5-1 STRING INSTRUCTIONS

The string instructions are summarized in Fig.5-1.Because the string instructions can operate on only a single byte or word unless they are used with the REP prefiks discussed in Sec. 5-2 they are often referred to as string primitives.

                 Figure 5-1 String primitives

   Name              Mnemonic and Format*    Description**

Move string             MOVS DST, SRC       ((DI)) <- ((SI))
                                            Byte operands	
                                            (SI) <- (SI)±1, (DI) <- (DI)±1
                                            Word operands
                                            (SI) <- (SI)±2, (DI) <- (DI)±2
Move byte string        MOVSB
Move word string        MOVSW

Compare string          CMPS SRC, DST       ((SI)) <- ((DI))
                                            Byte operands	
                                            (SI) <- (SI)±1, (DI) <- (DI)±1
                                            Word operands
                                            (SI) <- (SI)±2, (DI) <- (DI)±2
Compare byte string     CMPSB
Compare word string     CMPSW

Scan string             SCAS DST            Byte operand	
                                            ((AL)) - ((DI)), (DI) <- (DI)±1
                                            Word operand
                                            ((AX)) - ((DI)), (DI) <- (DI)±2
Scan byte string        SCASB
Scan word string        SCASW

Load string             LODS SRC            Byte operand	
                                            (AL) <- (SI), (SI) <- (SI)±1
                                            Word operand
                                            (AX) <- (SI), (SI) <- (SI)±2
Load byte string        LODSB
Load word string        LODSW

Store string            STOS DST            Byte operand	
                                            ((DI)) <- ((AL)), (DI) <- (DI)±1
                                            Word operand
                                            ((DI)) <- ((AX)), (DI) <- (DI)±2
Store byte string       STOSB
Store word string       STOSW

        *The B suffix indicates byte operands and the W suffix indicates.
        **Incrementing (+) is used if DF=0 and decrementing (-) is used
        if DF=1.

        Flags: CMPS and SCAS affect all condition flags and 
               MOVS,LODS and STOS affect no flags.
        Addresing modes: Operands are implied.

There are five basic primitives and eachmay appear in one of the following three forms:
Operation Operand(s)
or
OperationB
or
OperationW
In the first form is used, whether bytes or words are to be operated on is determined implicitly by the type of the operand(s).The second and third forms explicitly indicate byte and word operations, respectively.
For instance, if DF=0, then

                               MOVSB