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Maths (Arithmetic): LL38

[NES version, Bank 1]

Name: LL38 [Show more] Type: Subroutine Category: Maths (Arithmetic) Summary: Calculate (S A) = (S R) + (A Q)
Context: See this subroutine in context in the source code References: This subroutine is called as follows: * LL51 calls LL38 * LL9 (Part 5 of 12) calls LL38

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 .LL40 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