Skip to navigation

Maths (Arithmetic): DORND

[BBC Micro cassette version, Loader]

Name: DORND [Show more] Type: Subroutine Category: Maths (Arithmetic) 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: * PLL1 (Part 1 of 3) calls DORND * PLL1 (Part 2 of 3) calls DORND * PLL1 (Part 3 of 3) 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. This is a simplified version of the DORND routine in the main game code. It swaps the two calculations around and omits the ROL A instruction, but is otherwise very similar. See the DORND routine in the main game code for more details.
.DORND LDA RAND+1 \ r1´ = r1 + r3 + C TAX \ r3´ = r1 ADC RAND+3 STA RAND+1 STX RAND+3 LDA RAND \ X = r2´ = r0 TAX \ A = r0´ = r0 + r2 ADC RAND+2 STA RAND STX RAND+2 RTS \ Return from the subroutine