
   BCC &1972:INC &73  \new scr=new scr + 8
72:LDA &7A:CLC:ADC #8:STA &7A  \point to next column
   BCC &197D:INC &7B  \old scr=old scr + 8
7D:INC &74:BNE &1983:INC &75  \new data=new data + 1, next byte of new sprite
83:INC &76:BNE &1989:INC &77  \old data=old data + 1, next byte of old sprite
89:DEX:BEQ &198F:JMP &1919  \loop till all rows done
8F:PLA:STA &7B:PLA:STA &7A  \restore original old scr
   PLA:STA &73:PLA:STA &72  \restore original new scr
   INC &71:LDA &71:AND #7:BEQ &19AC  \bottom of char row
   INC &72:BNE &19B9:INC &73  \new scr=new scr+1
B9:JMP &19B9
AC:LDA &72:CLC:ADC #&79:STA &72
   LDA &73:ADC #2:STA &73  \new scr = new scr + &279
B9:INC &7C:LDA &7C:AND #7:BEQ &19C8  \bottom of char row
   INC &79:BNE &19D5:INC &7B
   RTS
C8:LDA &7A:CLC:ADC #&79:STA &7A
   LDA &7B:ADC #2:STA &7B  \old scr = old scr + &279
D8:RTS

\ *** move player ship ***
D6:BRK  \copy of shipx
   LDA &E00:STA &19D6
   LDX &11E1  \get INKEY code for right
   JSR &1A2A:CPY #&FF:BNE &19F4  \not pressed
   INC &E00:LDA &E00:CMP #140:BNE &19F4  \not at maximum right
   DEC &E00  \put back to left
F4:LDX &11E0  \get INKEY code for left
   JSR &1A2A:CPY #&FF:BNE &1A0B  \not pressed
   DEC &E00


PAGE &1A
--------
01:LDA &E00:CMP #10:BNE &1A0B  \ship not at maximum left
   INC &E00  \put back to right
0B:LDA &E00:CMP &19D6:BEQ &1A22  \ship hasn't moved so don't print
   LDA #&00:STA &82:LDA #&0E:STA &83:JSR &181B  \print ship
   JSR &1F58  \check for collision
   RTS
22:JSR &1F58  \check if collided with egg
   LDX #6:JMP &22CE  \delay

\ *** check for keypress ***
\ entry: X=INKEY number
\ exit: A and X preserved
2A:PHA:TXA:PHA
   LDA #&81:LDY #&FF:JSR osbyte
   PLA:TAX:PLA
   RTS

\ *** move bird ***
\ entry: &8E/8F points to data block of bird to be moved
38:LDY #1:LDA (&8E),Y  \birdy =
   TAX:INX:TXA:STA (&8E),Y  \birdy + 1
   AND #15:BNE &1A48  \wings change position every 16th line
   JSR &1ACA  \swap bird sprite
48:LDY #1:LDA (&8E),Y:CMP #236:BNE &1A5F  \birdy <> 236
   JSR &1EAE  \put egg
   LDY #1:LDA #8:STA (&8E),Y  \set birdy=8
   JSR &1FDE  \swoop sound
5F:LDY #14:LDA (&8E),Y
   CLC:ADC #32:AND #&F0
   CMP #32:BEQ &1A95:BMI &1A83
   LDY #0:LDA (&8E),Y:SEC:SBC #1:STA (&8E),Y  \birdx = birdx - 1
   CMP #2:BCS &1A95  \ birdx >= 2
   CLC:ADC #1:STA (&8E),Y:JMP &1A95
83:LDY #0:LDA (&8E),Y:SEC:ADC #0:STA (&8E),Y  \birdx = birdx + 1
   CMP #147:BCC &1A95  \ birdx <= 147
   SEC:SBC #1:STA (&8E),Y
95:JSR &1EA3  \print egg
   JSR &2464  \check collision
   LDY #0:LDA (&8E),Y:CMP &E00:BEQ &1AB7  \birdx = shipx
   BCC &1AB8  \birdx < shipx
   LDY #14:LDA (&8E),Y:CLC:ADC #1:STA (&8E),Y
   CMP #32:BNE &1AB7
   LDA #31:STA (&8E),Y
B7:RTS

B8:LDY #14:LDA (&8E),Y
   CLC:SBC #0:STA (&8E),Y
   CMP #224:BNE &1AB7
   LDA #225:STA (&8E),Y:RTS

\ *** change bird wing position ***
\ if bird sprite addr is &1526 change to &1588 and vice versa
CA:LDY #4:LDA (&8E),Y  \get sprite data hi
   CMP #&26:BNE &1AD7
   LDA #&88:STA (&8E),Y:RTS
D7:LDA #&26:STA (&8E),Y:RTS
 
DC:LDA &E10:BNE &1B29  \player bullet x is active
   LDA #129:LDX #0:LDY #0:JSR osbyte
   CPY #0:BNE &1AFB  \key not pressed
   CPX &11E2:BEQ &1B0B  \fire key pressed
   CPX #ASC"P":BEQ &1B00:CPX #ASC"p":BEQ &1B00  \pause pressed
   LDX #1:JMP &22CE  \delay to slow down game


PAGE &1B
--------
00:JSR &2000  \clear kbd
   LDA #7:JSR oswrch:JMP &2405  \beep and wait for key press

\ *** init player bullet ***
0B:LDA &E00:CLC:ADC #3:STA &E10  \bulletx at shipx+3
   LDA #230:STA &E11 \bullety at 230
   LDA #&80:STA &E1A  \old screen &80xx
   LDA #&10:STA &82:LDA #&0E:STA &83  \point to &E10
   JMP &181B  \and call sprite routine to print bullet

\ *** move player bullet up screen ***
29:DEC &E11  \bullety = bullety -1
   LDA &E11:CMP #11:BCS &1B4E  \bullety >= 11 so at top of scr yet
   LDA #&10:STA &82:LDA #&0E:STA &83  \point to &E10
   JSR &181B  \erase bullet
   LDA #0:STA &E10  \signal bullet now inactive
   JMP &2000  \clear keybd
4E:LDA &E19:STA &70:LDA &E1A:STA &71  \old screen
   JSR &1B69  \move bullet up
   LDA &70:STA &E19
   LDA &71:STA &E1A:JSR &231E
   RTS

\ *** print bullet at addr in &70/1 ***
\ uses &70/1 = addr of top of bullet
\ &72 = pixel code to be written
\ &73/4 = &70/1 + &280, addr of bottom of bullet
69:LDA &70:AND #7:BNE &1B7F  \in row
   LDA &70:SEC:SBC #&79:STA &70
   LDA &71:SBC #2:STA &71  \screen=screen + &279
   JMP &1B8C
7F:LDA &70:SEC:SBC #1:STA &70
   LDA &71:SBC #0:STA &71  \screen=screen - 1
   LDA &E1B:CMP #1:BNE &1B9A
   LDA #21:STA &72  \code for black-white
   JMP &1B9E
9A:LDA #42:STA &72  \code for white-black
   DEC &E18:LDA &E18:AND #7:STA &E18  \old row no
   LDY #0:LDA &72:EOR (&70),Y:STA (&70),Y  \print top pixel of bullet
   LDA &70:CLC:ADC #&80:STA &73
   LDA &71:ADC #2:STA &74  \add &280 to get bottom of bullet
   EOR (&73),Y:STA (&73),Y  \erase bottom pixel of bullet
   RTS

\ *** init bird bullet ***
\ entry; &8E/F point to bird block
C5:LDY #2:LDA (&8E),Y:BNE &1BCC  \bird active
   RTS
CC:LDY #0:LDA (&8E),Y  \bird bulletx will be
   CLC:ADC #6:STA &74  \at birdx + 6
   LDA #&00:STA &70:STA &73
   LDA #&10:STA &71  \&70/71 points to 1st bullet block
   LDA &248F:AND #6:CLC:ADC #2:STA &72  \(level AND 6) + 2
E9:LDY #2:LDA (&70),Y  \get bullet state
   BNE &1BFE  \bullet block in use
   LDA &70:STA &75:LDA &71:STA &76
   LDA #1:STA &73  \signify free block found
FE:LDY #0


PAGE &1C
--------
00:LDA (&70),Y:CMP &74:BEQ &1C1B  \get
   LDA &70:CLC:ADC #16:STA &70  \point to
   LDA &71:ADC #0:STA &71  \next bullet block
   DEC &72:BNE &1BE9  \loop until block found
   LDA &73:BNE &1C1C  \and coords aren't same as another bullet
1B:RTS
   LDA #1:LDY #2:STA (&72),Y  \singal bullet active
   LDY #0:LDA &74:STA (&75),Y
   LDY #1:LDA (&8E),Y:CLC:ADC #6:STA (&75),Y  \store birdy+6
   LDY #4:LDA #&FD:STA (&75),Y
   INY:LDA #&16:STA (&75),Y  \bird bullet data @&16FD
   LDY #10:LDA #&80:STA (&75),Y  \old scr &80xx
   LDA &75:STA &82:LDA &76:STA &83
   JMP &181B

\ *** check bird bullets ***
4D:LDA #&00:STA &8B:LDA #&10:STA &8C  \&8B/8C point to 1st bullet block
   LDA #8:STA &8D  \count 8 blocks
59:LDY #2:LDA (&8B),Y:CMP #1:BEQ &1C91  \bullet active
   LDA &8B:CLC:ADC #16:STA &8B
   LDA &8C:ADC #0:STA &8C  \point to next block
   DEC &8D:BNE &1C59  \loop till all bullets checked
   RTS
73:LDY #1:LDA (&8B),Y:CMP #232:BCC &1C90  \bullety <= 232
   DEY:LDA (&8B),Y:CMP &E00:BCC &1C90  \bulletx < shipx
   CLC:SBC #6:CMP &E00:BCS &1C90  \bulletx + 5 > shipx
   PLA:PLA:PLA:PLA   \ship hit
90:RTS

\ move bird bullet, block addr in &8B/C
91:LDY #1:LDA (&8B),Y:CLC:ADC #1:STA (&8B),Y  \bullety=bullety + 1
   CMP #246:BCS &1D05  \bullety > 246
   LDY #9:LDA (&8B),Y:STA &70
   INY:LDA (&8B),Y:STA &71
   LDA &70:CLC:ADC #&80:STA &73  \bottom of bullet &280 bytes
   LDA &71:ADC #2:STA &74  \or 1 char row away from top of bullet
   INY:LDA (&8B),Y  \get old (x AND 1)
   CMP #1:BNE &1CC4  \=0 so even coord
   LDA #20:STA &72  \code for black-cyan
   JMP &1CC8
C4:LDA #40:STA &72  \code for cyan-black
C8:LDA &72:LDY #0:EOR (&70),Y:STA (&70),Y  \erase top of bullet
   LDA &72:EOR (&73),Y:STA (&73),Y  \add to bottom of bullet
   LDA &70:AND #7:CMP #7:BNE &1CEE  \not at bottom of row
   LDA &70:CLC:ADC #&79:STA &70
   LDA &71:ADC #2:STA &71  \scr = scr + &279
   JMP &1CF4
EE:INC &70:BNE &1CF4:INC &71  \scr = scr + 1
F4:LDY #9:LDA &70:STA (&8B),Y  \put new addr of bullet
   INY:LDA &71:STA (&8B),Y  \back in bird block
   JSR &1C73  \check if ship hit


PAGE &1D
--------
02:JMP &1C61  \loop to check next bullet
\ bullet has reached bottom of scr
05:LDY #1:SEC:SBC #1:STA (&8B),Y  \y=y-1
   LDA #&80:LDY #10:STA (&8B),Y  \old scr &80xx
   LDA &8B:STA &82:LDA &8C:STA &83
   JSR &181B  \print bullet
   LDY #2:LDA #0:STA (&8B),Y  \signal bullet inactive
   JMP &1C61  \loop to check next bullet

\*** init egg block ***
\entry: &70/1 = sprite addr
\ &74 = timer countdown value before change of state (e.g. egg -> explosion)
\ &78 = state: 1 for explosion or 2 for an egg
26:LDA #&80:STA &75:LDA #&10:STA &76  \first egg block @&1080
   LDA #16:STA &77  \count 16 blocks
32:LDY #2:LDA (&75),Y:CMP #0:BNE &1D69  \block occupied
   LDA &78:STA (&75),Y  \store timer
   LDA &74:INY:STA (&75),Y  \store state
   INY:LDA &70:STA (&75),Y  \store new
   INY:LDA &71:STA (&75),Y  \scr addr
   LDY #0:LDA &72:STA (&75),Y  \store eggx
   INY:LDA &73:STA (&75),Y
   LDY #10:LDA #&80:STA (&75),Y  \old screen &80xx
   LDA &75:STA &82:LDA &76:STA &83  \transfer egg block to sprite ptr
   JMP &181B  \print egg
69:LDA &75:CLC:ADC #16:STA &75
   LDA &76:ADC #0:STA &76  \point to next block
   DEC &77:BNE &1D32  \loop till all blocks checked
   RTS

\ *** countdown timer for egg or explosion ***
7B:LDA #&80:STA &75:LDA #&10:STA &76  \1st egg block &1080
   LDA #16:STA &77  \count 16 blocks
   LDY #2:LDA (&75),Y:BEQ &1DEE  \not active
   INY:LDA (&75),Y:SEC:SBC #1:STA (&75),Y  \timer = timer -1
   BNE &1DEE  \not 0 yet
   DEY:LDA (&75),Y:CMP #1:BEQ &1DAA  \status=1
   CMP #2:BEQ &1DAA  \status=2
   SEC:SBC #1:STA (&75),Y:JMP &1DEE
AA:LDY #10:LDA #&80:STA (&75),Y  \old screen &80xx
   LDA &75:STA &82:LDA &76:STA &83  \egg block addr to &82/83
   LDA &75:PHA:LDA &&76:PHA  \preserve current block addr
   LDA &77:PHA  \and timer
   JSR &181B  \erase egg
   PLA:STA &77  \restore timer
   PLA:STA &66:PLA:STA &75  \and addr
   LDY #2:LDA (&75),Y:CMP #2:BEQ &1DDC  \status=2 so do explosion
   LDA #0:STA (&75),Y  \status=1
   JMP &1DEE
DC:LDA #0:STA (&75),Y
   LDY #0:LDA (&75),Y:STA &72
   INY:LDA (&75),Y:STA &73:JSR &1EE6  \print explosion
EE:LDA &75:CLC:ADC #16:STA &75  \point to next
   LDA &76:ADC #0:STA &76  \egg block
   DEC &77:BNE &1D87  \loop till egg blocks checked
   RTS


PAGE &1E
--------
\ *** fancy colours ***
00:LDA &200F:AND #15:BEQ &1E08  \change colour every 16th count
   RTS
08:LDA #0:STA &70
0C:LDA &200F:AND #&F0:LSRA:LSRA:LSRA:LSRA
   CLC:ADC &70:AND #3:PHA 
   LDA #19:JSR oswrch  \VDU 19,
   LDA &70:CLC:ADC #8:JSR oswrch  \ ?&70+8,
   PLA:JSRoswrch  \((count DIV 16)+?&70) AND 3 
   LDA #0:JSR oswrch:JSR oswrch:JSR oswrch
   INC &70:LDA &70:CMP #4:BNE &1E0C  \loop till colours 8,9,10,11 are done
   LDA #19:JSR oswrch  \VDU19,
   LDA &200F:LSRA:LSRA:LSRA:LSRA:AND #3 \
   CLC:ADC #1:JSR oswrch  \(?&200F DIV 16) AND 3 + 1
   LDA #0:JSR oswrch:JSR oswrch:JSR oswrch
   RTS

\ *** print hi score ***
64:LDA #31:JSR oswrch:LDA #14:JSR oswrch:LDA #0:JSR oswrch  \VDU31,14,0
   LDA &2490:STA &70:LDA &2491:STA &71  \hi score -> &70/71
   JSR &21FC  \print number
   LDA #17:JSR oswrch:LDA #7:JMP oswrch  \COLOUR 7

\ *** VDU 19,2,X;0; ***
8A:LDA #19:JSR oswrch:LDA #2:JSR oswrch  \VDU 19,2,
   TXA:JSR oswrch:LDA #0:JSR oswrch:JSR oswrch:JMP oswrch  \X,0,0,0

A3:LDA &11E3:BEQ &1EB4
   RTS
B4:JSR &1FA7  \egg sound
   LDY #0:LDA (&8E),Y:CLC:ADC #4:STA &72  \birdx + 2
   INY:LDA (&8E),Y:STA &73  \birdy
   LDA #3:STA &78
   LDA #&4C:STA &70:LDA #&16:STA &71  \egg sprite @&164C
   JMP &1D26  \insert into egg block

\ *** exploding egg/bird ***
D8:LDY #0:LDA (&8E),Y:CLC:ADC #2:STA &72 \birdx + 2
   INY:LDA (&8E),Y:STA &73  \birdy
E6:LDA #50:STA &74  \timer
   LDA #1:STA &78  \egg status=1
   LDA #&39:STA &70:LDA #&17:STA &71  \explosion sprite @&1739
   JMP &1D26  \insert into block

\ *** ship explosion ***
F9:LDA #&00:STA &82:LDA #&0E:STA &83  \point to ship block


PAGE &1F
--------
01:LDA #&80:STA &E0A  \old addr =&80xx
   JSR &181B  \erase ship
   LDA &E00:SBC #4:STA &73  \shipx-4
   LDA #90:STA &74  \timer
   LDA #1:STA &78  \status
   LDA #&6B:STA &70:LDA #&17:STA &71  \explosion sprite @&176B
   JSR &1D26
   LDA #7:JSR &1FC3  \do VDU19,0,7;0;
   LDX #20:JSR &22CE  \delay
   JSR &2451  \explosion sound
   LDA #2:STA &200F
   LDA #90
45:LDX #13:JSR &22CE \delay
   INC &200F:PHA:JSR &1E00  \fancy colours
   PLA:SEC:SBC #1:BNE &1F45
   RTS

\ *** check if ship has collided with egg ***
58:LDA #&80:STA &70:LDA #&10:STA &71  \first egg block @&1080
   STA &72  \count 16 eggs
62:LDY #2:LDA (&70),Y:CMP #2:BEQ &1F71  \get status
   CMP #3:BEQ &1F71  \1 if exploding
   JMP &1F8C  \so loop to next block
71:LDA &E00:CLC:ADC #5   \shipx + 5
   DEY:DEY:CMP (&70),Y:BCC &1F8C  \without 5 pixels
   LDA (&70),Y:CLC:ADC #4  \get eggx + 4
   CMP &E00:BEQ &1F8C \
   PLA:PLA:PLA:PLA:RTS  \collision
8C:LDA &70:CLC:ADC #16:STA &70  \point to next block
   LDA &71:ADC #0:STA &71
   DEC &72:BNE &1F62  \loop till all eggs examined against ship
   RTS

\ *** make level complete sound, data @&26F7 ***
9E:LDX #&F7:LDY #&26:LDA #7:JMP &1FB0

\ *** make egg sound, data @&270D ***
A7:LDX #&0D:LDY #&27:LDA #7:JMP &1FB0

\ *** sound routine ***
B0:CMP #7:BEQ &1FB7:JMP osword  \A<>7 for e.g. defining envelope
B7:PHA:LDA &11E4:BEQ &1FBF  \if silence flag =0 make sound
   PLA:RTS  \else just return
BF:PLA:JMP osword

\ *** VDU19,0,A;0; ***
C3:PHA:LDA #19:JSR oswrch:LDA #0:JSR oswrch  \VDU19,0,
   PLA:JSR oswrch  \A,
   LDA #0:JSR oswrch:JSR oswrch:JSR oswrch:RTS  \0,0,0

\ *** make bird swoop sound on channel A ***
DE:CLC:ADC #&10:STA &2716  \add &10 to force sound
   LDX #&16:LDY #&27:LDA #7:JMP &1FB0

\ *** choose if bird will drop bullet ***
ED:LDY #7:LDA (&8E),Y  \get birdy
   AND #7:BNE &1FFF  \not multiple of 8
   LDA &FE44  \get a 'changing number'
   CMP #63:BCC &1FFF  \exit if  <=63
   JSR &1BC5  \else init bird bullet
FF:RTS


PAGE &20
--------
00:LDA #15:LDX #0:LDY #0:JMP osbyte  \clear kbd
09:BRK  \1st bird no in flight
0A:BRK  \bird x dir +/-2
0B
0C
0D:BRK  \no birds killed
0E:BRK  \2nd bird no in flight
0F:BRK  \delay for star backdrop
10:BRK  \ level AND 1
11:
12:

13:JSR &2000  \clear kbd
   LDA #2:STA &200A  \set bird dx = 0
   STA &200D  \no birds killed = 0
   STA &2011
   STA &200C
   LDA #31:STA &200E  \bird count =31
   LDA &248F:AND #1:STA &2010  \get (level AND 1)
   LDA #2:JSR &1FDE  \bird swoop sound on ch2
   LDA &2010:BEQ &2045  \even level so don't
   LDA #3:JSR &1FDE  \do bird swoop sound on ch3
45:LDA #0:STA &2009  \bird to fly = 0
   INC &2011
4D:INC &200F  \inc star delay
   JSR &1E00  \change colours
   JSR &1ADC  \check fire and pause
   JSR &1D7B  \egg timer
   JSR &241F  \check escape
   JSR &19D7  \move ship
   JSR &1ADC  \check fire and pause again
   LDA #2:STA &2012
   LDA &200F:AND #1:BEQ &2071:JMP &2807  \move stars every 2nd count
71:LDA &200D:JSR &22E3  \calc bird block address
   LDY #2:LDA (&8E),Y:BNE &208F  \extant
   INC &200D  \increase 1st bird no in flight
   LDA #2:JSR &1FDE  \swoop next bird on ch2
   LDA &200D:CMP #30:BNE &2071  \30 birds not killed yet
   JMP &216D  \next level
8F:JSR &1A38  \bird flies down
   LDA &2463:BEQ &2098  \loop till dead flag is TRUE
   RTS  \dead so return
98:JSR &1FED  \choose rnd to see if bird will fire
   LDA #3:STA &2012
   JSR &1C4D  \check bullets
   JSR &1ADC  \check fire and pause
A6:LDA &2010:BEQ &20DB  \even level
   LDA &200E:JSR &22E3  \calc bird block address
   LDY #2:LDA (&8E),Y:BNE &20C9  \extant
   DEC &200E  \decrease 2nd bird no in flight
   LDA #3:JSR &1FDE  \swoop next bird on ch3
   LDA &200E:CMP #&FF:BNE &20A6
   JMP &216D  \next level
C9:JSR &1A38  \bird flies down
   LDA &2463:BEQ &20D2  \dead = FALSE
   RTS  \player killed
D2:JSR &1FED  \choose rnd to see if bird will fire
   JSR &1C4D  \check bullets
   JSR &1ADC  \check fire and pause
DB:LDA &248F:CMP #6:BCC &20E5  \level <= 6
   JSR &1C4D  \check bullets
E5:LDA &200F:AND #3:BEQ &20EF
   JMP &204D
EF:LDA &2009:CMP &200D:BEQ &213D
   LDA &2010:BEQ &2104  \even level (easy)
   LDA &2009:CMP &200E:BEQ &213D
   
   
PAGE &21
--------
04:LDA &200A:JSR &22E3  \calc bird block address
   JSR &1ACA  \change wing position
   LDY #2:LDA (&8E),Y:BEQ &213A  \bird not present
13:LDY #0:LDA (&8E),Y:CLC:ADC &200A  \birdx = birdx + dx
   CMP #6:BCC &2156  \birdx <= 6 so change dx
   CMP #144:BCS &2167  \birdx >= 144 so change dx
   LDA &8E:STA &82:LDA &8F:STA &83  \point to current bird block
   JSR &181B  \print bird
   LDA &FE44:CMP #150:BCC &213A  \rnd <= 150
   JSR &1BC5  \init bullet
3A:JSR &1C4D  \check bullets
3D:INC &2009:CMP #30:BEQ &2153
   JSR &22E3  \calc bird block address
   LDY #2:LDA (&8E),Y:BEQ &213D  \not present
   JMP &204D                               
53:JMP &2045
56:LDA &200A:CMP #2:BNE &2165  \dx=-2
   LDA #-2:STA &200A  \set dx=-2
   JMP &2113
65:LDA #2:STA &200A  \set dx=2
   JMP &2113

\ *** next level! ***
6D:INC &248F  \level = level + 1
   LDA &248F:AND #1:BNE &217D  \(level AND 1) <> 0
   INC &E02  \award extra life after even level
   JSR &1F9E  \make whacky sound
7D:LDA &248F:CMP #8:BNE &2189  \level <> 8
   LDA #6:STA &248F  \reset level to 6
89:JSR &2520  \set screen for next lot
   JMP &2013  \back to main loop

\*** add A to score ***
8F:CLC:ADC &248B:STA &248B
   LDA &248C:CMP &2491:BCC &21CC
   BNE &21B3
   LDA &2490:CMP &248B:BCC &21B3
   JMP &21CC
B3:LDA &248B:STA &2490
   LDA &248C:STA &2491  \hiscore = score
   LDA #17:JSR oswrch:LDA #12:JSR oswrch  \COLOUR 12
   JSR &1E64  \print hi score
CC:LDA #31:JSR oswrch:LDA #2:JSR oswrch:LDA #0:JSR oswrch  \VDU31,2,0
   LDA #17:JSR oswrch  \COLOUR
   LDA #7:SEC:SBC &248F  \7 - (score AND &FF)
   CLC:ADC #1:CMP #8:BNE &21EF  \if colour=8
   LDA #3 \then colour=3
EF:JSR oswrch
   LDA &248B:STA &70:LDA &248C:STA &71
FC:LDY #0
FE:LDX #0

                       
PAGE &22
--------
00:SEC:SBC &26DF,Y:STA &70  \subtract power of 10
   LDA &71:INY:SBC &26DF,Y:STA &71
   BCC &2217  \number's gone < 0
   STA &71:INX:DEY:JMP &2200
17:DEY:LDA &70:ADC &26DF,Y:STA &70  \make no +ve again
   TXA:ORA #ASC"0":JSR &2237  \print digit
   INY:INY:CPY #8:BCC &21FE  \loop till digits checked
   LDA &70:ORA #ASC"0":JSR &2237
   LDA #ASC"0":JMP &2237  \add extra 0 on end of score
37:CMP #ASC"0":BNE &2240  \check if 0
   LDA #ASC"O":JMP oswrch  \if so substitute for O and print
40:JMP oswrch

\ *** init player ship ***
43:LDA #0:STA &E10  \player bullet x =0
   LDA #&80:STA &E1A  \old screen addr =&80xx
   LDA #76:STA &E00  \player x =76
   LDA #235:STA &E01  \player y =235
   LDA #&07:STA &E04:LDA #&17:STA &E05  \player ship data @&1707
   LDA #&00:STA &82  \point to player ship block
   LDA #&0E:STA &83  \in &82/83
   JMP &181B  \and print ship

7A:LDA #0:STA &E10  \set player bullet x =0 (not present)
   LDA #&80:STA &E1A  \bullet address &80xx
   LDA #&F3:STA &E14:LDA #&16:STA &E15  \bullet sprite @&16F3
   RTS

\ *** set up mode 2 screen ***
8F:LDA #&21:STA &80:LDA #&26:STA &81
   JSR &2307  \Mode 2 + off cursor data @&2621
   JSR &2804  \print starry backdrop
   LDA #17:JSR oswrch  \COLOUR
   LDA &E02:AND #7:JSR oswrch  \ (lives AND 7)
   LDA &E02:CLC:ADC #ASC"0":JSR oswrch  \print lives
   LDA #0:JSR &218F
   JSR &1E64  \print hi score
   LDA &248F:BEQ &22C9
   AND #3:CLC:ADC #3:TAX:JMP &1E8A  \change colour 2
C9:LDX #2:JMP &1E8A  \colour 2 = black

\ *** delay routine ***
\ entry: X=arbitrary counter
\ exit: AXY intact
CE:PHA:TXA:PHA:TYA:PHA
D3:LDA #0
D5:SEC:SBC #1:BNE &22D5:DEX:BNE &22D3
   PLA:TAY:PLA:TAX:PLA:RTS

\ *** calculate bird block ***
\ entry: A=bird no
\ exit: &8E/F= &E20 + A*16
E3:STA &8E:LDA #0:STA &8F
   LDA &8E
   ASLA:ROL &8F:ASLA:ROL &8F
   ASLA:ROL &8F:ASLA:ROL &8F
   STA &8E  \*16
   LDA #&20:CLC:ADC &8E:STA &8E
   LDA &8F:ADC #&0E:STA &8F  \ + &E20
   RTS


PAGE &23
--------
\*** print text ***
\entry: &80/81 point to text terminated by &FF
\exit A and Y intact
07:PHA:TYA:PHA:LDY #0
0C:LDA (&80),Y:CMP #&FF:BNE &2316  \not &FF yet
   PLA:TAY:PLA:RTS
16:JSR oswrch:INY:JMP &230C  \print and jump back into loop

\ *** check if player bullet has hit bird ***
1D:BRK
1E:LDA #0:STA &231D
   LDA &E11:STA &70  \get bullet y in &70
   CLC:ADC #8:STA &71  \bulletx+8
   LDA #&20:STA &8E:LDA #&0E:STA &8F  \first bird block @&E20
   LDY #2:LDA (&8E),Y  \get bird status
   BEQ &2347  \not present
   LDY #0:LDA (&8E),Y:CMP &E10  \get bird x and cf player bullet x
   BCS &2347 \its > than
   JMP &234A
47:JMP &23EA  \next bird
4A:LDA #10:CLC:ADC (&8E),Y \add 10 to bird x
   CMP &E10:BCS &2357  \its >
   JMP &23EA  \next bird
57:LDY #1:LDA (&8E),Y:CLC:ADC #6  \get bird y + 6
   CMP &E11:BCS &2366  \it's >
   JMP &23EA  \next bird
66:LDA (&8E),Y:CMP &71:BCS &2363  \bird y > bullet y + 6
   LDA &8E:STA &82:LDA &83:STA &83  \bird block -> &82/83
   LDY #0:STA (&8E),Y  \signal bird now inactive
   LDY #10:LDA #&80:STA (&8E),Y  \new bird address =&80xx
   JSR &181B  \erase bird
   JSR &2000  \clear kbd
   LDA #&10:STA &82:LDA #&0E:STA &83  \set ptr to player bullet block
   LDA #&80:STA &E1A  \new address =&80xx
   JSR &181B  \erase bullet
   LDA &231D:CMP &200D:BEQ &23B6
   LDA &2010:BEQ &23AB
   LDA &200E:CMP &231D:BEQ &23B6
AB:JSR &1ED8  \eggsplosion
   LDA #1:JSR &281F:JMP &23E2  \add 10 to score

B6:LDA #&8A:STA &70:LDA #&16:STA &71  \'20' sprite @&168A
   LDY #0:LDA (&8E),Y:CLC:ADC #3:STA &72  \birdx + 20
   INY:LDA (&8E),Y:SEC:SBC #6:STA &73  \birdy - 6
   LDA #1:STA &78  \status=1=exploding
   JSR &1D26  \find free space in egg blocks
   LDA #2:JSR &218F  \add 20 to score
   JSR &1ED8  \print
E2:LDA #0:STA &E10  \signal player bullet inactive
   JMP &2424

EA:LDA &8E:CLC:ADC #16:STA &8E \
   LDA &8F:ADC #0:STA &8F  \point to next bird block
   INC &213D  \increment count
   LDA &231D:CMP #30:BNE &2402  \loop till all birds checked
   RTS                                                    
02:JMP &2335

   
PAGE &24
--------
02:JMP &2335

05:JSR osrdch  \GET
   CMP #27:BEQ &240D  \escape pressed
   RTS
0D:LDA #126:JSR osbyte  \clear escape
   JMP &1800  \and start over

24:LDX #&A6:LDY #&26:LDA #7:JMP &1FB0  \killed bird sound @&26A6

\ *** define envelopes ***
\ (could have been done with less hassle in a Basic loader)
2D:LDX #&AF:LDY #&26:LDA #8:JSR &1FB0  \env 1
   LDX #&BE:LDY #&26:LDA #8:JSR &1FB0  \env 2
   LDX #&E8:LDY #&26:LDA #8:JSR &1FB0  \env 3
   LDX #&F1:LDY #&27:LDA #8:JMP &1FB0  \env 4

51:LDX #&0D:LDY #&26:LDA #7:JSR &1FB0  \player explodes sound @&260D
   LDX #&D6:LDY #&26:LDA #7:JMP &1FB0  \and &26D6

\ *** check to see if bird has hit player ***
63:BRK  \dead flag
64:LDY #0:STY &2463
   INY:LDA (&8E),Y  \get bird y coord
   BPL &2489  \<128 so is nowhere near player
   CMP #232:BMI &2489  \above player and out of harm's way
   CMP #243:BPL &2489  \below player on screen
   DEY:LDA (&8E),Y  \get x coord
   SEC:SBC &E00  \subtract player x coord
   CLC:ADC #10:CMP #17:BCS &2489  \not within 17 pixels of player x
   LDA #1:STA &2463  \within 17 pixels, set dead flag = 1
89:RTS

8A:EQUW 0  \current score
8F:BRK  \colour cycle
90:EQUW 0  \hi score

\ *** set keys and draw alternate title screen ***
\ note this is never used!
92:LDA #12:LDX #0:LDY #0:JSR osbyte  \autorepeat off
   LDA #0:STA &11E3:STA &11E4:STA &11E5
   LDA #&9E:STA &11E0  \INKEY code for Z (-98)
   LDA #&BD:STA &11E1  \INKEY code for X (-67)
   LDA #ASC"?":STA &11E2  \fire key ASCII code
   LDA #&37:STA &80:LDA #&26:STA &81:JSR &2307  \text at &2637
   JSR &2405  \check if escape pressed
   JSR &1803:JMP &24C0 

\ *** zeroise score, define envelopes, init birds ***
C9:LDY #0
   LDA #&8A:STA &70:LDA #&24:STA &71  \&70/71=&248A
   LDX #6
D5:LDA #0:STA (&70),Y:INY:DEX:BNE &24D5  \zeroise score + level no
   JSR &242D  \define envelopes
   LDA #3:STA &E02  \number of lives=3
   JSR &2520  \initialise birds
   JMP &24FA

EB:JSR &228F  \set up mode 2 screen
   JSR &2589  \init egg data blocks
   JSR &2243  \init and print ship
   JSR &25B3  \init bird positions
   JSR &25F9  \print birds
FA:JSR &2013  \main game routine
                                                       
\ routines that pull 2 addrs of stack (see &1C4D and &1F58)
\ will return here where...


PAGE &25
--------
00:DEC &E02  \decrement lives
   BNE &24EB  \until all lives lost
   JSR &2000  \clear kbd
   LDA #&2E:STA &80:LDA #&27:STA &81:JSR &2307  \game over message @&272E
   LDA &248B:STA &11E5:LDA &248C:STA &11E6  \copy score
   RTS

20:JSR &228F  \set up mode 2
   JSR &25D2  \set all birds present
   JSR &227A  \init player bullet data block
   JSR &2589  \init egg data blocks
   JSR &25F9  \print birds
   LDA &248F:BEQ &2546  \don't pause on first level
   AND #1:BNE &2546  \don't pause on odd levels
   LDX #0:JSR &22CE:JSR &22CE:JSR &22CE:JSR &22CE  \long pause
46:JMP &2243  \init player ship

\ *** init a line of birds ***
\ Entry: &70/71 contains address of bird block, &73=y coord of birds
49:LDX #6  \set up for 6 birds
   LDA #16:STA &74  \first bird on line at x=16
4F:LDY #0:LDA &74:STA (&70),Y  \store x coord
   INY:LDA &73:STA (&70),Y  \store y from &73
   LDY #10:LDA #&80:STA (&70),Y  \set old addr to &80xx
   LDA #1:LDY #14:STA (&70),Y  \signal bird present
   LDY #4:LDA #&26:STA (&70),Y  \set sprite
   INY:LDA #&15:STA (&70),Y  \address to &1526 (bird data)
   LDA &70:CLC:ADC #16:STA &70  \point to next
   LDA &71:ADC #0:STA &71  \bird data block
   LDA &74:CLC:ADC #16:STA &74  \next bird 16 pixels to the right
   DEX:BNE &254F  \loop until all bird attributes set
   RTS

\ *** reset attributes in egg/explosion block ***
49:LDA #&00:STA &70:LDA #&10:STA &71  \first egg block @&1000
   LDA #24:STA &72  \there can be up to 24 eggs on screen
95:LDY #2:LDA #0:STA (&70),Y  \signal egg not present
   LDY #10:LDA #&80:STA (&70),Y  \set old addr to &80xx
   LDA &70:CLC:ADC #16:STA &70  \point to next
   LDA &71:ADC #0:STA &71  \egg data block
   DEC &72:BNE &2595  \loop until all egg attributes set
   RTS

\ *** init all lines of birds ***
B3:LDA #&20:STA &70:LDA #&0E:STA &71  \first bird block @&0E20
   LDA #5:STA &72  \5 lines of birds
   LDA #16:STA &73  \y coord of first line is 16
C3:JSR &2549  \print a line of birds
   LDA &73:CLC:ADC #12:STA &73  \next birds are 12 pixels down
   DEC &72:BNE &25C3  \loop till all lines printed
   RTS

\ *** print birds and signify all present ***
D2:JSR &25B3  \init birds
   LDA #&20:STA &70:LDA #&0E:STA &71  \first bird block @&0E20
   LDA #30:STA &72
E1:LDY #2:LDA #1:STA (&70),Y  \signal bird present
   LDA &70:CLC:ADC #16:STA &70  \point to next
   LDA &71:ADC #0:STA &71  \bird block
   DEC &72:BNE &25E1  \loop till all birds done
   RTS

F9:LDA #&20:STA &82:LDA #&0E:STA &83  \first bird block @&E20


PAGE &26
--------
01:LDA #30:STA &84  \count 30 birds
05:LDY #2:LDA (&82),Y:BEQ &260E  \bird not present
   JSR &181B  \print bird
0E:LDA &82:CLC:ADC #&10:STA &82
   LDA &83:ADC #0:STA &83  \update pointer to next bird block
   DEC &84:BNE &2605  \loop till all birds printed
   RTS:RTS

\ data for MODE 2 play screen and titles
21:EQUB 22,2, 31,10,0  \MODE 2:VDU 31,10,0
   EQUS "HI:"
   EQUB 30, 23,0,10,32,0,0,0,0,0,0 \home cursor, cursor off
   EQUB 0,&FF

\ data for alternative title screen, never actually seen!
37:EQUB 22,7, 10,10,10,130,141
   EQUS STRING$(9," ")+"** SWOOP **"+STRING$(9," ")
   EQUB 10,13,130,141
   EQUS STRING$(9," ")+"** SWOOP **"+STRING$(9," ")
   EQUB 10,10,10,10,13, 134
   EQUS "  Z left   X right  ? fire"
   EQUB 10,10,10,13
   EQUS "   Press SPACE to play."
   EQUB &FF

\ sounds data
A6:EQUW &10,1,6,16  \bird dead
AF:EQUB 1,1,0,0,0,0,0,0,126,-1,-1,-1,126,100  \ENVELOPE 1
BE:EQUB 2,1,-1,-1,-1,65,65,70,1,0,0,0,1,1  \ENVELOPE 2
CD:EQUW &110,-15,7,30  \
D6:EQUW &111,2,200,30  \player dead
DF:EQUW 10000,1000,100,10  \another tens table used at &218F
E8:EQUB 3,1,-15,-15,-15,255,255,255,126,0,0,-126,126,100 \ENV.3
F7:EQUW &12,3,128,50  \level complete


PAGE &27
--------
0D:EQUW 1,3,140,2  \egg
16:EQUW &12,4,175,65  \bird swooping
1F:EQUB 4,6,-1,-1,-1,80,80,96,126,-2,0,-1,90,40  \ENVELOPE 4

\ GAME OVER message
2E:EQUB 31,5,14
   EQUS "Game Over"
   EQUB &FF

\hi score names and scores
\1st 14 bytes is name + CR, last 2 bytes is score
\(divided by 10 since extra 0 always printed)
80:EQUS "David Elliot ":EQUB 13:EQUW 578  \hi score of 5780
90:EQUS STRING$(16,CHR$0)
A0:EQUS STRING$(16,CHR$0)
B0:EQUS STRING$(16,CHR$0)
C0:EQUS STRING$(16,CHR$0)
D0:EQUS STRING$(16,CHR$0)


PAGE &28
--------
00:EQUB 33  \change this to any number <>33 for the alternate title scr
01:JMP &280C 
04:JMP &299A  \not used (move backdrop)
07:JMP &29CA \not used (move stars)
0A:EQUB 31  \number of stars in backdrop
0C:LDA #0:STA &11E3:STA &11E4  \set silence flag = FALSE
   LDA #&96:STA &11E0  \INKEY code for Z
   LDA #&BD:STA &11E1  \INKEY code for X
   LDA #&2F:STA &11E2  \ASCII code for ?
23:JSR &2A73 \go into mode 7
   JSR &2853 \draw mode 7 scr
29:JSR &1803 \ -> &24C9 -> main game
   JSR &28F1:CMP #5:BEQ &284A  \check hi score for place in table  
33:JSR &2B41  \clear kbd
   LDA #&66:STA &80:LDA #&2B:STA &81
   JSR &2A35  \print 'You are in the top 5'
   JSR &2977  \input player name
   JSR &2A59  \check loc &FF for Escape
   JMP &2823  \until the cows come home
\not in the top 5
4A:JSR &2B41  \clear kbd
   JSR &2A49  \get a key
   JMP &2829  \until the cows come home

\ *** draw mode 7 scr ***
53:LDA #0:STA &70:STA &72 \&70/71=&7C00 and &72/73=&2C00
   LDA #&7C:STA &71:LDA #&2C:STA &73
   LDA #&FF:STA &74:LDA #4:STA &75:LDY #0
6B:LDA (&72),Y:STA (&70),Y
   INC &70:BNE &2875:INC &71  \scr=scr+1
75:INC &72:BNE &287B:INC &73  \dat=dat+1
7B:DEC &74:BNE &286B:DEC &75:BNE &286B  \loop till 4 pages done
\ print hi score table
83:LDA #0
85:PHA:JSR &2A9A  \print a hi score
   PLA:CLC:ADC #1:CMP #5:BNE &2885  \until all 5 done
\ check Y/N keypress for sound options 
   LDA &11E4:BEQ &28B1:JMP &28C4  \check sound flag
99:JSR &2A49  \get keypress
   CMP #ASC"Y":BEQ &28B1:CMP #ASC"y":BEQ &28B1  \Y or y?
   CMP #ASC"N":BEQ &28C4:CMP #ASC"n":BEQ &28C4  \N or n?
   CMP #32:BNE &2899  \spacebar not pressed so loop back to &2899
B1:LDA #&4A:STA &80:LDA #&2B:STA &81  \YES message at &284A
   JSR &2A33  \print that message
   LDA #0:STA &11E4:JMP &2899  \set sound flag FALSE and jump back
C4:LDA #&52:STA &80:LDA #&2B:STA &81:JSR &2A33  \NO message at &2852
   LDA #1:STA &11E4:JMP &2899  \set sound flag TRUE and jump back

\ *** clear name in hi score table ***
\ &8E/F point to data of position in table
D7:LDA #0:PHA:JSR &2A7E
   LDA #0:LDY #0
E1:STA (&8E),Y:INY:CPY #16:BNE &28E1
   PLA:CLC:ADC #1:CMP #5:BNE &28D9:RTS

\ *** check score against those in table ***
F1:LDA #&80:STA &70:LDA #&27:STA &71  \&70/71=&2780
   LDA #0:STA &72:LDY #15:LDA &11E6


PAGE &29
--------
02:CMP (&70),Y:BEQ &2918:BCS &2922
08:LDA &70:CLC:ADC #16:STA &70  \next position
   INC &72:LDA &72:CMP #5:BNE &28FD  \loop till score >= hi byte in table
   RTS  \or 5th place reached
18:DEY:LDA &11E5
   CMP (&70),Y:BEQ &2908:BCC &2908  \low byte of score <= lo in table
22:JSR &2935  \
   LDY #15:LDA &11E6:STA (&70),Y  \store hi byte of score
   DEY:LDA &11E5:STA (&70),Y  \store lo byte
   LDA #0:RTS
35:LDA &72:CMP #4:BEQ &2966:STA &73:LDA #4
3F:PHA:JSR &2A7E
   LDA &8E:STA &8C:LDA &8F:STA &8D  \copy from position in &8D/E
   PLA:PHA:SEC:SBC #1:JSR &2A7E  \to pos in &8E/F
   LDY #0
55:LDA (&8E),Y:STA (&8C),Y  \move scores down 1 position
   INY:CPY #16:BNE &2955
   PLA:SEC:SBC #1:CMP &72:BNE &293F
66:LDA &72:JSR &2A7E
   LDY #0:LDA #20
6F:STA (&8E),Y
   INY:CPY #14:BNE &296F:RTS

\ *** set up buffer for typing name ***
77:LDA &72:JSR &2A7E
   LDA &8E:STA &78:LDA &8F:STA &79  \buffer pointed to by &78/79
   LDA #13:STA &7A  \max chars allowed is 13
   LDA #32:STA &7B  \min ascii char allowed is 32
   LDA #127:STA &7C  \max ascii char is 127
   LDA #0:LDX #&78:LDY #0  \set up registers
   JSR osword:RTS  \call OSWORD 0 to input chars

\ *** print starry backdrop ***
9A:LDA #&A0:STA &70:LDA #&2B:STA &71  \&70/71=&2BA0
   LDA &280A:STA &72  \number of stars
   LDY #0:LDA (&70),Y:STA &73  \star's address on mode 2
   INY:LDA (&70),Y:STA &74  \screen in &73/74
   INY:LDA (&70),Y:STA &75  \pixel value
A7:LDY #0:LDA (&73),Y:EOR &75:STA (&73),Y  \put star on screen
   INC &70:INC &70:INC &70  \point to next address and value
   DEC &72:BNE &29A7:RTS  \loop until all stars done then return

\ *** move starry backdrop ***
CA:LDA #&A0:STA &70:LDA #&2B:STA &71  \&70/71=&28A0
   LDA &280A:STA &72  \number of stars
D7:LDY #0:LDA (&70),Y:STA &73:  \get
   INY:LDA (&70),Y:STA &74  \star's address
   INY:LDA (&70),Y:STA &75  \pixel value
   LDY #0:LDA (&73),Y:EOR &75:STA (&73),Y  \EOR it to screen
   LDA &73:AND #7:CMP #7  \bottom of char row?
   BEQ &2A00  \yes
   INC &73:BCC &2A0D:INC &74  \no, just add 1 to scr addr
   JMP &2A0D  \and continue


PAGE &2A
--------
\ *** scroll starry backdrop cont'd ***
00:LDA &73:CLC:ADC #&79:STA &73
   LDA &74:ADC #2:STA &74  \scr=scr+&279
0D:LDA &74:CMP #&80  \check hi byte of screen
   BCC &2A17  \less than &8000 so still on screen
   SBC #&4E:STA &74  \wrap round to top of screen
17:LDY #0:LDA (&73),Y:EOR &75:STA (&73),Y
   LDA &73:STA (&70),Y  \put pixel value back in table
   INC &70:INC &70:INC &70  \point to next addr and value
   DEC &72:BNE &29D7:RTS  \loop till all stars done and return

\ *** message print ***
\ Entry: &80/81 point to text terminated by &FF.
\ Exit: A and Y preserved
33:PHA:TYA:PHA:LDY #0
38:LDA (&80),Y:CMP #&FF:BNE &2A42  \not yet &FF
   PLA:TAY:PLA:RTS  \restore and return
42:JSR oswrch:INY:JMP &2A38  \print it and jump back into loop

\ *** get a key, eq of GET in Basic ***
49:JSR osrdch:CMP #27  \escape?
   BEQ &2A51  \yes, deal with it
   RTS  \no, return with value in A
51:LDA #126:JSR osbyte:JMP &1800  \clear escape and start over

\ *** check location &FF ***
\ will be -ve if Escape pressed   
59:LDA &FF:BMI &2A51:RTS  \branch to routine which clears escape

\ *** delay routine ***
\ Entry: X=arbitrary counter
\ Exit: A X and Y preserved
5E:PHA:TXA:PHA:TYA:PHA   \preserve AXY
63:LDA #0
65:SEC:SBC #1:BNE &2A65
   DEX:BNE &2A63
   PLA:TAY:PLA:TAX:PLA:RTS  \restore and return

\ *** do MODE 7 ***
73:LDA #&5A:STA &80:LDA #&2B:STA &81  \&2B5A contains 22,7,
   JMP &2A33  \do MODE 7 via message routine

\ check position in hi score table
7E:CMP #5:BCC &2A8B  \A<5
\ set pointer to '6th' position
   LDA #&D0:STA &8E:LDA #&27:STA &8F  \&27D0 addr of 6th hi name
   RTS

\ *** calculate address of hiscore name ***
\ Entry: A=rank of hi name required
8B:ASLA:ASLA:ASLA:ASLA
   ADC #&80:STA &8E:LDA #&27:STA &8F  \&2780 + A*16
   RTS

\ *** print a high score ***
99:BRK  \workspace
9A:STA &2A99  \store rank temporarily
   JSR &2A7E  \is it in '6th' place?
A0:LDA #31:JSR oswrch:LDA #3:JSR oswrch  \
   LDA #15:CLC:ADC &2A99:JSR oswrch  \VDU 31,3,15+rank
   LDY #0
B5:LDA (&8E),Y:JSR oswrch  \get char from hi name
   CMP #13:BEQ &2AC3  \CR is end of hi name
   INY:CPY #14:BNE &2AB5  \loop till 14 chars of name printed
C3:LDA #31:JSR oswrch:LDA #18:JSR oswrch  \
   LDA &2A99:CLC:ADC #15:JSR oswrch  \VDU 31,18,15+rank
   LDY #14:LDA (&8E),Y:STA &70  \put score in &70/71
   INY:LDA (&8E),Y:STA &71
   LDA &70:ORA &71:BEQ &2AEA
   JSR &2AEE 
EA:LDA &2A99
   RTS

\ *** print 16bit number with leading spaces ***
\ Entry: &70/71 contain number
EE:LDY #0
   LDA #1:STA &72  \set leading zero as true initially
F4:LDA #0
F6:LDA &70:SEC:SBC &2B5D,Y:STA &70  \subtract power of ten from table
   LDA &71


PAGE &2B
--------
00:INY:SBC &2B5D,Y:BCC &2B0D
   STA &71:INX:DEY:JMP &2AF6
0D:DEY:LDA &70:ADC &2B5D,Y:STA &70  \restore to +ve
   JSR &2B28  \print digit
   INY:INY:CPY #8:BCC &2AF4
   LDX &70:JSR &2B28
   LDA #&30:JMP oswrch  \add extra 0 after number

28:TXA:CMP #0:BNE &2B36  \is digit 0? 
   LDA &72:BEQ &2B3C  \leading space flag=0 if first non0 printed
   LDA #32:JMP oswrch  \print leading space
36:PHA:
   LDA #0:STA &72  \leading space = FALSE now non0 number has been printed
   PLA:ORA #&30:JMP oswrch

\ *** clear keyboard buffer ***
41:LDA #15:LDX #0:LDY #0:JMP osbyte

\ YES message (see &28B1)
4A:EQUB 31,32,22
   EQUS "YES"
   EQUB 30,&FF

\ NO message (see &28C4)
52:EQUB 31,32,22
   EQUS "NO "
   EQUB &30,&FF

\ VDU 22,7 data (see &2B73)
  EQUB 22,7,&FF

\ powers of tens table
5D:EQUW 10000,1000,100,10

\ message inviting player to enter name
66:EQUB 31,0,16
   EQUS "You are in the TOP"
   EQUB 10,10,13
   EQUS "FIVE"
   EQUB 10,10,13
   EQUS "Name?"
   EQUB 8, 23,0,10,103,0,0,0,0,0,0  \backspace + cursor on
   EQUB 0,&FF

\ data for starry backdrop
\ there are 31 stars, the number held at &280A
\ each item of data comprises a mode 2 address and a pixel value
\ see &299A (set up stars) and &29CA (move stars)

A0:EQUW &42A8:EQUB 21
   EQUW &4E53:EQUB 21
   EQUW &3F60:EQUB 21
   EQUW &5712:EQUB 21
   EQUW &4DBC:EQUB 21
   EQUW &4B91:EQUB 21
   EQUW &49C4:EQUB 21
   EQUW &6417:EQUB 21
   EQUW &5A41:EQUB 21
   EQUW &68BC:EQUB 21
   EQUW &52C5:EQUB 21
   EQUW &6FA3:EQUB 21
   EQUW &4D30:EQUB 21
   EQUW &69EC:EQUB 21
   EQUW &7491:EQUB 21
   EQUW &59B6:EQUB 21
   EQUW &6320:EQUB 21
   EQUW &7137:EQUB 21
   EQUW &3A86:EQUB 21
   EQUW &775E:EQUB 21
   EQUW &4D69:EQUB 21
   EQUW &3A2A:EQUB 21
   EQUW &3E34:EQUB 21
   EQUW &6D62:EQUB 21
   EQUW &4F05:EQUB 21
   EQUW &6734:EQUB 21
   EQUW &7345:EQUB 21
   EQUW &6D65:EQUB 21
   EQUW &4098:EQUB 21
   EQUW &3E25:EQUB 21
   EQUW &6693:EQUB 21

PAGE &28
--------
00:150,154,
02:EQUS "                                      "
28:EQUB 150,154,160,240,240,160,146,224,240,160,
32:EQUS "  "
34:EQUB 224,240,145,
37:EQUS "  "
39:EQUB 224,240,176,
3C:EQUS "  "
3E:EQUB 148,
3F:EQUS "  "
41:EQUB 240,240,
43:EQUS "  "
45:EQUB 147,240,240,240,176,
4A:EQUS "      "
50:EQUB 154,150,254,191,175,253,146,234,255,160,
5A:EQUS "  "
5C:EQUB 234,255,145,
5F:EQUS " "
60:EQUB 248,255,175,255,244,148,
66:EQUS " "
67:EQUB 224,254,191,239,253,176,147,255,191,175,239,253,176,
74:EQUS "    "
78:EQUB 150,154,255,181,
7C:EQUS " "
7D:EQUB 162,146,234,255,
81:EQUS " "
82:EQUB 224,
83:EQUS " "
84:EQUB 234,255,145,232,255,
89:EQUS "  "
8B:EQUB 162,255,180,148,254,183,
91:EQUS " "
92:EQUB 160,235,253,147,255,181,160,
99:EQUS " "
9A:EQUB 255,181,
9C:EQUS "    "
A0:EQUB 150,154,171,255,244,160,146,234,255,232,255,253,234,255,145,234,255,
B1:EQUS "   "
B4:EQUB 255,181,148,255,181,
B9:EQUS "  "
BB:EQUB 234,255,147,255,253,252,254,191,161,
C4:EQUS "    "
C8:EQUB 150,154,
CA:EQUS " "
CB:EQUB 162,239,253,146,234,255,255,183,255,255,255,145,234,255,
D9:EQUS "   "
DC:EQUB 255,181,148,255,181,
E1:EQUS "  "
E3:EQUB 234,255,147,255,183,163,161,
EA:EQUS "      "
F0:EQUB 154,150,244,
F3:EQUS " "
F4:EQUB 234,255,146,234,255,191,160,170,255,255,145,162,255,244,
02:EQUS " "
03:EQUB 248,255,161,148,235,253,176,224,254,183,147,255,181,
10:EQUS "        "
18:EQUB 150,154,171,255,255,167,146,234,255,161,160,
23:EQUS " "
24:EQUB 235,255,145,
27:EQUS " "
28:EQUB 162,239,255,191,161,
2D:EQUS " "
2E:EQUB 148,160,171,255,255,167,
34:EQUS " "
35:EQUB 147,255,181,
38:EQUS "           "
43:EQUB 160,
44:EQUS "         "
4D:EQUB 160,
4E:EQUS "               "
5D:EQUB 160,
5E:EQUS "          "
68:EQUB 131,
69:EQUS " Z=left  X=right  ?=fire   P=pause.                                            "
B8:EQUB 134,
B9:EQUS " Watch out for the exploding  eggs.                                            "
08:EQUB 132,157,
0A:EQUS " "
0B:EQUB 130,135,
0D:EQUS "  ** Top Five **    "
21:EQUB 131,
22:EQUS "Written by "
2D:EQUB 156,
2E:EQUS "  "
30:EQUB 148,255,130,
33:EQUS "    Name:      Score:"
48:EQUB 132,157,132,157,
4C:EQUS "         "
55:EQUB 156,
56:EQUS "  "
58:EQUB 148,255,131,
5B:EQUS "                     "
70:EQUB 132,157,134,
73:EQUS " David    "
7D:EQUB 156,
7E:EQUS "  "
80:EQUB 148,255,130,
83:EQUS "                     "
98:EQUB 132,157,134,
9B:EQUS " Elliot   "
A5:EQUB 156,
A6:EQUS "  "
A8:EQUB 148,255,130,
AB:EQUS "                     "
C0:EQUB 132,157,131,
C3:EQUS "          "
CD:EQUB 156,
CE:EQUS "  "
D0:EQUB 148,255,130,
D3:EQUS "                     "
E8:EQUB 132,157,131,
EB:EQUS " Elliot   "
F5:EQUB 156,
F6:EQUS "  "
F8:EQUB 148,255,130,
FB:EQUS "                     "
10:EQUB 132,157,131,
13:EQUS "Software  "
1D:EQUB 156,
1E:EQUS "  "
20:EQUB 132,157,130,
23:EQUS "                                  "
45:EQUB 156,
46:EQUS "  "
48:EQUB 131,157,129,
4B:EQUS "  Copyright 1982 Program Power    "
6D:EQUB 156,
6E:EQUS "  "
70:EQUB 129,157,131,
73:EQUS "Do you want sound (Y/N)?   =      "
95:EQUB 156,
96:EQUS "  "
98:EQUB 130,157,136,132,
9C:EQUS "     Press SPACE to play.        "
BD:EQUB 156,
BE:EQUS "                     "
D3:EQUB 160,
D4:EQUS "  "