Maths (Geometry): SCALEX

       Name: SCALEX                                                  [Show more]
       Type: Subroutine
   Category: Maths (Geometry)
    Summary: Scale the x-coordinate in A to 32 + 0.75 * A
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * TT103 calls SCALEX
             * TT14 calls SCALEX
             * TT22 calls SCALEX


 This routine (and the related SCALEY and SCALEY2 routines) are called from
 various places in the code to scale the value in A. This code is different in
 the Apple II and BBC Master versions, and allows coordinates to be scaled
 correctly on different platforms.

 In the Apple version, the scaling routines scale screen coordinates in the
 system charts by 3/4 in each direction. This enables the charts to fit into
 the smaller screen size  when compared to the BBC Micro versions, while still
 leaving the dashboard on-screen (the Commodore 64 version also has a smaller
 screen, but it removes the dashboard from the screen to make more room).

 The scaling routines do the following:

   * SCALEX scales an x-coordinate to 32 + 0.75 * x

   * SCALEY scales a y-coordinate to 0.375 * y

   * SCALEY2 scales a y-coordinate to 0.75 * y


.SCALEX

 JSR SCALEY2            ; Set A = 0.75 * A

 ADC #32                ; Set A = A + 32
                        ;       = 32 + 0.75 * A
                        ;
                        ; The addition works because SCALEY2 clears the C flag

 RTS                    ; Return from the subroutine