What is REPE in assembly language?
REPE and REPZ are mnemonics for the same prefix; they stand for Repeat if Equal and Repeat if Zero respectively. REPE/REPZ causes the succeeding string instruction to be repeated as long as the comapred bytes or words are equal (ZF = 1) and CX is not yet counted down to zero.
What is Rep x86 assembly?
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 the difference between Rep and REPE?
REP/REPE/REPZ/REPNE /REPNZ–Repeat String Operation 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.
What is the function of different types of Rep prefix?
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.
What does the REP prefix accomplish and what type of instruction is it used with?
REP is an operational repeat prefix coded before the string instructions MOVS and STOS. Load the CX with a count prior to execution. For each execution of the string instruction, REP decrements CX by 1 and repeats the operation until the CX is 0, at which point processing continues with the next instruction.
What is Movsb microprocessor?
The MOVSB instruction tells the assembler to move data as bytes; the MOVSW implies the string is to be moved as words.
How do I use Scasb?
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. found!
What is Jnz in assembly language?
The JNZ instruction transfers control to the specified address if the value in the accumulator is not 0. If the accumulator has a value of 0, the next instruction is executed. Neither the accumulator nor any flags are modified by this instruction.
What is Repne Scasb?
A common use of the REPNE SCASB instruction is to determine the length of a string.
What is microprocessor string?
MicroprocessorMicrocontroller8086. String is a group of bytes/words and their memory is always allocated in a sequential order. String is either referred as byte string or word string. Here we will see some instructions which are used to manipulate the string related operations.
How do I use Movsb?
- The MOVSB instruction. 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.
- The MOVSW instruction.
- The MOVSD instruction.
- REP Prefix.
What is the use of 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.
What is Lodsb Assembly?
The size of the source and destination operands is selected with the mnemonic: LODSB (byte loaded into register AL), LODSW (word loaded into AX), or LODSD (doubleword loaded into EAX).
What is JZ in assembly language?
jz is “jump if zero”. cmp subtracts its two operands, and sets flags accordingly. (See here for reference.) If the two operands are equal, the subtraction will result in zero and the ZF flag will be set.
What is STD in microprocessor?
This instruction is used to set the Direction Flag to 1 so that SI/DI will automatically decrement when one of the string instructions executes.
What is microprocessor CLD?
Description. Clears the direction flag; affects no other flags or registers. Causes all subsequent string operations to increment the index registers, (E)SI and/or (E)DI, used during the operation.
What is Stosb microprocessor?
The STOS instruction copies a byte (word) from AL (AX) to a memory location stored in [ES:DI]. DI is automatically incremented after the copy, the DF determines the increment/decrement. In effect, it replaces a string element with a byte (word) from AL (AX).
Why is x86 assembly language programming so messy?
The topic of x86 assembly language programming is messy because: There are many different assemblers out there: MASM, NASM, gas, as86, TASM, a86, Terse, etc. All use radically different assembly languages.
What are the different types of assemblers and assembly languages?
There are many different assemblers out there: MASM, NASM, gas, as86, TASM, a86, Terse, etc. All use radically different assembly languages. There are differences in the way you have to code for Linux, OS/X, Windows, etc. Many different object file formats exist: ELF, COFF, Win32, OMF, a.out for Linux, a.out for FreeBSD, rdf, IEEE-695, as86, etc.
What is the x86 architecture?
The x86 architecture is the most popular architecture for desktop and laptop computers. Let’s see how we can program in assembly language for processors in this family. This document contains very brief examples of assembly language programs for the x86.
Are there any notes on DOS assembly language programs?
We will even include a section on DOS assembly language programs for historical interest. These notes are not intended to be a substitute for the documentation that accompanies the processor and the assemblers, nor is it intended to teach you assembly language.