Two-Pass Assembler in C (with Macro Preprocessor)

A modular assembler written in C, developed as a university systems-programming lab project.
The tool processes assembly source files, expands macros (pre-assembler stage), and then performs a two-pass assembly to generate machine-code output files and symbol usage reports.

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Pipeline Overview

1. Pre-Processor (Macro Expansion)

   • Reads the original .as file
   • Expands macros
   • Produces a preprocessed .am file

2. First Pass

   • Parses each line and validates syntax
   • Builds/updates the symbol table
   • Computes instruction/data memory layout and addresses

3. Second Pass

   • Resolves symbols (labels, externals, entries)
   • Encodes instructions/data into machine words
   • Writes final output files

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Input / Output Files

Input

• *.as - assembly source file (original input)

Outputs (as applicable)

• *.am - preprocessed file after macro expansion
• *.ob - object file (encoded machine words; format per course specification)
• *.ent - entry labels (generated only if .entry labels exist)
• *.ext - external label usages (generated only if .extern symbols are used)

If assembly fails due to errors, the program typically avoids generating partial/invalid output files.

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Build

Requirements

• GCC (or compatible compiler)
• GNU Make
• Linux/macOS (recommended)

Clean

make clean

The project is intended to compile cleanly with strict warning flags (e.g., -Wall -ansi -pedantic) as required by the course workflow

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Run

Run the assembler by providing one or more base filenames (without extension). For each name, the program reads .as and produces outputs for that input.

./assembler inputMain valid_input empty

This expects files like:

• inputMain.as
• valid_input.as
• empty.as

And may generate:

• inputMain.am/.ob/.ent/.ext (depending on directives and symbol usage)

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Repository Structure

.

├── assembler.c # main driver (preprocess → first pass → second pass)

├── assembler.h

│

├── pre_proc.c # macro expansion → emits .am

├── pre_proc.h

├── pre_proc_errors.c # preprocessing validation / error scenarios

│

├── first_pass.c # first pass (symbol table, layout, validation) - my part

├── first_pass_helpers.c

├── first_pass_helpers.h

├── first_pass_error_checks.c

├── first_pass_error_checks.h

│

├── second_pass.c # second pass (resolution + output) - partner’s part

├── second_pass.h

├── translation_unit.c # encoding / translation utilities - partner’s part

├── translation_unit.h

│

├── structs.c # shared data structures

├── functions.c # shared utilities

│

├── inputMain.as # examples / tests

├── inputMain.am

├── inputMain.ob

├── inputMain.ent

├── inputMain.ext

│

├── valid_input.as

├── valid_input.am

├── valid_input.ob

│

├── valid_macro.* # macro tests

├── invalid_macro.as

│

├── empty.* # edge cases

├── error_second_pass.* # second-pass error cases

└── first_pass__errors. # first-pass error cases (e.g., .as / .am variants)