Table of Contents

Original Lesson

 

Assembly Language Programming
Lesson 1: Bits!
By Robert M (adapted by Duane Alan Hahn)

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.

 

 

 

 

 

Materials Needed

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:

 

1. What does it mean when the light is on?
2. 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.

 

 

 

 

 

Answers

The key points that you need to take away from this exercise are:

 

1. The light is either on or off, there are only 2 possible conditions for the light. The light represents a single digital bit.
2. 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"

3. 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?

 

1. What does it mean when the light is on?

It means what ever you the programmer want it to mean is true.

2. 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.

 

 

 

 

Next Lesson >

 

 

 

 

 

Lesson Links

Lesson 1: Bits!

Lesson 2: Enumeration

Lesson 3: Codes

Lesson 4: Binary Counting

Lesson 5: Binary Math

Lesson 6: Binary Logic

Lesson 7: State Machines

 

 

 

 

 

Useful Links

Atari Roots by Mark Andrews (Online Book)

This book was written in English, not computerese. It's written for Atari users, not for professional programmers (though they might find it useful).

 

 

Machine Language For Beginners by Richard Mansfield (Online Book)

This book only assumes a working knowledge of BASIC. It was designed to speak directly to the amateur programmer, the part-time computerist. It should help you make the transition from BASIC to machine language with relative ease.

The Six Instruction Groups

The 6502 Instruction Set broken down into 6 groups.

6502 Instruction Set

Nice, simple instruction set in little boxes (not made out of ticky-tacky).

 

 

The Second Book Of Machine Language by Richard Mansfield (Online Book)

This book shows how to put together a large machine language program. All of the fundamentals were covered in Machine Language for Beginners. What remains is to put the rules to use by constructing a working program, to take the theory into the field and show how machine language is done.

6502 Instruction Set

An easy-to-read page from The Second Book Of Machine Language.

 

 

6502 Instruction Set with Examples

A useful page from Assembly Language Programming for the Atari Computers.

 

 

6502.org

Continually strives to remain the largest and most complete source for 6502-related information in the world.

NMOS 6502 Opcodes

By John Pickens. Updated by Bruce Clark.

 

 

Guide to 6502 Assembly Language Programming by Andrew Jacobs

Below are direct links to the most important pages.

Registers

Goes over each of the internal registers and their use.

Instruction Set

Gives a summary of whole instruction set.

Addressing Modes

Describes each of the 6502 memory addressing modes.

Instruction Reference

Describes the complete instruction set in detail.

 

 

Stella Programmer's Guide

HTMLified version.

 

 

Nick Bensema's Guide to Cycle Counting on the Atari 2600

Cycle counting is an important aspect of Atari 2600 programming. It makes possible the positioning of sprites, the drawing of six-digit scores, non-mirrored playfield graphics and many other cool TIA tricks that keep every game from looking like Combat.

 

 

How to Draw A Playfield by Nick Bensema

Atari 2600 programming is different from any other kind of programming in many ways. Just one of these ways is the flow of the program.

 

 

Cart Sizes and Bankswitching Methods by Kevin Horton

The "bankswitching bible." Also check out the Atari 2600 Fun Facts and Information Guide and this post about bankswitching by SeaGtGruff at AtariAge.

 

 

Atari 2600 Specifications

Atari 2600 programming specs (HTML version).

 

 

Atari 2600 Programming Page (AtariAge)

Links to useful information, tools, source code, and documentation.

 

 

MiniDig

Atari 2600 programming site based on Garon's "The Dig," which is now dead.

 

 

TIA Color Charts and Tools

Includes interactive color charts, an NTSC/PAL color conversion tool, and Atari 2600 color compatibility tools that can help you quickly find colors that go great together.

 

 

The Atari 2600 Music and Sound Page

Adapted information and charts related to Atari 2600 music and sound.

 

 

Game Standards and Procedures

A guide and a check list for finished carts.

 

 

Stella

A multi-platform Atari 2600 VCS emulator

 

 

Harmony Cartridge

Quickly and easily test your games on a real Atari 2600 or play hundreds of your favorite games.

 

 

Melody Boards

Melody boards are stripped-down Harmony boards that can be distributed on their own dedicated cartridges.

 

 

batari Basic Commands

If assembly language seems a little too hard, don't worry. You can always try to make Atari 2600 games the faster, easier way with batari Basic.

Back to Top

 

Background Color Changer