Building an Assembler: A Journey into Low-Level Programming

In the second installment of his series on programs that create programs, Brian Robert Callahan presents a meticulous approach to constructing an assembler for the Zilog Z80 CPU architecture. Building upon his previous work on a disassembler, Callahan embarks on what he acknowledges will be a more substantial undertaking, aiming to create an educational tool that prioritizes clarity and understanding over theoretical perfection.

The article establishes a clear philosophical framework for the project, emphasizing practical learning over academic rigor. Callahan explicitly draws inspiration from Jack Crenshaw's "Let's Build a Compiler" series, stating his intention to "completely ignore the more theoretical aspects of the subject." This approach positions the assembler as both a programming exercise and an educational tool, designed to be accessible to those with minimal prior experience.

Technically, the assembler targets the Intel 8080 instruction set rather than the native Z80 assembly language, despite being intended for Z80 processors. This pragmatic choice stems from the Z80's binary compatibility with the 8080 and the relative simplicity of the 8080 assembly syntax. As Callahan explains, "the Z80 has a different assembly language because Intel copyrighted the 8080 mnemonics," making the 8080 assembly language more straightforward to parse while still producing code that the Z80 can execute.

The development environment is deliberately minimal, utilizing the D programming language with any standard compiler. Callahan notes that the assembler will eventually grow to approximately 1300 lines of code, organized as a single file for simplicity. This approach contrasts with more complex toolchains but aligns with the educational goal of creating something approachable and understandable.

A significant portion of the article is dedicated to defining the structure of assembly language and the requirements for the assembler. Callahan breaks down the assembly instruction format into three primary components: [label:], [op [arg1[, arg2]]], and [; comment]. This structural analysis provides the foundation for the parser that will be implemented in subsequent articles.

The assembler's requirements are thoughtfully balanced between completeness and accessibility. Callahan identifies four minimum requirements: generating correct object code, rejecting incorrect inputs, supporting comments, and providing basic facilities like DB (define byte), EQU (equation), and ORG (origin). These features represent the essential building blocks for practical assembly programming while avoiding unnecessary complexity.

Notably, Callahan explicitly states what the assembler will not prioritize: perfection, speed, or external validation. This deliberate limitation of scope allows the project to remain focused on its educational objectives. As he explains, "Who cares if it is perfect and if someone else likes it? It is going to fulfill its job of taking assembly code and producing the correct machine code translation."

The article concludes with the implementation of the main function, which handles file input and output preparation. The code demonstrates how to read an assembly file line by line and prepare a corresponding output file with the .com extension, which was the standard executable format for CP/M systems running on Z80 processors.

This article represents a thoughtful approach to teaching low-level programming concepts through practical implementation. By focusing on a well-defined, constrained problem space and emphasizing understanding over optimization, Callahan creates an accessible entry point into the complex world of program transformation tools. The educational value lies not only in the technical details of assembler implementation but in the demonstrated approach to problem-solving and software design.