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57
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1 ; 6809 CRC32 with tests
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2 ;
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3 ; Johann E. Klasek, j AT klasek at
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4 ;
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5 ; Testprogram, previous version submitted to http://beebwiki.mdfs.net/index.php/CRC-32#6809
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6
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7 org $100
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8 lds #$8000
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9
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10 ; Calculate a ZIP 32-bit CRC from data in memory. This code is as
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11 ; tight and nearly as fast as it can be, moving as much code out of inner
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12 ; loops as possible. With the included optimisation, moving the whole
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13 ; CRC in registers, the performane gain on average data is only slight
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14 ; (estimated 2% but at losing clarity of implementation;
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15 ; worst case gain is 18%, best case worsens at 29%)
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16 ;
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17 ; On entry, crc..crc+3 = incoming CRC
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18 ; reg. U = start address of data
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19 ; reg. X = number of bytes
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20 ; On exit, crc..crc+3 = updated CRC
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21 ; reg. U = points to first byte behind data
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22 ; reg. X = 0
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23 ; reg. Y = 0
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24 ;
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25 ; Value order in memory is H,L (big endian)
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26 ;
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27 ; Multiple passes over data in memory can be made to update the CRC.
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28 ; For ZIP, initial CRC must be $FFFFFFFF, and the final CRC must
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29 ; be EORed with $FFFFFFFF before being stored in the ZIP file.
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30 ; Total 47 bytes (if above parameters are located in direct page).
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31 ;
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32 ; ZIP polynomic, reflected (bit reversed) from $04C11DB7
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33 CRCHH EQU $ED
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34 CRCHL EQU $B8
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35 CRCLH EQU $83
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36 CRCLL EQU $20
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37 CRCINITH EQU $FFFF
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38 CRCINITL EQU $FFFF
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39
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40 ; CRC 32 bit in DP (4 bytes)
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41 crc EQU $80
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42
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43 ldu #s1 ; start address in u
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44 ldb ,u+ ;
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45 clra ; length in d
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46 leax d,u ;
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47 pshs x ; end address +1 to TOS
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48 ldd #CRCINITL
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49 std crc+2
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50 ldx #CRCINITH
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51 stx crc
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52 ; d/x contains the CRC
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53 bl:
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54 eorb ,u+ ; XOR with lowest byte
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55 ldy #8 ; bit counter
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56 rl:
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57 exg d,x
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58 rl1:
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59 lsra ; shift CRC right, beginning with high word
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60 rorb
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61 exg d,x
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62 rora ; low word
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63 rorb
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64 bcc cl
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65 ; CRC=CRC XOR polynomic
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66 eora #CRCLH ; apply CRC polynomic low word
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67 eorb #CRCLL
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68 exg d,x
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69 eora #CRCHH ; apply CRC polynomic high word
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70 eorb #CRCHL
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71 leay -1,y ; bit count down
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72 bne rl1
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73 exg d,x ; CRC: restore correct order
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74 beq el ; leave bit loop
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75 cl:
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76 leay -1,y ; bit count down
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77 bne rl ; bit loop
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78 el:
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79 cmpu ,s ; end address reached?
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80 bne bl ; byte loop
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81
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82 std crc+2 ; CRC low word
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83 stx crc ; CRC high word
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84
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85
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86 realexit:
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87 sync
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88
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89
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90 s1: fcb 19,"An Arbitrary String"
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91 ; CRC=$90415518
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92
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93 s2: fcb 26,"ZYXWVUTSRQPONMLKJIHGFEDBCA"
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94 ; CRC32=$6632024D
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95
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96 enddata
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97
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98 end
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