Maths (Arithmetic): Absolute16Bit

       Name: Absolute16Bit                                           [Show more]
       Type: Subroutine
   Category: Maths (Arithmetic)
    Summary: Calculate the absolute value (modulus) of a 16-bit number
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * FollowGazeVector (Part 5 of 5) calls Absolute16Bit
             * GetPolygonLines (Part 6 of 6) calls Absolute16Bit
             * ProcessTileData calls Absolute16Bit


 This routine sets (A T) = |A T|.

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

 This routine is from Revs, Geoff Crammond's previous game.


 Arguments:

   (A T)                The number to make positive

   N flag               Controls the sign to be applied:

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

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

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

                            * N flag clear to calculate (A T) * 1

                            * N flag set to calculate (A T) * -1


.Absolute16Bit

 BPL MainTitleLoop-1    \ If the high byte in A is already positive, return from
                        \ the subroutine (as MainTitleLoop-1 contains an RTS)

                        \ Otherwise fall through into Negate16Bit to negate the
                        \ number in (A T), which will make it positive, so this
                        \ sets (A T) = |A T|