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BBC Micro Elite

Maths (Arithmetic): LL38 (6502SP version)

Name: LL38 [View in context] Type: Subroutine Category: Maths (Arithmetic) Summary: Calculate (S A) = (S R) + (A Q)
Calculate the following between sign-magnitude numbers: (S A) = (S R) + (A Q) where the sign bytes only contain the sign bits, not magnitudes. Returns: C flag Set if the addition overflowed, clear otherwise
.LL38 EOR S \ If the sign of A * S is negative, skip to LL35, as BMI LL39 \ A and S have different signs so we need to subtract LDA Q \ Otherwise set A = R + Q, which is the result we need, CLC \ as S already contains the correct sign ADC R RTS \ Return from the subroutine .LL39 LDA R \ Set A = R - Q SEC SBC Q BCC P%+4 \ If the subtraction underflowed, skip the next two \ instructions so we can negate the result CLC \ Otherwise the result is correct, and S contains the \ correct sign of the result as R is the dominant side \ of the subtraction, so clear the C flag RTS \ And return from the subroutine \ If we get here we need to negate both the result and \ the sign in S, as both are the wrong sign PHA \ Store the result of the subtraction on the stack LDA S \ Flip the sign of S EOR #%10000000 STA S PLA \ Restore the subtraction result into A EOR #%11111111 \ Negate the result in A using two's complement, i.e. ADC #1 \ set A = ~A + 1 RTS \ Return from the subroutine