What Is the Data Structure of Assembler?


Angela Bailey

The data structure of assembler is an important concept to understand for anyone interested in low-level programming. Assembler, also known as assembly language, is a low-level programming language that is closely related to machine code. It uses mnemonic codes and symbols to represent instructions and data, making it easier for programmers to work with the underlying hardware.

Assembler Instructions

An assembler program consists of a series of instructions that are executed sequentially. These instructions can be classified into different types based on their functionality:

  • Data Transfer Instructions: These instructions are used to move data between registers and memory locations. They include instructions like MOV, LDR, and STR.
  • Arithmetic and Logical Instructions: These instructions perform arithmetic and logical operations on data.

    Examples include ADD, SUB, AND, and OR.

  • Control Flow Instructions: These instructions control the flow of execution based on certain conditions. They include conditional branches like BNE (Branch if Not Equal) and unconditional branches like B.
  • Stack Instructions: Assembler provides instructions to manipulate the stack, such as PUSH, POP, and SUB SP, SP, #4.

Data Representation in Assembler

Data in assembler is represented using various data types. The most common types include:

  • Byte: A byte is the smallest addressable unit of memory in assembler. It can store values from 0 to 255.
  • Word: A word is typically 2 bytes long and can store larger values.
  • Doubleword: A doubleword is usually 4 bytes long and can store even larger values.

Assembler also provides directives to declare and initialize data. These directives include .data, .byte, .word, and .space.

Symbol Table

A symbol table is a data structure used by the assembler to keep track of symbols defined in the program. Symbols can be labels, variables, or constants.

The symbol table stores information about each symbol, such as its name, value, size, and type. This information is used during the assembly process to resolve symbols and generate the corresponding machine code.


Consider the following assembler code:

x: .word 10
y: .byte 20text
    LDR R0, x   ; Load the value of x into register R0
    MOV R1, #1  ; Move the immediate value 1 into register R1
    ADD R0, R0, R1 ; Add the values of R0 and R1 and store the result in R0
    STR R0, y   ; Store the value of R0 into memory location yend

In this example, we have defined two symbols in the data section: ‘x’ and ‘y’. The symbol table keeps track of these symbols along with their respective types and sizes. During assembly, when these symbols are referenced in instructions, the assembler resolves them to their corresponding memory locations.


The data structure of assembler plays a crucial role in the assembly process. It provides a way to represent instructions and data using mnemonic codes and symbols.

Understanding the different types of instructions, data representation, and the symbol table is essential for writing efficient and error-free assembler programs. By utilizing these elements, programmers can create powerful low-level code that interacts directly with the hardware.

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