Extra tracks: Multiply8x8Signed

       Name: Multiply8x8Signed                                       [Show more]
       Type: Subroutine
   Category: Extra tracks
    Summary: Multiply two 8-bit numbers, one of which is signed
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * CalcSegmentVector calls Multiply8x8Signed


 This routine calculates the following, retaining the sign in A.

   A = A * U / 256

 Specifically, if the last instruction to affect the N flag before the call is
 an LDA instruction, and A is signed, then set:

   A = |A| * U * abs(A)
     = A * U / 256

 So this multiplies A and U, retaining the sign in A.


.Multiply8x8Signed

 PHP                    \ Store the N flag on the stack, as set by the LDA just
                        \ before the call, so this equals abs(A)

 JMP MultiplyHeight+11  \ Jump into the MultiplyHeight routine to do this:
                        \
                        \   JSR Absolute8Bit      \ Set A = |A|
                        \
                        \   JSR Multiply8x8       \ Set (A T) = A * U
                        \                         \           = |A| * U
                        \                         \
                        \                         \ So A = |A| * U / 256
                        \
                        \   PLP                   \ Retrieve sign in N, which we
                        \                         \ set to abs(A) above
                        \
                        \   JSR Absolute8Bit      \ Set A = |A| * abs(A)
                        \                         \       = A * U / 256
                        \
                        \   RTS                   \ Return from the subroutine
                        \
                        \ So this sets A = A * U while retaining the sign in A