MOV Instruction in x86-64 Assembly

One of the most common instructions in x86-64 assembly is MOV, which allows data to be moved from one location to another.

MOV (commonly pronounced ‘move’) is one of the most common assembly instructions, is relatively simple to learn and is also quite powerful. It can be found in virtually all programs, and provides a great introduction to assembly syntax.

MOV is a binary operation, meaning that it takes two operands. The value of the second operand is copied to the first:

MOV <operand_1> <operand_2>

In this example, the value of operand_2 is copied into operand_1.

Note: MOV copies the value of the second operand; it doesn’t actually affect its’ value.

For that reason, MOV can be considered to perform a ‘copy’ rather than a true ‘move’. In the above example, operand_2 isn’t affected by the operation; only operand_1 is.

This article will walk you through the MOV instruction and its importance, rules and constraints, usage, and some real-world examples.

MOV Instruction Examples

Let’s analyze a few code snippets to better grasp the functionality of the MOV instruction in x86-64 assembly. We’ll cover cases of moving an integer to a register, register to register, and register to memory.

Integer to Register

The following example shows the simplest case, that of moving a value directly into a register.

MOV AX, 0x5

In this instance, the hexadecimal value of 5 will be transferred into the AX register. Hexadecimal is commonly used for data representation in assembly, because two hex digits can conveniently represent one byte. One hex digit (as in this example) is called a ‘nibble‘.

Following this MOV operation, the AX register will contain the hex value ‘5’.

Register to Register

Next, take a look at a slightly more complex example where we move data from one register to another:

MOV BX, AX

Here, the current value contained in the AX register is moved to the BX register. The BX register is unaffected.

Following this operation, the AX and BX registers will contain the same value.

This showcases the ability of the MOV instruction to facilitate data transfer between registers, a common requirement in numerous scenarios.

Register to Memory

In this example, we are moving data from a register to a memory location.

MOV [BX], AX

In this case, the value residing in the AX register is moved to a specific memory location, one that is pointed to by the BX register.

Note that we do this using square bracket notation to indicate that we are using the register contents as a pointer. The value stored in the BX register will be interpreted as a memory location.

Each of these examples demonstrates the versatility and adaptability of the MOV instruction. They illustrate the various ways in which this instruction can be used to fulfill a range of programming needs.

Constraints of the MOV instruction

The x86-64 assembly mov instruction operates within specific parameters and boundaries. It’s important to be aware of these, as they guide the effective usage of this command. To start, the source and destination data sizes must match. Whether you’re dealing with 8, 16, 32, or 64 bits, the sizes must be the same. This ensures data integrity and prevents any loss or misinterpretation of data during the move operation.

Another vital rule of the mov instruction involves memory. You can’t directly transfer data from one memory location to another using MOV. Instead, you’ll need an intermediate register to carry out this task. In other words, the data needs to move from the source memory location to a register, and then from the register to the destination memory location.

Having a firm understanding of these constraints not only helps you use the mov instruction effectively and helps prevent common errors.

Advanced Usage of the MOV Instruction

Delving further into the capabilities of the Assembly MOV instruction, it’s worth noting that its functionality extends beyond the basics. There are numerous advanced applications of the MOV instruction in the realm of x64 assembly, making it indispensable to programmers.

One common application is in system calls, or calls to other other external functions such as those in libc. These calls are performed by first moving values that correspond with the system call itself and arguments into the appropriate registers. Once the values are loaded into the registers using MOV, the call can be performed.

Another common application is general data manipulation. Using the MOV instruction, a programmer can transport data into registers or memory, and then perform other operations to modify that data as needed. MOV facilitates the handling of data structures, and it can simplify complex programming tasks. While this functionality seems obvious, it’s importance is tough to overstate, and it’s one of the reasons that MOV can be found in virtually every assembly program.

Another application of the MOV instruction lies in its interaction with control registers. Control registers hold critical system information, and MOV can be used to move data to and from these registers, providing an interface for accessing and manipulating system-level data.

The MOV instruction is also a useful tool for debugging, a key aspect of every programmer’s workflow. It allows for the transfer of data to specific locations, which can be examined to identify and resolve bugs.

Moreover, the MOV instruction can also play a role in conditional checking. By manipulating flags with MOV, we can set up conditions that influence the execution flow of your program, contributing to its dynamism and adaptability.