String manipulation instructions in 8086 microprocessor
In this Article, We will be going through the String Manipulation Instruction, We will start our Article with the Introduction of the String Manipulation Instruction in the 8086, Then We will go through the different String Manipulation Instructions with some Examples, and At last, we will conclude our Article with Some FAQs.
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.
FAQs on String manipulation instructions in 8086 microprocessor
How are string manipulation instructions used in assembly language programming?
In assembly language programming, string manipulation instructions are used to implement various string operations directly at the machine level. Programmers can combine these instructions with control flow structures and other operations to implement complex string manipulation algorithms.
Can string manipulation instructions handle strings of variable length?
Yes, string manipulation instructions are designed to handle strings of variable length.
What are some examples of string manipulation instructions in the 8086 microprocessor?
Examples of string manipulation instructions include MOVSB, CMPSB, SCASB, LODSB, STOSB and other Instructions.
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