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BBC Micro Elite

Maths (Arithmetic): ROOT

Name: ROOT [View in context] Type: Subroutine Category: Maths (Arithmetic) Summary: Calculate ZP = SQRT(ZP(1 0))
Calculate the following square root: ZP = SQRT(ZP(1 0)) This routine is identical to LL5 in the main game code - it even has the same label names. The only difference is that LL5 calculates Q = SQRT(R Q), but apart from the variables used, the instructions are identical, so see the LL5 routine in the main game code for more details on the algorithm used here.
.ROOT LDY ZP+1 \ Set (Y Q) = ZP(1 0) LDA ZP STA Q \ So now to calculate ZP = SQRT(Y Q) LDX #0 \ Set X = 0, to hold the remainder STX ZP \ Set ZP = 0, to hold the result LDA #8 \ Set P = 8, to use as a loop counter STA P .LL6 CPX ZP \ If X < ZP, jump to LL7 BCC LL7 BNE LL8 \ If X > ZP, jump to LL8 CPY #64 \ If Y < 64, jump to LL7 with the C flag clear, BCC LL7 \ otherwise fall through into LL8 with the C flag set .LL8 TYA \ Set Y = Y - 64 SBC #64 \ TAY \ This subtraction will work as we know C is set from \ the BCC above, and the result will not underflow as we \ already checked that Y >= 64, so the C flag is also \ set for the next subtraction TXA \ Set X = X - ZP SBC ZP TAX .LL7 ROL ZP \ Shift the result in Q to the left, shifting the C flag \ into bit 0 and bit 7 into the C flag ASL Q \ Shift the dividend in (Y S) to the left, inserting TYA \ bit 7 from above into bit 0 ROL A TAY TXA \ Shift the remainder in X to the left ROL A TAX ASL Q \ Shift the dividend in (Y S) to the left TYA ROL A TAY TXA \ Shift the remainder in X to the left ROL A TAX DEC P \ Decrement the loop counter BNE LL6 \ Loop back to LL6 until we have done 8 loops RTS \ Return from the subroutine