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Elite on the BBC Micro

Maths (Arithmetic): SPS2, Moved [Elite-A, Flight]

Name: SPS2, Moved [Show more] Type: Subroutine Category: Maths (Arithmetic) Summary: Calculate (Y X) = A / 10
Context: See this subroutine in context in the source code References: No direct references to this subroutine in this source file

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
\ --- Mod: Original Acornsoft code moved elsewhere: ---> \.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 \ --- End of moved code ------------------------------->