Maths (Arithmetic): Multiply8x16Signed

       Name: Multiply8x16Signed                                      [Show more]
       Type: Subroutine
   Category: Maths (Arithmetic)
    Summary: Multiply an 8-bit and a 16-bit number and apply a sign to the
             result
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * ApplySpinYaw calls Multiply8x16Signed
             * ScaleTyreForces calls Multiply8x16Signed


 This routine calculates:

   (A T) = (A T) * abs(A) * Y / 256

 where A is the last variable to be loaded before the subroutine call. So if
 the call follows an LDA instruction, for example, the following is calculated
 if A is positive:

   (A T) = (A T) * Y / 256

 and the following is calculated if A is negative:

   (A T) = -(A T) * Y / 256


 Arguments:

   (A T)                A 16-bit signed number

   Y                    An unsigned number

   N flag               Controls the sign to be applied:

                          * N flag clear to calculate (A T) * Y / 256

                          * N flag set to calculate -(A T) * Y / 256


.Multiply8x16Signed

 PHP                    \ Store the N flag on the stack

 JSR Absolute16Bit      \ Set the sign of (A T) to that of the N flag argument

 STA V                  \ Set (V T) = (A T)

 STY U                  \ Set U = Y

 JSR Multiply8x16       \ Set (U T) = U * (V T) / 256
                        \           = Y * (A T) / 256

 LDA U                  \ Set (A T) = (U T)
                        \           = Y * (A T) / 256

 PLP                    \ Retrieve the N flag from the stack

 JSR Absolute16Bit      \ Set the sign of (A T) to that of the N flag argument

 RTS                    \ Return from the subroutine