String Instructions

What are String Manipulation Instructions in 8086 microprocessors?

String Manipulation Instructions in the 8086 microprocessor architecture are the set of Instructions that are used to manipulate strings in memory. The String manipulation Instructions offer different functionality such as copying, Searching, and Modifying Strings of data. Key String manipulation instruction in 8086 instruction sets includes different instructions such as MOVSB, CMPSB, SCASB, LODSB, STOSB, and other instructions which are going to be discussed further.

Different String Manipulation Instructions

The string is a series of data bytes or words available in memory at consecutive locations. It is either referred as byte string or a word string. Their memory is always allocated in a sequential order. Instructions used to manipulate strings are called string manipulation instructions. Following is the table showing the list of string manipulation instructions:

OPCODE	OPERAND	EXPLANATION	EXAMPLE
REP	instruction	repeat the given instruction till CX != 0	REP MOVSB
REPE	instruction	repeat the given instruction while CX = 0	REPE
REPZ	instruction	repeat the given instruction while ZF = 1	REPZ
REPNE	instruction	repeat the given instruction while CX != 0	REPNE
REPNZ	instruction	repeat the given instruction while ZF = 0	REPNZ
MOVSB	none	moves contents of byte given by DS:SI into ES:DI	MOVSB
MOVSW	none	moves contents of word given by DS:SI into ES:DI	MOVSW
MOVD	none	moves contents of double word given by DS:SI into ES:DI	MOVD
LODSB	none	moves the byte at address DS:SI into AL; SI is incr/decr by 1	LODSB
LODSW	none	moves the word at address DS: SI into AX; SI is incr/decr by 2	LODSW
LODSD	none	moves the double word at address DS:SI into EAX; SI is incr/decr by 4	LODSD
STOSB	none	moves contents of AL to byte address given by ES:DI; DI is incr/dec by 1	STOSB
STOSW	none	moves the contents of AX to the word address given by ES:DI; DI is incr/decr by 2	STOSW
STOSD	none	moves contents of EAX to the DOUBLE WORD address given by ES:DI; DI is incr/decr by 4	STOSD
SCASB	none	compares byte at ES:DI with AL and sets flags according to result	SCASB
SCASW	none	compares word at ES:DI with AX and sets flags	SCASW
SCASD	none	compares double word at ES:DI with EAX and sets flags	SCASD
CMPSB	none	compares byte at ES:DI with byte at DS:SI and sets flags	CMPSB
CMPSW	none	compares word at ES:DI with word at DS:SI and sets flags	CMPSW
CMPSD	none	compares double word at ES:DI with double word at DS:SI and sets flags	CMPSD

Examples of String manipulation instructions in 8086 microprocessor

Given below are some Examples of the String manipulation instructions in 8086 microprocessor

Example of REP with MOVSB Instruction

MOV AX, 7000H    Assign source segment address to AX
MOV DS, AX          Load source segment address into DS
MOV AX, 8000H   Assign destination segment address to AX
MOV CX, 0E0H    Move the length of the string to the counter register CX
MOV SI, 3000H    Assign source index address to SI
MOV DI, 4000H   Assign destination index address to DI
CLD                       Ensure auto-increment mode is set by clearing the direction flag
REP MOVSB         Repeat the move byte from source to destination instruction CX times

Example of REPE With CMPSB Instruction

MOV AX, SEG_STRING1         Move the segment address of STRING1 to AX
MOV DS, AX                             Load it to DS
MOV AX, SEG_STRING2        Move the segment address of STRING2 to AX
MOV ES, AX                              Load it to ES
MOV SI, OFFSET_STRING1   Move the offset of STRING1 to SI
MOV DI, OFFSET_STRING2  Move the offset of STRING2 to DI
MOV CX, 020H                       Move the length of the strings to CX
CLD                                         Clear the direction flag, set auto-increment mode
REPE CMPSB                         Compare the 020H bytes of STRING1 and STRING2,While they are Equal,
                                                 If mismatch is found modify the flags and proceed with further Execution

NOTE: If Both Strings are Equal,CX becomes ZERO,the ZF is set otherwise ZF is reset

Example of REPNE With SCASW Instruction

MOV AX, SEGMENT_STR      Move the segment address of the String to AX
MOV ES, AX                             Load it to ES
MOV DI, OFFSET_STR           Move the offset of String to DI
MOV CX, 020H                        Move the length of the String to CX
MOV AL, WORD_TO_FIND   The word to be scanned for is loaded into AL
CLD                                           Clear the direction flag
REPNE SCASW                        Scan the 020H words of the String until a match to the word is found

Example of LODSB Instruction

MOV CX, 10                      Set CX to the number of bytes to be read
MOV SI, OFFSET_STR    Set SI to point to the start of the string
MOV DI, OFFSET_BUF   Set DI to point to the destination buffer
CLD                                   Clear the direction flag for auto-increment

READ_LOOP:
    LODSB                         Load a byte from the memory location pointed to by SI into AL, and increment SI
    STOSB                         Store the byte in AL at the memory location pointed to by DI, and increment DI
    LOOP READ_LOOP   Decrement CX and loop back if CX is not zero
    HLT                               Halt the processor (assembly language instruction for stopping execution)

Conclusion

In this Article, We have gone through the String manipulation in the 8086 microprocessor which provides efficiently handling operations on strings of data stored in the memory. These instructions enable us to copying, searching and comparing Function in the memory. Also we have gone through the different examples of the Instructions which provides us the clear understanding of the String manipulation instructions set.