Assembly Language Programming
Lesson 1: Bits!
By Robert M (adapted by Duane Alan Hahn, a.k.a. Random Terrain)
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Page Table of Contents
Original Lesson
This course assumes no prior knowledge of computer programming. While the examples given in the course are targeted at the 650X family of processors, the ideas presented will apply to assembly language programming and often programming in general.
Please feel free to posts comments or questions into the Lesson threads. If you wish to ask a private question don't hesitate to send me a PM.
The assembler we will be using for this course is DASM. We won't need the assembler for the first several sessions, I will provide links for downloading and installing DASM. DASM is available for DOS, Windows (in a DOS window), Amiga, and Macintosh (OS-9 or OS-X). So the development tools will be available on many platforms.
The Most Important Thing You Need to Know about Computers
Imagine you have a black box with a single light bulb sticking out of it. Sometimes the light is on, sometimes it is off. Please answer these questions:
- What does it mean when the light is on?
- What does it mean when the light is off?
Until you can answer these questions, programming computers will never quite make sense. Everything else we do in this course will be built on this radical idea. Please take a moment to consider these questions. There are no wrong answers, its just a mental exercise to broaden your mind.
The key points that you need to take away from this exercise are:
- The light is either on or off, there are only 2 possible conditions for the light. The light represents a single digital bit.
- The two states of the bit can represent ANY TWO OPPOSING conditions:
1 or 0
YES or NO
"The Player is alive" or "The player is Dead"
"The Fire button is pressed" or "The fire button is not pressed" - All information in a digital computer at its lowest level is composed of bits. There is no piece of information smaller than a single bit.
So how does this relate back to the questions I asked above?
- What does it mean when the light is on?
It means what ever you the programmer want it to mean is true. - What does it mean when the light is off?
The opposing condition for the meaning you give to the bit is true.
By common convention. the values 1 and 0 are used to represent the states of bits. One usually means on, yes, or true. Zero usually means off, no, or false. Notice that I said "by convention" and "usually". You could just as well use "A" and "B" or "Zip" and "Zap", but this makes it hard to talk with others about bits, so we will use 1 and 0 in this class.
This is the great secret of all computers and computer programming in general. When you program in assembly language, you have complete control/responsibility 
to provide the meaning of the values of the bits that make up your program. If you want a bit to mean "The Dragon is awake" when it is 1 and "The dragon is asleep" when it is zero, that is fine. Just understand that the meaning you give to the bit is completely your own invention and when the user pulls out cartridge with your program and puts another one in, that program will apply a completely different meaning to the EXACT SAME BIT.
Other Assembly Language Tutorials
Be sure to check out the other assembly language tutorials and the general programming pages on this web site.
Amazon: Atari 2600 Programming (#ad)
Amazon: 6502 Assembly Language Programming (#ad)
Atari 2600 Programming for Newbies (#ad)
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Lesson 1: Bits!
Atari 2600 BASIC
If assembly language is too hard for you, try batari Basic. It's a BASIC-like language for creating Atari 2600 games. It's the faster, easier way to make Atari 2600 games.
Disclaimer
View this page and any external web sites at your own risk. I am not responsible for any possible spiritual, emotional, physical, financial or any other damage to you, your friends, family, ancestors, or descendants in the past, present, or future, living or dead, in this dimension or any other.
Use any example programs at your own risk. I am not responsible if they blow up your computer or melt your Atari 2600. Use assembly language at your own risk. I am not responsible if assembly language makes you cry or gives you brain damage.