Maths (Arithmetic): MLTU2, Removed

       Name: MLTU2, Removed                                          [Show more]
       Type: Subroutine
   Category: Maths (Arithmetic)
    Summary: Calculate (A P+1 P) = (A ~P) * Q
  Deep dive: Shift-and-add multiplication
    Context: See this subroutine in context in the source code
 References: No direct references to this subroutine in this source file


 Do the following multiplication of an unsigned 16-bit number and an unsigned
 8-bit number:

   (A P+1 P) = (A ~P) * Q

 where ~P means P EOR %11111111 (i.e. P with all its bits flipped). In other
 words, if you wanted to calculate &1234 * &56, you would:

   * Set A to &12
   * Set P to &34 EOR %11111111 = &CB
   * Set Q to &56

 before calling MLTU2.

 This routine is like a mash-up of MU11 and FMLTU. It uses part of FMLTU's
 inverted argument trick to work out whether or not to do an addition, and like
 MU11 it sets up a counter in X to extract bits from (P+1 P). But this time we
 extract 16 bits from (P+1 P), so the result is a 24-bit number. The core of
 the algorithm is still the shift-and-add approach explained in MULT1, just
 with more bits.


 Returns:

   Q                    Q is preserved


 Other entry points:

   MLTU2-2              Set Q to X, so this calculates (A P+1 P) = (A ~P) * X


                        \ --- Mod: Code removed for Elite-A: ------------------>

\ STX Q                 \ Store X in Q
\
\.MLTU2
\
\ EOR #%11111111        \ Flip the bits in A and rotate right, storing the
\ LSR A                 \ result in P+1, so we now calculate (P+1 P) * Q
\ STA P+1
\
\ LDA #0                \ Set A = 0 so we can start building the answer in A
\
\ LDX #16               \ Set up a counter in X to count the 16 bits in (P+1 P)
\
\ ROR P                 \ Set P = P >> 1 with bit 7 = bit 0 of A
\                       \ and C flag = bit 0 of P
\
\.MUL7
\
\ BCS MU21              \ If C (i.e. the next bit from P) is set, do not do the
\                       \ addition for this bit of P, and instead skip to MU21
\                       \ to just do the shifts
\
\ ADC Q                 \ Do the addition for this bit of P:
\                       \
\                       \   A = A + Q + C
\                       \     = A + Q
\
\ ROR A                 \ Rotate (A P+1 P) to the right, so we capture the next
\ ROR P+1               \ digit of the result in P+1, and extract the next digit
\ ROR P                 \ of (P+1 P) in the C flag
\
\ DEX                   \ Decrement the loop counter
\
\ BNE MUL7              \ Loop back for the next bit until P has been rotated
\                       \ all the way
\
\ RTS                   \ Return from the subroutine
\
\.MU21
\
\ LSR A                 \ Shift (A P+1 P) to the right, so we capture the next
\ ROR P+1               \ digit of the result in P+1, and extract the next digit
\ ROR P                 \ of (P+1 P) in the C flag
\
\ DEX                   \ Decrement the loop counter
\
\ BNE MUL7              \ Loop back for the next bit until P has been rotated
\                       \ all the way
\
\ RTS                   \ Return from the subroutine

                        \ --- End of removed code ----------------------------->