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BBC Micro Elite

Maths (Arithmetic): MLTU2

Name: MLTU2 [View in context] Type: Subroutine Category: Maths (Arithmetic) Summary: Calculate (A P+1 P) = (A ~P) * Q
Do the following multiplication of an unsigned 16-bit number and an unsigned 8-bit number: (A P+1 P) = (A ~P) * Q where ~P means P EOR %11111111 (i.e. P with all its bits flipped). In other words, if you wanted to calculate &1234 * &56, you would: * Set A to &12 * Set P to &34 EOR %11111111 = &CB * Set Q to &56 before calling MLTU2. This routine is like a mash-up of MU11 and FMLTU. It uses part of FMLTU's inverted argument trick to work out whether or not to do an addition, and like MU11 it sets up a counter in X to extract bits from (P+1 P). But this time we extract 16 bits from (P+1 P), so the result is a 24-bit number. The core of the algorithm is still the shift-and-add approach explained in MULT1, just with more bits. Returns: Q Q is preserved Other entry points: MLTU2-2 Set Q to X, so this calculates (A P+1 P) = (A ~P) * X
STX Q \ Store X in Q .MLTU2 EOR #%11111111 \ Flip the bits in A and rotate right, storing the LSR A \ result in P+1, so we now calculate (P+1 P) * Q STA P+1 LDA #0 \ Set A = 0 so we can start building the answer in A LDX #16 \ Set up a counter in X to count the 16 bits in (P+1 P) ROR P \ Set P = P >> 1 with bit 7 = bit 0 of A \ and C flag = bit 0 of P .MUL7 BCS MU21 \ 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 MU21 \ to just do the shifts ADC Q \ Do the addition for this bit of P: \ \ A = A + Q + C \ = A + Q ROR A \ Rotate (A P+1 P) to the right, so we capture the next ROR P+1 \ digit of the result in P+1, and extract the next digit ROR P \ of (P+1 P) in the C flag DEX \ Decrement the loop counter BNE MUL7 \ Loop back for the next bit until P has been rotated \ all the way RTS \ Return from the subroutine .MU21 LSR A \ Shift (A P+1 P) to the right, so we capture the next ROR P+1 \ digit of the result in P+1, and extract the next digit ROR P \ of (P+1 P) in the C flag DEX \ Decrement the loop counter BNE MUL7 \ Loop back for the next bit until P has been rotated \ all the way RTS \ Return from the subroutine