Introduction
First, let me introduce this book by telling you what I will teach you. By the end of this book, you will have enough information to write any text-based console program in the form of a 16-bit DOS (Disk Operating System) “.com” file.
The “.com” file was a format used by all versions of MS-DOS, and even supported on Windows up to XP. It has no header information and is limited to 64 kilobytes of memory. Rather than viewing the limitation as a weakness, I view it as a strength because it forces me to be a better programmer and squeeze the most out of every byte.
Required Knowledge
To get the most out of this book, some background on the Binary and Hexadecimal numeral systems is going to be helpful, but this is not strictly required because I will be providing functions you can use in your code that will convert between decimal (base ten), binary (base two), and hexadecimal (base 16).
However, I would say that experience in at least one programming language is necessary for an understanding of terminology like “arrays”, “pointers”, “addresses”, “integers”, “floating point”, etc. I recommend the C Programming Language as a start. C++ is also a good starting language, but it tends to abstract details away that directly apply to Assembly Language, which is the lowest level a human can go for understanding a computer.
Low Level
Low level is a term that confuses people. People think something high-level is better than low-level. In simple terms, humans consider themselves superior to machines and therefore think themselves higher or more important because of their abstract thought.
A computer thinks only in terms of numbers. A computer may not understand “high-level” abstractions such as love, religion, philosophy, etc, but that is not its job. A computer must add, subtract, multiply, and divide. These are the four arithmetic functions that many humans struggle with.
Therefore, I ask you, between a human and a computer, who is really low level or high level? In the age of Artificial Intelligence taking over human jobs and beating humans at Chess, we would all do well to take this question seriously.
I wrote this book because I think like a machine, and I hope to help others think this way because it is the best way to learn programming and control your computer by writing Assembly Language programs, or to go back to your favorite programming language with a greater understanding of why things work as they do.
Why DOS?
DOS is not at all like Windows or Linux, because it comes from an older time when people were expected to read books, and even video games often came in the form of source code published in books. Therefore, I have decided to dedicate this book to the Disk Operating System, more commonly called DOS, and made famous by MS-DOS, which was Microsoft’s version that people in the 80s and 90s remember. Later on, I plan to write a book on programming on Linux using similar but modern methods.
Online Example Programs
Although you can retype every example program from this book and try to run it in your DOS emulator. I also provide the examples as downloads from my Github repository for teaching Assembly.
My samples are free and under the “GNU General Public License v3.0” because my intention is to make Assembly Language easy to learn for everyone without any restrictions. My hope is that others find the joy of programming in DOS, no matter whether they learn it from me or whether they learned it from someone else who may have picked up a few things from me.