Maths (Arithmetic): MultiplyBy1Point5

       Name: MultiplyBy1Point5                                       [Show more]
       Type: Subroutine
   Category: Maths (Arithmetic)
    Summary: Multiply a 16-bit signed number by 1.5
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * ApplySpinYaw calls MultiplyBy1Point5
             * ScaleTyreForces calls MultiplyBy1Point5


 Calculate the following:

   (A T) = (U T) * 1.5


 Arguments:

   (U T)                A 16-bit signed number


.MultiplyBy1Point5

 LDA U                  \ Set A = U, the high byte of (U T)

 CLC                    \ Clear the C flag, to use when A is positive

 BPL mulh1              \ If A is positive, jump to mulh1 to skip the following
                        \ instruction

 SEC                    \ Set the C flag, to use when A is negative

.mulh1

                        \ By this point, the C flag contains the sign bit of A
                        \ (set if negative, clear if positive)

 ROR A                  \ Shift A to the right, inserting the C flag into bit 7
                        \ to retain the correct sign

 PHA                    \ Store the result on the stack, so the stack contains
                        \ the top byte of (U T), shifted right by one place

 LDA T                  \ Set A = T, the low byte of (U T)

 ROR A                  \ Shift A to the right, so the stack and A contain the
                        \ high and low bytes of (U T) >> 1, so if S is the value
                        \ on top of the stack, we have:
                        \
                        \   (S A) = (U T) >> 1

 CLC                    \ Set (A T) = (S A) + (U T)
 ADC T                  \           = (U T) >> 1 + (U T)
 STA T                  \           = (U T) * 1.5
                        \
                        \ starting with the low bytes

 PLA                    \ And then the high bytes
 ADC U

 RTS                    \ Return from the subroutine