Maths (Arithmetic): Scale16Bit

       Name: Scale16Bit                                              [Show more]
       Type: Subroutine
   Category: Maths (Arithmetic)
    Summary: Scale up a 16-bit value by 2^5
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * ApplySkidForces calls Scale16Bit
             * GetTyreForces calls Scale16Bit


 Scale up a 16-bit value by 2^5, capping the result at the maximum possible
 positive value of &7Fxx, and ensuring that the result is positive.


 Arguments:

   Y                    The offset of the variable to scale

                          * 8 = xVelocity

                          * 9 = zVelocity


 Returns:

   (NN MM)              The scaled value


.Scale16Bit

 LDA xPlayerSpeedHi,Y   \ Set (A MM) to the variable pointed to by Y, which we
 STA MM                 \ call variableY
 LDA xPlayerSpeedTop,Y

 BPL scal1              \ If the top byte in A is positive, jump to scal1 to
                        \ skip the following

 LDA #0                 \ Negate (A MM), starting with the low bytes
 SEC
 SBC MM
 STA MM

 LDA #0                 \ And then the high bytes, so we now have:
 SBC xPlayerSpeedTop,Y  \
                        \   (A MM) = |variableY|

.scal1

 LDY #5                 \ Set Y = 5, to act as a shift counter in the following
                        \ loop

.scal2

 ASL MM                 \ Set (A MM) = (A MM) << 1
 ROL A

 BMI scal4              \ If bit 7 of the top byte in A is set, jump to scal4
                        \ to stop shifting and set the top byte to the largest
                        \ possible positive top byte

 DEY                    \ Decrement the shift counter

 BNE scal2              \ Loop back until we have left-shifted five times

.scal3

 STA NN                 \ Set (NN MM) = (A MM) to return as the result

 RTS                    \ Return from the subroutine

.scal4

 LDA #%01111111         \ Set A = %01111111 to act as the largest possible
                        \ positive top byte

 BNE scal3              \ Jump to scal3 to return (NN MM)