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Elite on the BBC Micro

Maths (Arithmetic): SQUA2

[6502 Second Processor version, Loader 1]

Name: SQUA2 [Show more] Type: Subroutine Category: Maths (Arithmetic) Summary: Calculate (A P) = A * A Deep dive: Shift-and-add multiplication
Context: See this subroutine in context in the source code References: This subroutine is called as follows: * PLL1 (Part 1 of 3) calls SQUA2 * PLL1 (Part 2 of 3) calls SQUA2 * PLL1 (Part 3 of 3) calls SQUA2

Do the following multiplication of signed 8-bit numbers: (A P) = A * A This uses a similar approach to routine SQUA2 in the main game code, which itself uses the MU11 routine to do the multiplication. However, this version first ensures that A is positive, so it can support signed numbers.
.SQUA2 BPL SQUA \ If A > 0, jump to SQUA EOR #&FF \ Otherwise we need to negate A for the SQUA algorithm CLC \ to work, so we do this using two's complement, by ADC #1 \ setting A = ~A + 1 .SQUA STA Q \ Set Q = A and P = A STA P \ Set P = A LDA #0 \ Set A = 0 so we can start building the answer in A LDY #8 \ Set up a counter in Y to count the 8 bits in P LSR P \ Set P = P >> 1 \ and C flag = bit 0 of P .SQL1 BCC SQ1 \ If C (i.e. the next bit from P) is set, do the CLC \ addition for this bit of P: ADC Q \ \ A = A + Q .SQ1 ROR A \ Shift A right to catch the next digit of our result, \ which the next ROR sticks into the left end of P while \ also extracting the next bit of P ROR P \ Add the overspill from shifting A to the right onto \ the start of P, and shift P right to fetch the next \ bit for the calculation into the C flag DEY \ Decrement the loop counter BNE SQL1 \ Loop back for the next bit until P has been rotated \ all the way RTS \ Return from the subroutine