"� Main/shadow screen byte swap
� by Richard Carter
� (c) The Micro User
(� March 1991
2� 130:� code% &FF
<� pass=0 � 2 � 2:P%=code%
FPT pass:.start LDX #0
P#LDA #0:STA &70:LDA #&30:STA &71
ZLDA #&4D:STA &72:STA &73
d$.loop JSR swapscreen:CLC:LDA &70
n$ADC &73:STA &70:BCC loop:INC &71
xLDA &71:CMP #&80:BNE loop
�LDA #&30:STA &71:LDA #19
�JSR &FFF4:DEC &72:LDA &72
�BNE loop:RTS
�#.swapscreen LDA (&70,X):STA &74
�LDA &FE34:� #2:CMP #2
�BEQ bitclear:SEC:LDA &FE34
�SBC #2:STA &FE34
�.bitclear LDA (&70,X):PHA
�LDA &74:STA (&70,X):CLC
�LDA &FE34:ADC #2:STA &FE34
�PLA:STA (&70,X):RTS:]:�
�2� t=1 � 20:� 3,�(7):� 85,�(1279),�(1023):�:� 2
�W� t=1 � 20:� 3,�(7):� 85,�(1279),�(1023):�:� 23,1,0;0;0;0;:�:� start:� � �(500):� 0
�

00000000  0d 00 0a 22 f4 20 4d 61  69 6e 2f 73 68 61 64 6f  |...". Main/shado|
00000010  77 20 73 63 72 65 65 6e  20 62 79 74 65 20 73 77  |w screen byte sw|
00000020  61 70 0d 00 14 17 f4 20  62 79 20 52 69 63 68 61  |ap..... by Richa|
00000030  72 64 20 43 61 72 74 65  72 0d 00 1e 18 f4 20 28  |rd Carter..... (|
00000040  63 29 20 54 68 65 20 4d  69 63 72 6f 20 55 73 65  |c) The Micro Use|
00000050  72 0d 00 28 10 f4 20 4d  61 72 63 68 20 31 39 39  |r..(.. March 199|
00000060  31 0d 00 32 15 eb 20 31  33 30 3a de 20 63 6f 64  |1..2.. 130:. cod|
00000070  65 25 20 26 46 46 0d 00  3c 1d e3 20 70 61 73 73  |e% &FF..<.. pass|
00000080  3d 30 20 b8 20 32 20 88  20 32 3a 50 25 3d 63 6f  |=0 . 2 . 2:P%=co|
00000090  64 65 25 0d 00 46 1b 5b  4f 50 54 20 70 61 73 73  |de%..F.[OPT pass|
000000a0  3a 2e 73 74 61 72 74 20  4c 44 58 20 23 30 0d 00  |:.start LDX #0..|
000000b0  50 23 4c 44 41 20 23 30  3a 53 54 41 20 26 37 30  |P#LDA #0:STA &70|
000000c0  3a 4c 44 41 20 23 26 33  30 3a 53 54 41 20 26 37  |:LDA #&30:STA &7|
000000d0  31 0d 00 5a 1c 4c 44 41  20 23 26 34 44 3a 53 54  |1..Z.LDA #&4D:ST|
000000e0  41 20 26 37 32 3a 53 54  41 20 26 37 33 0d 00 64  |A &72:STA &73..d|
000000f0  24 2e 6c 6f 6f 70 20 4a  53 52 20 73 77 61 70 73  |$.loop JSR swaps|
00000100  63 72 65 65 6e 3a 43 4c  43 3a 4c 44 41 20 26 37  |creen:CLC:LDA &7|
00000110  30 0d 00 6e 24 41 44 43  20 26 37 33 3a 53 54 41  |0..n$ADC &73:STA|
00000120  20 26 37 30 3a 42 43 43  20 6c 6f 6f 70 3a 49 4e  | &70:BCC loop:IN|
00000130  43 20 26 37 31 0d 00 78  1d 4c 44 41 20 26 37 31  |C &71..x.LDA &71|
00000140  3a 43 4d 50 20 23 26 38  30 3a 42 4e 45 20 6c 6f  |:CMP #&80:BNE lo|
00000150  6f 70 0d 00 82 1c 4c 44  41 20 23 26 33 30 3a 53  |op....LDA #&30:S|
00000160  54 41 20 26 37 31 3a 4c  44 41 20 23 31 39 0d 00  |TA &71:LDA #19..|
00000170  8c 1d 4a 53 52 20 26 46  46 46 34 3a 44 45 43 20  |..JSR &FFF4:DEC |
00000180  26 37 32 3a 4c 44 41 20  26 37 32 0d 00 96 10 42  |&72:LDA &72....B|
00000190  4e 45 20 6c 6f 6f 70 3a  52 54 53 0d 00 a0 23 2e  |NE loop:RTS...#.|
000001a0  73 77 61 70 73 63 72 65  65 6e 20 4c 44 41 20 28  |swapscreen LDA (|
000001b0  26 37 30 2c 58 29 3a 53  54 41 20 26 37 34 0d 00  |&70,X):STA &74..|
000001c0  aa 19 4c 44 41 20 26 46  45 33 34 3a 80 20 23 32  |..LDA &FE34:. #2|
000001d0  3a 43 4d 50 20 23 32 0d  00 b4 1e 42 45 51 20 62  |:CMP #2....BEQ b|
000001e0  69 74 63 6c 65 61 72 3a  53 45 43 3a 4c 44 41 20  |itclear:SEC:LDA |
000001f0  26 46 45 33 34 0d 00 be  14 53 42 43 20 23 32 3a  |&FE34....SBC #2:|
00000200  53 54 41 20 26 46 45 33  34 0d 00 c8 1d 2e 62 69  |STA &FE34.....bi|
00000210  74 63 6c 65 61 72 20 4c  44 41 20 28 26 37 30 2c  |tclear LDA (&70,|
00000220  58 29 3a 50 48 41 0d 00  d2 1b 4c 44 41 20 26 37  |X):PHA....LDA &7|
00000230  34 3a 53 54 41 20 28 26  37 30 2c 58 29 3a 43 4c  |4:STA (&70,X):CL|
00000240  43 0d 00 dc 1e 4c 44 41  20 26 46 45 33 34 3a 41  |C....LDA &FE34:A|
00000250  44 43 20 23 32 3a 53 54  41 20 26 46 45 33 34 0d  |DC #2:STA &FE34.|
00000260  00 e6 1b 50 4c 41 3a 53  54 41 20 28 26 37 30 2c  |...PLA:STA (&70,|
00000270  58 29 3a 52 54 53 3a 5d  3a ed 0d 00 f0 32 e3 20  |X):RTS:]:....2. |
00000280  74 3d 31 20 b8 20 32 30  3a e6 20 33 2c b3 28 37  |t=1 . 20:. 3,.(7|
00000290  29 3a f0 20 38 35 2c b3  28 31 32 37 39 29 2c b3  |):. 85,.(1279),.|
000002a0  28 31 30 32 33 29 3a ed  3a eb 20 32 0d 00 fa 57  |(1023):.:. 2...W|
000002b0  e3 20 74 3d 31 20 b8 20  32 30 3a e6 20 33 2c b3  |. t=1 . 20:. 3,.|
000002c0  28 37 29 3a f0 20 38 35  2c b3 28 31 32 37 39 29  |(7):. 85,.(1279)|
000002d0  2c b3 28 31 30 32 33 29  3a ed 3a ef 20 32 33 2c  |,.(1023):.:. 23,|
000002e0  31 2c 30 3b 30 3b 30 3b  30 3b 3a f5 3a d6 20 73  |1,0;0;0;0;:.:. s|
000002f0  74 61 72 74 3a f5 20 fd  20 a6 28 35 30 30 29 3a  |tart:. . .(500):|
00000300  fd 20 30 0d ff                                    |. 0..|
00000305