Maths (Geometry): SCALEY2

       Name: SCALEY2                                                 [Show more]
       Type: Subroutine
   Category: Maths (Geometry)
    Summary: Scale the y-coordinate in A to 0.75 * A
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * SCALEX calls SCALEY2
             * TT105 calls SCALEY2
             * TT14 calls SCALEY2
             * TT23 calls SCALEY2


 This routine (and the related SCALEY and SCALEX 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.

 The original source contains the comment "SCALE Scans by 3/4 to fit in".

 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


 Returns:

   C flag               The C flag is cleared


.SCALEY2

 STA T3                 ; Set A = (A / 4) - A
 LSR A                  ;       = -0.75 * A
 LSR A                  ;
 SEC                    ; This also clears the C flag, as the subtraction will
 SBC T3                 ; underflow

 EOR #$FF               ; Negate A, so A = 0.75 * A
 ADC #1                 ;
                        ; The addition works because we cleared the C flag
                        ; above, and the ADC also clears the C flag, as the
                        ; addition will never overflow

 RTS                    ; Return from the subroutine