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What is mov in Assembly?

Published in Assembly Instruction 4 mins read

The mov instruction is a fundamental operation in assembly language used to move data between different locations accessible by the CPU.

As noted in the reference, the mov instruction copies data from one location to another. It is arguably one of the most frequently used instructions in assembly programming because it is the primary way to initialize or change the value of registers and memory locations.

How the mov Instruction Works

At its core, mov performs a simple data transfer. It takes a source operand and a destination operand. The value of the source operand is copied into the destination operand. It's important to note that this is a copy operation, not a move in the sense of cutting and pasting; the source operand retains its original value.

The general syntax typically looks like this (depending on the specific assembly syntax, e.g., Intel vs. AT&T):

mov destination, source

The size of the data being moved (e.g., 8-bit byte, 16-bit word, 32-bit doubleword, 64-bit quadword) is usually inferred by the destination operand's size (if it's a register) or explicitly specified.

Operands: Where Data Can Be Moved

The mov instruction can work with several types of locations:

  • Registers: High-speed storage areas directly on the CPU.
  • Memory Locations: Addresses in the main system memory (RAM).
  • Immediate Values: Literal, constant data hardcoded directly into the instruction.

However, mov cannot directly copy data between two memory locations in a single instruction on most architectures. Data typically needs to be moved from memory into a register, and then from the register into another memory location. Similarly, it cannot move data directly from one immediate value to another (which doesn't make sense anyway).

Valid Operand Combinations

Here's a simplified look at common valid mov operand combinations:

Destination Source Description Example (Intel Syntax, 32-bit)
Register Immediate Value Load a constant value into a register mov eax, 12345h
Register Register Copy value from one register to another mov ebx, eax
Register Memory Location Load value from memory into a register mov ecx, [some_address]
Memory Location Register Store value from a register into memory mov [another_address], edx
Memory Location Immediate Value Store a constant value into memory mov [yet_another_address], 0

Note: The examples above use 32-bit registers (like EAX) and assume memory addresses are being referred to.

Why mov is Essential

The mov instruction is the bedrock of many assembly operations because:

  • Initialization: It's used to set initial values in registers and memory.
  • Data Loading: It loads data from memory into registers for processing (arithmetic, logical operations, etc.).
  • Data Saving: It saves results from registers back into memory.
  • Argument Passing: It's often used to place function arguments into specific registers or memory locations before calling a function.
  • Return Values: It can be used to place function return values into registers.

Without the ability to move data between these locations, the CPU couldn't access the data it needs to process or store the results of its operations.

Example Usage

Let's look at a tiny snippet illustrating mov:

section .data
    my_variable dw 10 ; Define a word variable in memory, initialized to 10

section .text
    global _start

_start:
    mov ax, [my_variable] ; Move the value from my_variable (memory) into the AX register
                          ; AX now holds the value 10

    mov bx, 5             ; Move the immediate value 5 into the BX register
                          ; BX now holds the value 5

    add ax, bx            ; Add the value in BX to AX (AX becomes 15)

    mov [my_variable], ax ; Move the new value from AX back into my_variable (memory)
                          ; my_variable in memory now holds the value 15

    ; ... rest of the program (e.g., exit)

In this simple sequence, mov is used three times: to load data from memory into a register, to load an immediate value into a register, and to store data from a register into memory.

In summary, mov is the fundamental data transfer instruction in assembly, indispensable for managing data flow between registers, memory, and immediate values, enabling all subsequent computational tasks.

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