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What is the REP prefix instruction?

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Eleanor Gray

Published Feb 28, 2026

What is the REP prefix instruction?

REP is a prefix written before one of the string instructions. It is used for repeating an instruction count number of times, where count is stored in the CX register. After every operation the CX register is decremented and the zero flag is tested; the process continues till CX = 0.

Furthermore, is Rep a prefix?

The REP (repeat), REPE (repeat while equal), REPNE (repeat while not equal), REPZ (repeat while zero), and REPNZ (repeat while not zero) mnemonics are prefixes that can be added to one of the string instructions. The REP prefixes apply only to one string instruction at a time.

One may also ask, which is a prefix instruction? Here is a list of instruction prefixes: Section override prefixes cs, ds, ss, es, fs, gs. These are automatically added by specifying using the section:memory-operand form for memory references. The bus lock prefix lock inhibits interrupts during execution of the instruction it precedes.

Secondly, what does Rep instructions do?

Description. Use the rep (repeat while equal), repnz (repeat while nonzero) or repz (repeat while zero) prefixes in conjunction with string operations. Each prefix causes the associated string instruction to repeat until the count register (CX) or the zero flag (ZF) matches a tested condition.

What is Rep Assembly?

A representative assembly is a political institution in which a number of persons representing the population or privileged orders within the population of a state come together to debate, negotiate with the executive (originally the king or other ruler) and legislate.

What is Rep prefix explain with an example?

REP is a prefix written before one of the string instructions. It is used for repeating an instruction count number of times, where count is stored in the CX register.

Is Rep short for represent?

Rep is short for representative: His rep said he would return to work immediately.

What is reps word?

: reputation especially : status in a group (such as a gang) rep. noun (2) Definition of rep (Entry 2 of 9) : representative sales reps.

What does Rep mean in gaming?

Rep means reputation. Every time you get 25 posts in the forums you'll receive one reputation power. It takes 100 reputation points earned from others to gain one reputation power.

What does Rep mean in steam?

it means positive reputation. its always a good thing to have, especially in the TF2 trade market.it means your trustworthy and friendly, and people will trade with you more often.

How do you use rep in a sentence?

Use “rep” in a sentence | “rep” sentence examples

You need to speak to the students' rep.
I'd been working as a sales rep for a photographic company.
Darren has been working as a rep.
The company's marketing rep was giving out pens and mugs – the usual freebies.
The play was premiered at the Birmingham Rep in 1999.

What are the call and ret instructions and explain how they work?

Two instructions control the use of assembly-language procedures: CALL pushes the return address onto the stack and transfers control to a procedure. RET pops the return address off the stack and returns control to that location.

What is Scasb instruction?

The SCAS instruction is used for searching a particular character or set of characters in a string. The data item to be searched should be in AL (for SCASB), AX (for SCASW) or EAX (for SCASD) registers. The string to be searched should be in memory and pointed by the ES:DI (or EDI) register.

What is Movsb?

The MOVSB (move string, byte) instruction fetches the byte at address SI, stores it at address DI and then increments or decrements the SI and DI registers by one.

Why REP prefix is generally not used with LODS instruction?

LODS is generally used in a loop and not with the REP prefix since the value previously loaded in the register is overwritten if the instruction is repeated and only the last value of the block remains in the register.

How do I use Cmpsb?

The CMPSB(W) instruction can be used to compare a byte(word) in one string (DS:offset in SI) with a byte (word) in another string (ES:offset in DI). The comparison is executed by subtracting the byte (word) in DI from the byte (word) in SI.

What is machine language instruction format?

• A machine language instruction format has one or more number of fields associated with it. •The first field is called as operation code field or op-code field, which indicates the. type of operation to be performed by the CPU.

What is LDS instruction?

This instruction copies a word from two memory locations into the register specified in the instruction. It then copies a word from the next two memory locations into the DS(ES) register. It is useful for pointing to SI(DI) and DS(ES) at the start of a string before using a string instruction.

What is STD in assembly language?

STD: std is used to set the direction flag to a 1 so that SI and/or DI will automatically be decremented to point to the next string element when one of the string instruction executes. If the direction flag is set SI/DI will be decremented by 1 for byte strings and 2 for word strings.

What is loop instruction in assembly language?

The LOOP instruction assumes that the ECX register contains the loop count. When the loop instruction is executed, the ECX register is decremented and the control jumps to the target label, until the ECX register value, i.e., the counter reaches the value zero.

What is the lock prefix?

The LOCK prefix is typically used with the BTS instruction to perform a read-modify-write operation on a memory location in shared memory environment. The integrity of the LOCK prefix is not affected by the alignment of the memory field. Memory locking is observed for arbitrarily misaligned fields.

What is Vex encoding?

The VEX prefix (from "vector extensions") and VEX coding scheme are comprising an extension to the x86 and x86-64 instruction set architecture for microprocessors from Intel, AMD and others.

What is Repz RETQ?

So, repz is a prefix that repeats the following instruction until some register is 0. Also, it only works on string instructions; otherwise the behavior is undefined.

What is segment override prefix in 8086?

A segment override prefix allows any segment register (DS, ES, SS, or CS) to be used as the segment when evaluating addresses in an instruction.

How do you give rotation count in rotate instructions if count is greater than 1?

If number of shifts are greater than 1 then shift count must be loaded in CL register and CL must be placed in the count position of the instruction. ROR Instruction : ROR destination, count. This Rotate Instruction in 8086 with example all bits in a specified byte or word to the left some number of bit positions.

How many bytes are used to encode x86 instructions?

An x86-64 instruction may be at most 15 bytes in length. It consists of the following components in the given order, where the prefixes are at the least-significant (lowest) address in memory: Legacy prefixes (1-4 bytes, optional) Opcode with prefixes (1-4 bytes, required)

What is difference between div and IDIV?

The DIV instruction divides unsigned numbers, and IDIV divides signed numbers. The dividend is the number to be divided, and the divisor is the number to divide by. The quotient is the result. The divisor can be in any register or memory location but cannot be an immediate value.

Which is the oldest representative assembly in the world?

The oldest representative assembly in the world is House of Commons.

Which is the first example of a representative assembly?

In Jamestown, Virginia, the first elected legislative assembly in the New World—the House of Burgesses—convenes in the choir of the town's church.

What is test EAX EAX?

test is like bitwise and except it only sets the flags. Anding a value with itself gives the same value, so test eax, eax sets the flags based on whatever eax contains. ZF is set when the result of an operation is zero. jne jumps when ZF is not set.

How does test work in assembly?

The TEST instruction performs an implied AND operation between corresponding bits in the two operands and sets the flags without modifying either operand. reg, mem, and immed can be 8, 16, or 32 bits. The CMP instruction sets the flags as if it had performed subtraction on the operand.

What is Movzbl Assembly?

? so movzbl means: move a byte with zero extension into 32-bit (“l”) register; last byte of. register is that byte, all others are zero. ? (zero extension means to put zeroes into the remaining unused bytes of the register)

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