Maths (Arithmetic): Absolute8Bit

       Name: Absolute8Bit                                            [Show more]
       Type: Subroutine
   Category: Maths (Arithmetic)
    Summary: Calculate the absolute value (modulus) of an 8-bit number
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * ApplyElevation (Part 2 of 5) calls Absolute8Bit
             * ApplyElevation (Part 4 of 5) calls Absolute8Bit
             * ApplyGrassOrTrack calls Absolute8Bit
             * ApplyTyreForces calls Absolute8Bit
             * ApplyWingBalance calls Absolute8Bit
             * BuildRoadSign calls Absolute8Bit
             * BuildVisibleCar calls Absolute8Bit
             * CheckForCrash calls Absolute8Bit
             * CompareSegments calls Absolute8Bit
             * GetTrackAndMarkers calls Absolute8Bit
             * MovePlayerOnTrack calls Absolute8Bit
             * MultiplyHeight calls Absolute8Bit
             * ProcessOvertaking (Part 2 of 3) calls Absolute8Bit
             * SetDriverSpeed calls Absolute8Bit


 This routine returns |A|.

 It can also return A * abs(n), where A is given the sign of n.


 Arguments:

   A                    The number to make positive

   N flag               Controls the sign to be applied:

                          * If we want to calculate |A|, do an LDA or equivalent
                            before calling the routine

                          * If we want to calculate A * abs(n), do a BIT n
                            before calling the routine

                          * If we want to set the sign of A, then call with:

                            * N flag clear to calculate A * 1

                            * N flag set to calculate A * -1


.Absolute8Bit

 BPL aval1              \ If A is positive then it already contains its absolute
                        \ value, so jump to aval1 to return from the subroutine

 EOR #&FF               \ Negate the value in A using two's complement, as the
 CLC                    \ following is true when A is negative:
 ADC #1                 \
                        \   |A| = -A
                        \       = ~A + 1

.aval1

 RTS                    \ Return from the subroutine