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Atari 2600 Programming for Newbies

Session 9: 6502 and DASM - Assembling the Basics

By Andrew Davie (adapted by Duane Alan Hahn)

Table of Contents

Original Session

This session we're going to have a look at the assembler "DASM", what it does, how it does it, why it does it, and how to get it to do it. Smile

 

The job of an assembler is to convert our source code into a binary image which can be run by the 6502. This conversion process ultimately replaces the mnemonics (the words representing the 6502 instructions we use when writing in assembler) and the symbols (the various names we use for things, such as labels to which we can branch, and various other things like the names of TIA registers, etc) with numerical values.

 

So ultimately, all the assembler needs to do is figure out a numerical value for all the things which become part of the binaryand place that value in the appropriate place in the binary.

 

 

 

 

 

NOP

We've already had a brief introduction to a 6502 instructionthe one called NOP. This is the no-operation instruction which simply takes 2 cycles to execute. Whenever we enter NOP into our source code, the assembler recognizes this as a 6502 instruction and inserts into the binary the value $EA. This shows that there can be a simple 1:1 relationship between source-code and the binary.

 

NOP is a single-byte instructionall it requires is the opcode, and the 6502 will happily execute it. Some instructions require additional 'parametersA value that is passed to a routine.

(Adapted from Webopedia.)
'the 'operandsIn all computer languages, expressions consist of two types of components: operands and operators. Operands are the objects that are manipulated and operators are the symbols that represent specific actions. For example, in the expression

5 + x

x and 5 are operands and + is an operator. All expressions have at least one operand.

(From Webopedia.)
'. The 6502 microprocessor can use an additional 1 or 2 bytes of operand data for some instructions, so the total number of bytes for a 6502 'instruction' can be 1, 2 or 3.

 

 

 

 

 

DASM

DASM is the assembler used by most (if not all) modern-day '2600 programmers. It is a multi-platform assembler written in 1988 by Matt Dillon (you should all find his email address and send him a "thank-you" sometime). It's a great tool.

 

DASM isn't just capable of assembling 6502 (and variant) codeit also has inbuilt capability to assemble code for several other microprocessors. Consequently, one of the very first things that it is necessary to do in our source code is tell DASM what processor the source code is written for. . .


     processor 6502

This should be just about the first line in any '2600 program you write. If you don't include it, DASM will probably get confused and spit out errors. That's simply because it is trying to assemble your code as if it were written for another processor.

 

We've just seen how mnemonics (the standard names for instructions) are converted into numerical values by the assembler. Another job the assembler does is convert labels and symbols into values. We've already encountered both of these in our previous sessions, but you may not be familiar with their names.

 

 

 

 

 

 

 

 

 

Summary

Soon we'll start playing with some TIA registers and seeing what happens to our screen when we do that! For now, though, make sure you are able to assemble and run the first kernel. If you have any problems, ask for assistance and I'm sure somebody will leap to your aid.

 

 

 

Other Assembly Language Tutorials

Be sure to check out the other assembly language tutorials and the general programming pages on this web site.

 

 

< Previous Session

 

 

Next Session >

 

 

 

 

Session Links

Session 1: Start Here

Session 2: Television Display Basics

Sessions 3 & 6: The TIA and the 6502

Session 4: The TIA

Session 5: Memory Architecture

Session 7: The TV and our Kernel

Session 8: Our First Kernel

Session 9: 6502 and DASM - Assembling the Basics

Session 10: Orgasm

Session 11: Colorful Colors

Session 12: Initialization

Session 13: Playfield Basics

Session 14: Playfield Weirdness

Session 15: Playfield Continued

Session 16: Letting the Assembler do the Work

Sessions 17 & 18: Asymmetrical Playfields (Parts 1 & 2)

Session 19: Addressing Modes

Session 20: Asymmetrical Playfields (Part 3)

Session 21: Sprites

Session 22: Sprites, Horizontal Positioning (Part 1)

Session 23: Moving Sprites Vertically

Session 24: Some Nice Code

Session 25: Advanced Timeslicing

 

 

 

 

Useful Links

Easy 6502 by Nick Morgan

How to get started writing 6502 assembly language. Includes a JavaScript 6502 assembler and simulator.

 

 

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 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 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.

 

 

Guide to 6502 Assembly Language Programming by Andrew Jacobs

Below are direct links to the most important pages.

 

 

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. It has a built-in debugger to help you with your works in progress or you can use it to study classic games.

 

 

JAVATARI

A very good emulator that can also be embedded on your own web site so people can play the games you make online. It's much better than JStella.

 

 

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.

 

 

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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.

 

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