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What are addressing modes in assembly language?

Rukhshan Haroon

Using different addressing modes, programmers can choose to reference data differently, as required by the program.

Most assembly instructions that require operands follow this syntax, where inst is the instruction name, dest denotes the destination, and src denotes the source:

inst dest src 

However, the operand may reside in memory, a register, or may be a constant. Depending on the type of operand an instruction is operating on, addressing in assembly language can be performed in three modes, as shown in the table below:

Mode

Operand Description

Register Addressing

Stored in a register.

Memory Addressing

Resides in memory

Immediate Addressing*

Is a constant or expression

Register addressing

The following mov instruction makes uses of register addressing to move the data referred to by esp into ecx:

mov ecx, esp

The addressing mode used here is register addressing as the destination operand is a register.

Another example of register addressing would be the inc instruction, which increments the value of a specified register.

inc eax

Memory addressing

In memory addressing, the second operand is a memory location and is usually represented by a variable name. Subsequently, this addressing mode requires access to the data segment of the program. Here is an example of a mov instruction that copies data items from the variable bill stored in the data segment to the register eax.

mov eax, bill

Immediate addressing

In immediate addressing, the second operand of the instruction is a constant. The first operand may reside in a register or the memory segment.

The following snippet of code demonstrates how immediate addressing may be used to change the value of a register and a memory location:

mov eax 37
add num 37

The value of the register eax and the memory location pointed to by the variable num (its previous value was 0) has been set to 37, which is % in ASCII.

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Rukhshan Haroon
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