What Data Structure Are Used by the First Pass of Assembler?


Scott Campbell

What Data Structures Are Used by the First Pass of Assembler?

The first pass of an assembler is a crucial step in the process of converting assembly language code into machine code. During this pass, the assembler scans through the source code and builds important data structures that are essential for subsequent passes. These data structures help in resolving symbols, managing memory, and generating the final object code.

Symbol Table

One of the key data structures used by the first pass of an assembler is the symbol table. The symbol table keeps track of all labels defined in the source code along with their corresponding addresses or values. It serves as a reference for resolving symbols during later passes.

The symbol table can be implemented using various data structures such as arrays, linked lists, or hash tables. Each entry in the symbol table typically consists of a label name and its associated value or address.

Literals Table

In addition to symbols, assemblers often encounter literals in assembly language programs. These literals are constants that are used within the program but not explicitly defined with labels. The first pass identifies such literals and creates a literals table to keep track of them.

The literals table records information about each literal encountered, including its value and address. This information is crucial for generating appropriate instructions or storing literal values in memory during subsequent passes.

Location Counter

Another essential data structure used by the first pass of an assembler is the location counter. The location counter keeps track of the current memory location being processed by the assembler during its scan through the source code.

The location counter is incremented as instructions and data are encountered, ensuring that each instruction or datum is assigned a unique address within memory. It also helps in calculating relative addresses and offsets for branch instructions.

Error Reporting Structures

During the first pass, assemblers need to detect and report various errors in the source code, such as undefined labels or invalid instructions. To facilitate error reporting, the first pass may utilize additional data structures like error tables or lists.

These error reporting structures store information about each encountered error, including the line number and a description of the error. This allows the assembler to provide informative error messages to help programmers identify and rectify their mistakes.


The first pass of an assembler involves the creation of several important data structures that lay the foundation for subsequent passes. The symbol table, literals table, location counter, and error reporting structures are key components used during this phase. Understanding these data structures helps in comprehending the inner workings of an assembler and enables efficient translation from assembly language code to machine code.

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