Mastering the Intel 8080: Build Your Own Disassembler!

Table of Contents

1. Introduction
2. Understanding the Basics of a Disassembler
3. Focus on the Intel 8080 Microprocessor
4. Implementing the Disassembler
5. Reading the File into Memory
6. Performing the Disassembly
7. Creating a Switch Statement
8. Implementing Instructions
9. Compiling and Running the Disassembler
10. Conclusion

💡 Introduction

In this article, we will take a closer look at the process of writing a disassembler. Although developing a disassembler for modern platforms like Intel or AMD processors can be a massive undertaking, we will focus on a simpler platform, the Intel 8080 microprocessor. This 8-bit chip, released in 1974, features specific characteristics that make it suitable for demonstration purposes. We will explore the architecture of the 8080, learn how instructions are encoded, and delve into the implementation of a disassembler for this platform. By the end of this article, you will have a clear understanding of how a disassembler works and how to implement one.

🔍 Understanding the Basics of a Disassembler

A disassembler is a software tool that converts machine code or object code back into assembly language or a high-level programming language. It allows us to reverse engineer executable files and understand their underlying instructions. By deconstructing the binary code, we can analyze the logic and functionality of a program.

🖥️ Focus on the Intel 8080 Microprocessor

To simplify the demonstration process, we will focus on the Intel 8080 microprocessor, released in 1974. This 8-bit chip features seven 8-bit general-purpose registers, including registers A, B, C, D, E, H, and L. Additionally, it has a 16-bit program counter (PC) register and a 16-bit stack pointer (SP) register. Each instruction in the 8080 is encoded using a single byte, with the architecture incorporating little-endian representation. Every operation can be followed by one or two bytes of data, such as memory addresses, port numbers, or operands.

🛠️ Implementing the Disassembler

To implement a disassembler for the Intel 8080, we will use the C programming language. We will create a disassemble function that takes in a buffer of bytes and the current program counter as input. This function will disassemble the instruction and return the new program counter location. We will also create a main function to handle file input, read the file into memory, and perform the disassembly process. The disassemble function will be responsible for decoding each instruction byte by byte.

📥 Reading the File into Memory

Before performing the disassembly, we need to read the file into a buffer in memory. To achieve this, we will determine the file size using the fseek and ftell functions. Then, we will allocate memory for the buffer based on the file size. Finally, we will read the file into the buffer and close the file.

⚙️ Performing the Disassembly

To perform the disassembly, we will initialize a program counter to track the current position in the buffer. We will use a while loop to iterate through the buffer bytes and disassemble each instruction. The program counter will be incremented based on the number of bytes associated with the instruction. We will use a switch statement to handle the different opcodes and print out the corresponding assembly instructions.

🔄 Creating a Switch Statement

The heart of the disassembler lies in a switch statement. Each case within the switch represents a specific opcode, which corresponds to an assembly instruction. The switch statement allows us to easily handle different opcodes and provide the corresponding disassembled instruction. We will also include a default case for unknown instructions.

⚙️ Implementing Instructions

Within the switch statement, we will implement multiple cases for different opcodes. Each case will print out the disassembled instruction and may include additional data if required. We will handle different instructions such as LXi, stacks, iNX, and more. By implementing these instructions, we will gradually build up the functionality of the disassembler.

⚙️ Compiling and Running the Disassembler

Once the disassembler is implemented, we can compile and run it using the C compiler. We will pass in the file we want to disassemble as a command-line argument. The disassembler will then read the file into memory, perform the disassembly, and print out the corresponding assembly instructions.

🏁 Conclusion

Writing a disassembler can be a complex task, especially for modern platforms with extensive instruction sets. However, by focusing on a simpler platform like the Intel 8080 microprocessor, we can gain a solid understanding of the disassembly process. Throughout this article, we have explored the basics of a disassembler, dived into the architecture of the 8080, and implemented a disassembler in C. By following the step-by-step process, you can create your own disassembler and learn more about the intricate workings of binary code.

Resources:

• Intel 8080 Instruction Set (Link to the reference used for the Intel 8080 instruction set)