FileReading.md

Reading from a File in a Verilog Testbench

Introduction

Reading from a file in Verilog is useful for automating testbenches, especially for feeding large sets of test inputs into a module. This tutorial explains how to read from a file line by line using standard Verilog file I/O functions.

Steps to Read a File in Verilog

1. Open the File

integer file;
file = $fopen("test_cases.txt", "r");
if (file == 0) begin
    $display("Error: Cannot open file!");
    $finish;
end

• $fopen("test_cases.txt", "r") opens the file in read mode.

• If the file fails to open (file == 0), it displays an error and stops execution.

2. Read the File Line by Line

while (!$feof(file)) begin
    integer scan_count;
    reg [8*100:1] line; // Buffer for the line content

    scan_count = $fgets(line, file);
    
    if (scan_count > 0) begin
        $display("Read Line: %s", line);
    end
end

• $feof(file) checks if the end of file (EOF) is reached.

• $fgets(line, file) reads a line from the file into the line buffer.

• If a valid line is read, it is displayed.

3. Extract Data from the Line

reg [31:0] instruction;
integer scan_count;

scan_count = $sscanf(line, "%d", instruction);
if (scan_count == 1) begin
    $display("Extracted Instruction: %d", instruction);
end

• $sscanf(line, "%d", instruction) extracts an integer value from line, and stores it in the instruction variable.

• If scan_count == 1, a valid integer was extracted.

4. Close the File

$fclose(file);

• $fclose(file) closes the file after reading is complete.

• Always close the file after reading to free system resources.

Key Functions for File Reading

Function	Description
$fopen("filename", "r)	Opens a file for reading. Returns file handle (0 if failed).
$feof(file)	Checks if the end of file is reached. Returns 1 if EOF, 0 otherwise.
$fgets(buffer, file)	Reads a line from the file into the buffer. Returns the number of bytes read.
$sscanf(buffer, format, data)	Extracts data from the buffer using the specified format. Returns the number of successful conversions.
$fclose(file)	Closes the file after reading is complete.