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BBC Micro Elite

Maths (Arithmetic): LL5

Name: LL5 [View in context] Type: Subroutine Category: Maths (Arithmetic) Summary: Calculate Q = SQRT(R Q)
Calculate the following square root: Q = SQRT(R Q)
.LL5 { LDY R \ Set (Y S) = (R Q) LDA Q STA S \ So now to calculate Q = SQRT(Y S) LDX #0 \ Set X = 0, to hold the remainder STX Q \ Set Q = 0, to hold the result LDA #8 \ Set T = 8, to use as a loop counter STA T .LL6 CPX Q \ If X < Q, jump to LL7 BCC LL7 BNE LL8 \ If X > Q, 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 - Q SBC Q TAX .LL7 ROL Q \ Shift the result in Q to the left, shifting the C flag \ into bit 0 and bit 7 into the C flag ASL S \ 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 S \ 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 T \ Decrement the loop counter BNE LL6 \ Loop back to LL6 until we have done 8 loops RTS \ Return from the subroutine }