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Universe: TT54

[6502 Second Processor version]

Name: TT54 [Show more] Type: Subroutine Category: Universe Summary: Twist the selected system's seeds Deep dive: Twisting the system seeds Galaxy and system seeds
Context: See this subroutine in context in the source code References: This subroutine is called as follows: * cpl calls TT54

This routine twists the three 16-bit seeds in QQ15 once. If we start with seeds s0, s1 and s2 and we want to work out their new values after we perform a twist (let's call the new values s0´, s1´ and s2´), then: s0´ = s1 s1´ = s2 s2´ = s0 + s1 + s2 So given an existing set of seeds in s0, s1 and s2, we can get the new values s0´, s1´ and s2´ simply by doing the above sums. And if we want to do the above in-place without creating three new s´ variables, then we can do the following: tmp = s0 + s1 s0 = s1 s1 = s2 s2 = tmp + s1 So this is what we do in this routine, where each seed is a 16-bit number.
.TT54 LDA QQ15 \ X = tmp_lo = s0_lo + s1_lo CLC ADC QQ15+2 TAX LDA QQ15+1 \ Y = tmp_hi = s1_hi + s1_hi + C ADC QQ15+3 TAY LDA QQ15+2 \ s0_lo = s1_lo STA QQ15 LDA QQ15+3 \ s0_hi = s1_hi STA QQ15+1 LDA QQ15+5 \ s1_hi = s2_hi STA QQ15+3 LDA QQ15+4 \ s1_lo = s2_lo STA QQ15+2 CLC \ s2_lo = X + s1_lo TXA ADC QQ15+2 STA QQ15+4 TYA \ s2_hi = Y + s1_hi + C ADC QQ15+3 STA QQ15+5 RTS \ The twist is complete so return from the subroutine