Building a MIPS Emulator (Part 1): Loading and Reading Raw Binary

[Lomanu4]

"I didn’t know how anything would work when I started—just had a vague sense I’d figure it out along the way."

Why I’m Doing This


I wanted to understand how computers really work—beyond just writing high-level code. I needed to deepen my knowledge and prove to myself that these low-level systems aren’t inherently difficult; you just need the right concepts.

So, I decided to build a MIPS emulator.

I broke it down into three parts:

1. Loading the binary code from a MIPS program
2. Decoding the binary instructions
3. Executing them

At first, I didn’t even know how to load a file, let alone parse instructions. So that’s where I began.

Step 1: Getting the Binary File


We need a binary file that contains MIPS instructions in raw binary format. There are other formats like ELF, but I chose to keep it simple with a .bin file.

Fun fact: I didn’t know until recently that .bin files can directly contain raw binary instructions. You learn something new every day.

To generate one, I used

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, a MIPS simulator written in Java. I wrote a simple program:


addiu $t0, $zero, 5 # $t0 = 5
addiu $t1, $zero, 7 # $t1 = 7
addu $t2, $t0, $t1 # $t2 = $t0 + $t1

Then I assembled it and used File → Dump Memory to export the instructions. Make sure you:

• Set the format to Binary
• Save it with a .bin extension

Step 2: Reading the Binary File in C


Once I had the .bin file, I wrote a simple C program to read and print each 32-bit MIPS instruction. Here's a snippet:


FILE *file = fopen(filename, "rb");

if (!file) {
perror("Failed to open file");
return 1;
}

The "rb" mode means we’re opening the file for reading in binary mode. If the file doesn’t exist, fopen() returns NULL.

Then I read the file 4 bytes at a time (since MIPS instructions are 32-bit = 4 bytes):


while (fread(&instruction, sizeof(u_int32_t), 1, file) == 1) {
printf("0x%08X | 0x%08X | %u\n", offset, instruction, instruction);
offset += 4;
}

I set the counter to one so I could overwrite whatever was in the instruction and store the value read from the file stream

Full Code


#include <stdio.h>
#include <stdint.h>

int loadMipsBinary(const char *filename) {
FILE *file = fopen(filename, "rb");

if (!file) {
perror("Failed to open file");
return 1;
}

printf("Address (byte offset) | Instruction (Hex) | Instruction (Dec)\n");
printf("-----------------------|---------------------|---------------------\n");

uint32_t instruction;
size_t offset = 0;

while (fread(&instruction, sizeof(uint32_t), 1, file) == 1) {
printf("0x%08X | 0x%08X | %u\n",
(unsigned int)offset, instruction, instruction);
offset += 4;
}

fclose(file);
return 0;
}

int main() {
loadMipsBinary("mipstest.bin");
return 0;
}

Let me know when you're ready for Part 2—decoding the binary into actual MIPS operations.

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