2. T opic: S TRING I NSTRUCTION P RESENTED B Y: N OOR FATIMA M AHA AKRAM ASIF

3.   An Instruction is binary pattern design in microprocessor to perform a specific  The entire group of instruction that the microprocessor support is called Instruction Set. Instruction Set

4.   Data Transfer Instruction.  Bit manipulation instruction.  Arithmetic instruction.  Program execution transfer instruction  String instruction  Process control insrtuction. CLASSIFICATION OF INSTRUCTION SET

5.   It is the classification of Instruction Set.  String in assembly language is just a sequentially stored bytes o words  By using these string instruction the size of the program is considerably reduced STRING INSTRUCTION

6.   A special prefix repeats the instruction in hardware called the REP Prefix  The REP Prefix allow these instruction to operate on a number of data element in one instruction  REP repeats the following string instruction CX times.  There are two REP Prefix  REPE  REPNE What is REP Prefix

7.   Firstly they all work on a block of data.  DI and SI are used to access memory. SI and DI are called Source Index and Destination Index because of string instruction. In this case override is possible.  Whenever a string instruction needs a memory destination, ES:DI holds the pointer to it. No override is possible in this case  Whenever a Byte register is needed, AL holds the value. Whenever a Word register is used AX holds the value.  If DF(Direction Flag) is Cleared, this is called auto-increment  If DF is Se t, this is called auto-decrement. EXPLANATION

8.   Cld ; clear the direction flag  Std ;set direction flag There are two Instruction

9.   STOS  LODS  SCAS  MOVS  CMPS TYPES OF STRING INSTRUCTION

10.   Stands for Store Information.  STOS transfer a byte or word from register AL or AX to string element addressed by ES:DL and update DI to point to the next location. STOS Memory AX

11. EXAMPLE OF STOS

12.   Stands for LOAD STRING.  LODS transfers a byte or word from the source location DS:SI to AL or AX and update SI to point to the next location.  LODS is generally used in a loop. LODS memory AX

13. EXAMPLE OF LODS

14.   Stands for SCAN STRING.  SCAS compares a source byte or word in register AL or AX with the destination string element addresses by ES:DI and update the flags.  The prefixes REPE (Repeat while equal) and REPNE (Repeat while not equal) are used with this instruction. SCAS memory AX OR AL Compare from

15. EXAMPLE OF SACS

16.   MOVS transfer a byte or word from the source location DS:SI to the destination ES:DI and updates SI and DI to point to the next location.  MOVS is used to move a block of memory. MOVS memory MOVE MOVE Date f

17. EXAMPLE OF MOVS

18.   Stands for COMPARE STRING.  CMPS compare two blocks of memory for equality or inequality of block.  CMPS subtracts the source location DS:SI from the destination location ES:DI.  Source and destination are unaffected. CMPS

19. EXAMPLE OF CMPS