6. Extended-Precision Integers and Shifting Multiple Doublewords

• Extended-precision integers can be stored as divided into an array of doublewords.

• The low-order doubleword is stored at the lowest address (using little-endian order).

• The following steps, for example, show how to shift an integer named dw_array one bit to the right:

      .CONST
          ARRAY_SIZE = 3
      .DATA           ; Begin data segment
          dw_array    DWORD   ARRAY_SIZE  dup(99999999h)  ; 1001 pattern...
      .CODE           ; Begin code segment
      ; Shift doublewords 1 bit to the right:
          mov esi, 0                     ; 10011001... Original pattern
          shr dw_array[ esi + 8 ], 1     ; 01001100... highest dword -> CF
          rcr dw_array[ esi + 4 ], 1     ; 11001100... CF -> middle dword -> CF
          rcr dw_array[ esi ], 1         ; 11001100... CF -> lowest dword -> CF
  

• Shifting any operand right by n bits divides the operand by 2ⁿ.

• The steps are:

  1. Set ESI to the offset of array.

  2. The high-order doubleword at [ ESI + 8 ] is shifted right,
     its lowest bit is copied into the Carry flag:

      

  3. The value at [ ESI + 4 ] is shifted right,

     1. the highest bit is filled from the Carry flag,

     2. the lowest bit is copied back into the Carry flag:

      

  4. The low-order doubleword at [ ESI ] is shifted right in a similar way.

  5. At the end, carry flag CF can be inspected for the remainder of the division.