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Elite on the BBC Micro

Utility routines: DORND [Elite-A, Encyclopedia]

Name: DORND [Show more] Type: Subroutine Category: Utility routines Summary: Generate random numbers Deep dive: Generating random numbers Fixing ship positions
Context: See this subroutine in context in the source code References: This subroutine is called as follows: * DETOK2 calls DORND * LL9 (Part 1 of 12) calls DORND * MT18 calls DORND * SUN (Part 3 of 4) calls DORND

Set A and X to random numbers (though note that X is set to the random number that was returned in A the last time DORND was called). The C and V flags are also set randomly. If we want to generate a repeatable sequence of random numbers, when generating explosion clouds, for example, then we call DORND2 to ensure that the value of the C flag on entry doesn't affect the outcome, as otherwise we might not get the same sequence of numbers if the C flag changes.
\ --- Mod: Original Acornsoft code removed: -----------> \.DORND2 \ CLC \ Clear the C flag so the value of the C flag on entry \ \ doesn't affect the outcome \ --- End of removed code -----------------------------> .DORND LDA RAND \ Calculate the next two values f2 and f3 in the feeder ROL A \ sequence: TAX \ ADC RAND+2 \ * f2 = (f1 << 1) mod 256 + C flag on entry STA RAND \ * f3 = f0 + f2 + (1 if bit 7 of f1 is set) STX RAND+2 \ * C flag is set according to the f3 calculation LDA RAND+1 \ Calculate the next value m2 in the main sequence: TAX \ ADC RAND+3 \ * A = m2 = m0 + m1 + C flag from feeder calculation STA RAND+1 \ * X = m1 STX RAND+3 \ * C and V flags set according to the m2 calculation RTS \ Return from the subroutine