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Elite on the BBC Micro

Maths (Arithmetic): SPS2 (Disc version)

Name: SPS2 [View in context] Type: Subroutine Category: Maths (Arithmetic) Summary: Calculate (Y X) = A / 10
Calculate the following, where A is a signed 8-bit integer and the result is a signed 16-bit integer: (Y X) = A / 10 Returns: C flag The C flag is cleared
.SPS2 ASL A \ Set X = |A| * 2, and set the C flag to the sign bit of TAX \ A LDA #0 \ Set Y to have the sign bit from A in bit 7, with the ROR A \ rest of its bits zeroed, so Y now contains the sign of TAY \ the original argument LDA #20 \ Set Q = 20 STA Q TXA \ Copy X into A, so A now contains the argument A * 2 JSR DVID4 \ Calculate the following: \ \ P = A / Q \ = |argument A| * 2 / 20 \ = |argument A| / 10 LDX P \ Set X to the result TYA \ If the sign of the original argument A is negative, BMI LL163 \ jump to LL163 to flip the sign of the result LDY #0 \ Set the high byte of the result to 0, as the result is \ positive RTS \ Return from the subroutine .LL163 LDY #&FF \ The result is negative, so set the high byte to &FF TXA \ Flip the low byte and add 1 to get the negated low EOR #&FF \ byte, using two's complement TAX INX RTS \ Return from the subroutine