.FMLTU EOR #%11111111 \ Flip the bits in A, set the C flag and rotate right, SEC \ so the C flag now contains bit 0 of A inverted, and P ROR A \ contains A inverted and shifted right by one, with bit STA P \ 7 set to a 1. We can now use P as our source of bits \ to shift right, just as in MU11, just with the logic \ reversed LDA #0 \ Set A = 0 so we can start building the answer in A .MUL3 BCS MU7 \ If C (i.e. the next bit from P) is set, do not do the \ addition for this bit of P, and instead skip to MU7 \ to just do the shifts ADC Q \ Do the addition for this bit of P: \ \ A = A + Q + C \ = A + Q ROR A \ Shift A right to catch the next digit of our result. \ If we were interested in the low byte of the result we \ would want to save the bit that falls off the end, but \ we aren't, so we can ignore it LSR P \ Shift P right to fetch the next bit for the \ calculation into the C flag BNE MUL3 \ Loop back to MUL3 if P still contains some set bits \ (so we loop through the bits of P until we get to the \ 1 we inserted before the loop, and then we stop) RTS \ Return from the subroutine .MU7 LSR A \ Shift A right to catch the next digit of our result, \ pushing a 0 into bit 7 as we aren't adding anything \ here (we can't use a ROR here as the C flag is set, so \ a ROR would push a 1 into bit 7) LSR P \ Fetch the next bit from P into the C flag BNE MUL3 \ Loop back to MUL3 if P still contains some set bits \ (so we loop through the bits of P until we get to the \ 1 we inserted before the loop, and then we stop) RTS \ Return from the subroutineName: FMLTU [Show more] Type: Subroutine Category: Maths (Arithmetic) Summary: Calculate A = A * Q / 256Context: See this subroutine in context in the source code Variations: See code variations for this subroutine in the different versions References: This subroutine is called as follows: * DOEXP calls FMLTU * LL51 calls FMLTU * LL9 (Part 5 of 12) calls FMLTU * MVEIT (Part 3 of 9) calls FMLTU

Do the following multiplication of two unsigned 8-bit numbers, returning only the high byte of the result: (A ?) = A * Q or, to put it another way: A = A * Q / 256

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Label MU7 is local to this routine

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Label MUL3 is local to this routine