String Instruction in 8086

String Instruction in 8086:

The String Instruction in 8086 are namely,

REP/REPE/REPZ/REPNE/REPNZ
MOVS/MOVSB/MOVSN
CMPS/CMPSB/CMPSW
SCAS/SCASB/SCASW
LODS/LODSB/LODSW
STOS/STOSB/STOSW

REP/REPE/REPZ/REPNE/REPNZ Instruction :

REP is a prefix which is written before one of the String Instruction in 8086. These instructions repeat until specified condition exists.

MOVS/MOVSB/MOVSW Instruction :

This instruction copies a byte or word from a location in the data segment to a location in the extra segment. The offset of the source byte or word in the data segment must be in the SI register. The offset of the destination in the extra segment must be contained in the DI register. For multiple byte or multiple word moves the number of elements to be moved is put in the CX register so that it can function as a counter. After the byte or word is moved SI and DI are automatically adjusted to point to the next source and the next destination. If the direction flag is 0, then SI and DI will be incremented by 1 after a byte move and they will incremented by 2 after a word move. If the DF is a 1, then SI and DI will be decremented by 1 after a byte move and they will be decremented by 2 after a word move. MOVS affects no flags.

The way to tell the assembler whether to code the instruction for a byte or word move is to add a “B” or a “W” to the MOVS mnemonic. MOVSB, for example, says move a string as bytes. MOVSW says move a string as words.

After move SI will be one greater than offset of last byte in source string. DI will be one greater than offset of last byte of destination string. CX will be 0.

CMPS/CMPSB/CMPSW Instruction :

A String Instruction in 8086 is a series of the same type of data items in sequential memory locations. The CMPS instruction can be used to compare a byte in one string with a byte in another string or to compare a word in one string with a word in another string. SI is used to hold the offset of a byte or word in the source string and DI is used to hold the offset of a byte or a word in the other string. The comparison is done by subtracting the byte or word pointed to by DI from the byte or word pointed to by SI. The AF, CF, OF, PF, SF, and ZF flags are affected by the comparison, but neither operand is affected.

After the comparison SI and DI will be automatically incremented or decremented according to direction flag to point to the next element in the two strings (if DF = 0, SI and DI ↑ ) CX functions as a counter which is decremented after each comparison. This will go on until CX = 0

SCAS/SCASB/SCASW Instruction :

SCAS compares a string byte with a byte in AL or a string word with word in AX. The instruction affects the flags, but it does not change either the operand in AL (AX) or the operand in the String Instruction in 8086. The string to be ‘scanned must be in the extra segment and DI must contain the offset of the byte or the word to be compared.

After the comparison DI will be automatically incremented or decremented according to direction flag, to point to the next element in the two strings (if DF = 0, SI and DI ↑ ) CX functions as a counter which is decremented after each comparison. This will go on until CX = 0. SCAS affects the AF, CF, OF, PF, SF and ZF flags.

SCASB says compare String Instruction in 8086 as bytes and SCASW says compare strings as words.

LODS/LODSB/LODSW Instruction :

This instruction copies a byte from a string location pointed to by SI to AL, or a word from a string location pointed to by SI to AX. LODS does not affect any flags. LODSB copies byte and LODSW copies a word.

STOS/STOSB/STOSW Instruction :

The STOS instruction copies a byte from AL or a word from AX to a memory location in the extra segment. DI is used to hold the offset of the memory location in the extra segment. After the copy, DI is automatically incremented or decremented to point to the next string element in memory. If the direction flag, DF, is cleared, then DI will automatically be incremented by one for a byte string or incremented by two for a word String Instruction in 8086. If the direction flag is set, DI will be automatically decremented by ono for a byte string or decremented by two for a word string. STOS does not affect any flags. STOSB copies byte and STOSW copies a word.