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Atari 2600 Programming for Newbies

Session 12: Initialization

By Andrew Davie (adapted by Duane Alan Hahn)

Table of Contents

Original Session

One of the joys of writing '2600 programs involves the quest for efficiencyboth in processing time used, and in ROM space required for the code. Every now and then, modern-day '2600 programmers will become obsessed with some fairly trivial task and try to see how efficient they can make it.

 

If you were about to go up on the Space Shuttle, you wouldn't expect them to just put in the key, turn it on, and take off. You'd like the very first thing they do is to make sure that all those switches are set to their correct positions. When our Atari 2600 (which, I might point out in a tenuous link to the previous sentence, is of the same vintage as the Space Shuttle) powers-up, we should assume that the 6502, RAM and TIA (and other systems) are in a fairly unknown state. It is considered good practice to initialize these systems. Unless you really, *really* know what you're doing, it can save you problems later on.

 

At the end of this session I'll present a highly optimized (and best of all, totally obscure Smile) piece of code which manages to initialize the 6502, all of RAM *and* the TIA using just 9 bytes of code-size. That's quite amazing, really. But first, we're going to do it the 'long' way, and learn a little bit more about the 6502 while we're doing it.

 

 

 

 

Initializing RAM

We've already been introduced to the three registers of the 6502A, X, and Y. X and Y are known as index registers (we'll see why, very soon) and A is our accumulatorthe register used to do most of the calculations (addition, subtraction, etc).

 

Let's have a look at the process of clearing (writing 0 to) all of our RAM. Our earlier discussions of the memory architecture of the 6502 showed that the '2600 has just 128 bytes ($80 bytes) of RAM, starting at address $80. So, our RAM occupies memory from $80 - $FF inclusive. Since we know how to write to memory (remember the "stx COLUBK" we used to write a color to the TIA background color register), it should be apparent that we could do this. . .


    lda #0            ; load the value 0 into the accumulator

    sta $80           ; store accumulator to location $80

    sta $81           ; store accumulator to location $81

    sta $82           ; store accumulator to location $82

    sta $83           ; store accumulator to location $83

    sta $84           ; store accumulator to location $84

    sta $85           ; store accumulator to location $85



; 119 more lines to store 0 into location $86 - $FC. . .



    sta $FD           ; store accumulator to location $FD

    sta $FE           ; store accumulator to location $FE

    sta $FF           ; store accumulator to location $FF

You're right, that's ugly! The code above uses 258 bytes of ROM (2 bytes for each store, and 2 for the initial accumulator load). We can't possibly afford thatand especially since I've already told you that it's possible to initialize the 6502 registers, clear RAM, *AND* initialize the TIA in just 9 bytes total!

 

The index registers have their name for a reason. They are useful in exactly the situation above, where we have a series of values we want to read or write to or from memory. Have a look at this next bit of code, and we'll walk through what it does. . .


    ldx #0

    lda #0

ClearRAM  sta $80,x

    inx

    cpx #$80

    bne ClearRAM

Firstly, this code is nowhere-near efficient, but it does do the same job as our first attempt and uses only 11 bytes. It achieves this saving by performing the clear in a loop, writing 0 (the accumulator) to one RAM location every iteration. The key is the "sta $80,x" line. In this "addressing mode", the 6502 adds the destination address ($80 in this exampleremember, this is the start of RAM) to the current value of the X registergiving it a final addressand uses that final address as the source/destination for the operation.

 

 

 

 

 

 

 

 

Initializing the TIA

Initializing the TIA is a similar process to initializing the RAMwe just want to write 0 to all memory locations from 0 to $7F (where the TIA lives!). This is safetrust meand we don't really need to know what we're writing to at this stage, just that after doing this the TIA will be nice and happy. We could do this in a second loop, similar to the first, but how about this. . .


    ldx #0

    lda #0

Clear

    sta $80,x     ; clear a byte of RAM

    sta 0,x       ; clear a byte of TIA register

    inx

    cpx #$80

    bne Clear

That's a perfectly adequate solution. Easy to read and maintain, and reasonably quick. We could, however, take advantage of the fact that RAM and the TIA are consecutive in memory (TIA from 0 - $7F, immediately followed by RAM $80 - $FF) and do the clear in one go. . .

 

 

 

 

 

Initializing RAM and the TIA


    ldx #0

    lda #0

Clear

    sta 0,x

    inx

    bne Clear

The above example uses 9 bytes, again, but now clears RAM and TIA in one 'go' by iterating the index register (which is the effective address when used in "sta 0,x") from 0 to 0 (ie: increments 256 times and then wraps back to 0 and the loop halts). This is starting to get into "elegant" territory, something the experienced guys strive for!

 

Furthermore, after this code has completed, X = 0 and A = 0a nice known state for two of the 3 6502 registers.

 

That's all I'm going to explain for the initialization at this stagewe should insert this code just after the "Reset" label and before the "StartOfFrame" label. This would cause the code to be executed only on a system reset, not every frame (as, every frame, the code branches back to the "StartOfFrame" for the beginning of the next frame).

 

 

 

 

 

Summary

Before we end today's session, though, I thought I'd share the "magical" 9-byte system clear with you. There's simply no way that I would expect you to understand this bit of code at the momentit pulls every trick in the bookbut this should give you some taste of just how obscure a bit of code CAN be, and how beautifully elegant clever coding can do amazing things.


   ; CLEARS ALL VARIABLES, STACK

   ; INIT STACK POINTER

   ; ALSO CLEARS TIA REGISTERS

   ; DOES THIS BY "WRAPPING" THE STACK - UNUSUAL



    LDX #0

    TXS

    PHA           ; BEST WAY TO GET SP=$FF, X=0



    TXA

CLEAR PHA

    DEX

    BNE CLEAR



   ; 9 BYTES TOTAL FOR CLEARING STACK, MEMORY

   ; STACK POINTER NOW $FF, A=X==0

Though the above was a truly magical piece of code, I've since developed an EIGHT byte solution to the problem of clearing RAM and initializing the stack and registers. Smile


        ldx #0

        txa

Clear   dex

        txs

        pha

        bne Clear

After the above, X=A=0, and all of RAM and the TIA has been initialized to 0, and the stack pointer is initialized to $FF. Amazing!

 

See you next time!

 

 

 

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.

 

 

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.

Try 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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