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2474
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1 ***************************************
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2
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3 * Subroutine to calc. a random number
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4 * Based on routine by L.A. Middaugh
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5 * The Rainbow Jan/85 p. 277
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6
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7 * Value truncated so that it is =>0 and <=passed value in D.
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8
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9 * OTHER MODULES NEEDED: none
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10
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11 * ENTRY: D=max value of number to be returned
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12 * EXIT: D = value
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13
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14
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15 nam Rnd
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16 ttl Assembler Library Module
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17
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18
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19 psect Rnd,0,0,0,0,0
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20
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21 vsect dp
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22
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23 SEED rmb 4
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24
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25 endsect
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26
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27 RND:
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28 pshs d,x,u
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29
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30 rnd0
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31 ldx #SEED point to seed
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32 ldb #8 number of shifts
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33 loop
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34 lda 3,x exclusive or bit 28 with 31
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35 rora
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36 rora
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37 rora
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38 eora 3,x
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39 rora result in carry
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40 rora
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41 ror 0,x rotate carry into bit0
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42 ror 1,x
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43 ror 2,x
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44 ror 3,x
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45 decb do 8 times
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46 bne loop
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47 ldd 1,x get rnd value
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48 bne trunc ensure we never return a 0
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49 inc 1,x fudge it so we get a non-zero
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50 inc 3,x
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51 bra rnd0
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52
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53 trunc
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54 cmpd ,s in range specified?
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55 bls exit yes
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56 subd ,s
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57 bra trunc
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58
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59 exit
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60 leas 2,s forget original D
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61 puls x,u,pc
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62
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63
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64 **********************************************
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65 *
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66 * Subroutine to seed the random number buffer
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67 * with the current system date
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68
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69 * ENTRY: none
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70 * EXIT: none
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71
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72 SEEDRND:
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73 pshs d,x
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74 leas -6,s make room for date
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75 tfr s,x point X to buffer
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76 os9 F$Time
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77 addd 4,x add min/secs to value in D
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78 addd <SEED add to orig value
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79 std <SEED set msb of seed
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80 addd <SEED+2 add lsb of seed to new msb
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81 std <SEED+2
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82 leas 6,s
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83 puls d,x,pc
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84
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85 endsect
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