SCASB            Scan String Byte                    Flags: O D I T S Z A P C
                                                            *       * * * * *
SCASB

          Logic:    CMP AL, (ES:DI)         ; Sets flags only
                    if DF = 0
                        DI . DI + 1
                    else
                        DI . DI - 1

    This instruction compares two bytes by subtracting the destination
    byte, pointed to by ES:DI, from AL. SCASB sets the flags according to
    the results of the comparison. The operands themselves are not
    altered. After the comparison, DI is incremented (if the direction
    flag is cleared) or decremented (if the direction flag is set), in
    preparation for comparing the next byte.

  --------------------------------------------------------------------------
   Operands                  Clocks   Transfers  Bytes   Example
      -                        15         1        1     SCASB
   (repeat)                9 + 15/rep   1/rep      1     REPNE SCASB
  --------------------------------------------------------------------------

       Notes:         SCAS is useful for searching a block for a given
                      byte or word value. Use CMPS, Compare String, if you
                      wish to compare two strings (or blocks) in memory,
                      element by element.

  -------------------------------- Example ---------------------------------

    The following example searches the 100-byte block starting at LOST_A
    for the character 'A' (65 decimal).

          MOV     AX, DS
          MOV     ES, AX          ;SCAS uses ES:DI, so copy DS to ES
          CLD                     ;Scan in the forward direction
          MOV     AL, 'A'         ;Searching for the lost 'A'
          MOV     CX,100          ;Scanning 100 bytes (CX is used by REPNE)
          LEA     DI, LOST_A      ;Starting address to DI
  REPNE   SCASB                   ;   ...and scan it.
          JE      FOUND           ;The Zero Flag will be set if we found
                                  ;   a match.
  NOTFOUND:       .               ;If we get here, no match was found
                  .
                  .
  FOUND:  DEC     DI              ;Back up DI so it points to the first
                  .               ;   matching 'A'
                  .

    Upon exit from the REPNE SCASB loop, the Zero Flag will be set if a
    match was found, and cleared otherwise. If a match was found, DI will
    be pointing one byte past the match location; the DEC DI at FOUND
    takes care of this problem.