Maths: Multiply16x16Bit0

       Name: Multiply16x16Bit0                                       [Show more]
       Type: Subroutine
   Category: Maths
    Summary: Multiply two 16-bit numbers that have their signs in bit 0
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * SetMatrices calls Multiply16x16Bit0
             * SetMatrixEntry calls Multiply16x16Bit0


 This routine multiplies two 16-bit numbers, both of which have their signs in
 bit 0 of the low byte. It calculates:

   (H G) = (J I) * (S R) >> 16

 The result in (H G) has the sign in bit 0 of the low byte.


 Arguments:

   (J I)                A signed 16-bit number, with the sign in bit 0 of I

   (S R)                A signed 16-bit number, with the sign in bit 0 of R


 Returns:

   (H G)                The result of the multiplication, with the sign in bit 0
                        of G

   K                    The sign of the result (in bit 0)


.Multiply16x16Bit0

 LDA R                  \ Extract the sign of (J I) * (S R) from bit 0 of I and
 EOR I                  \ bit 0 of R, and store the result in K
 AND #%00000001
 STA K

 JSR Multiply16x16      \ Calculate (H A) = (S R) * (J I) >> 16
                        \
                        \ and set the C flag if we need to increment H

 AND #%11111110         \ Set bit 0 of A to the sign we stored in K above, so
 ORA K                  \ (H A) has the correct sign of the multiplication

 STA G                  \ Set (H G) = (H A)
                        \           = (S R) * (J I) >> 16

 BCC mbit1              \ Increment the top byte in H if required
 INC H

.mbit1

 RTS                    \ Return from the subroutine