Atari 2600 Programming for Newbies

Session 24: Some Nice Code

By Andrew Davie (adapted by Duane Alan Hahn, a.k.a. Random Terrain)

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Page Table of Contents

Original Session

In tutorial 22, we learned that to horizontally position a sprite, we need to trigger the RESPx register at the appropriate position in the scanline, at which point the sprite will display immediately. To move to an arbitrary horizontal position, we need to trigger RESPx just before the TIA is displaying the appropriate color clock. Our solution has been to use the desired X-position of the sprite as the basis for a delay loop which starts at the beginning of a scanline, delays until roughly the correct position, adjusts the HMPx fine-tune horizontal position register and then 'hits' RESPx to immediately position the sprite.






Since the minimal time for a single loop iteration is 5 cycles (involving a register decrement, and a branch), and 5 cycles corresponds to 15 TIA color-clocks, it follows that our delay-loop approach can only position RESPx writes with an accuracy of 15 TIA color-clocks. This is fine, though, as the hardware capability of fine-positioning sprites by -8 to +7 pixels perfectly allows the correct position of the sprite to be established.


The approach taken previously has been to effectively divide the position by 15 (either through a table-lookup, or 'clever' code which simulated a divide by 15 using a divide by 16 (quick) + adjustment) and use that value as the iteration counter in a delay loop. This approach works, and has been fairly standard for a number of years. This is the approach presented in our earlier tutorial.


A posting to the [stella] list of an independent discovery of a 'new' method much improves on this technique. In actual fact, the technique was already known and documented in the list … but for various reasons these things don't always become well-known. The 'new' technique of horizontal positioning rolls the divide-by-15 and the delay loop into a single entity.


.Div15   sbc #15      ; 2

         bcs .Div15   ; 3(2)

Now that may not look like much, but it's absolutely brilliant! Every iteration through the loop, the accumulator is decremented by 15. When the subtraction results in a carry, the accumulator has gone 'past' 0, and our loop ends. Each iteration takes exactly 5 cycles (with an extra 2 cycles added for the initial 'sec' and one less for the final branch not taken). The real beauty of the code is that we also, 'for free', get the correct -8 to +7 adjustment for the fine-tuning of the position (which with a little bit of fine-tuning can be used for the HMP0 register)! Read the relevant post on [stella] here…


For this brilliant bit of coding, our thanks go to R. Mundschau…


; Positions an object horizontally
; Inputs: A = Desired position.
; X = Desired object to be positioned (0-5).
; scanlines: If control comes on or before cycle 73 then 1 scanline is consumed.
; If control comes after cycle 73 then 2 scanlines are consumed.
; Outputs: X = unchanged
; A = Fine Adjustment value.
; Y = the "remainder" of the division by 15 minus an additional 15.
; control is returned on cycle 6 of the next scanline.


            sta WSYNC                ; 00     Sync to start of scanline.

            sec                      ; 02     Set the carry flag so no borrow will be
                                     ;        applied during the division.

.divideby15 sbc #15                  ; 04     Waste the necessary amount of time
                                     ;        dividing X-pos by 15!

            bcs .divideby15          ; 06/07  11/16/21/26/31/36/41/46/51/56/61/66


            lda fineAdjustTable,y    ; 13 -> Consume 5 cycles by guaranteeing we
                                     ;       cross a page boundary

            sta HMP0,x


            sta RESP0,x              ; 21/ 26/31/36/41/46/51/56/61/66/71
                                     ; Set the rough position.


; This table converts the "remainder" of the division by 15 (-1 to -15) to the correct
; fine adjustment value. This table is on a page boundary to guarantee the processor
; will cross a page boundary and waste a cycle in order to be at the precise position
; for a RESP0,x write

            ORG $F000


            DC.B %01110000; Left 7 

            DC.B %01100000; Left 6

            DC.B %01010000; Left 5

            DC.B %01000000; Left 4

            DC.B %00110000; Left 3

            DC.B %00100000; Left 2

            DC.B %00010000; Left 1

            DC.B %00000000; No movement.

            DC.B %11110000; Right 1

            DC.B %11100000; Right 2

            DC.B %11010000; Right 3

            DC.B %11000000; Right 4

            DC.B %10110000; Right 5

            DC.B %10100000; Right 6

            DC.B %10010000; Right 7

fineAdjustTable EQU fineAdjustBegin - %11110001; NOTE: %11110001 = -15





Conceptual Negative Index

One interesting aspect of this code is the access to the table with a (conceptual) negative index (-1 to -15 inclusive). Negative numbers are represented in two's complement form, so -1 is %11111111 which is *exactly* the same as 255 (%11111111). So how can we use negative numbers as indexes? We can't! All indexing is considered to be with positive numbers. So if our index was -1, we would actually index 255 bytes past the beginning of our table. The neat bit of code at the bottom sets the conceptual start of our table to 241 bytes BEFORE the start of the actual data so that when we attempt to access the -15th element of the table, we ACTUALLY end up at the very first byte of the 'fineAdjustBegin' table. Likewise, when accessing the -1th element, we ACTUALLY access the last element of the table. It's all very neat!


Finally, since we need to account for every cycle in this code very carefully (as the horizontal position depends on exactly where we write the RESP0 value), we need to take into account the possibility that an extra cycle is being thrown in when we access fineAdjustTable,y and that access crosses a page boundary. By positioning the table being accessed exactly on a page boundary, the code guarantees that every access incurs an extra cycle 'penalty' and is therefore consistent for all cases.


I don't take any credit for this, I just admire it. I consider this a BRILLIANT bit of coding, so hats-off to R. Mundschau and thanks for sharing!





8-Byte System Clear

Another 'BRILLIANT' bit of code, but this time from yours truly, is the 8-byte system clear. We touched on this earlier in Session 12, but I thought I'd give a quick run-down on exactly how that code works…

        ldx #0 


Clear   dex 



        bne Clear

We assume that when this code starts, the system is in a totally unknown state. Firstly, X and A are set to 0, and we enter the loop.


The loop begins: X-register is decremented (to 255) and this value is placed in the stack pointer (now $FF) the accumulator(0) is then pushed onto the stack, so memory/hardware location $FF is set to 0, and the stack pointer decrements to $FE since the txs and pha don't affect the flags, the branch will be based on the decrement of the x register if non-zero, then we repeat the loop. 0 will be written to 256 consecutive memory locations starting with $FF and ending with 0 (inclusive). Loop will terminate after 256 iterations. On the final pass through, x would be decremented to 0, and this placed in the stack pointer. We then push the accumulator (0) onto the stack (which effectively writes it to memory (TIA) location 0) and as a consequence the stack pointer decrements (and wraps!) back to $FF.


At the conclusion of the above, X = 0, A = 0, SP = $FF, a near-perfect init!


That could be the best 8-bytes ever written. Wink







[This used to be the last session that Andrew Davie had time to work on. Now there is Session 25.]




Other Assembly Language Tutorials

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


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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 22: Sprites, Horizontal Positioning (Part 2)

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 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.

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.


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.




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.




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. Stella finally got Atari 2600 quality sound in December of 2018. Until version 6.0, the game sounds in Stella were mostly OK, but not great. Now it's almost impossible to tell the difference between the sound effects in Stella and a real Atari 2600.




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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In Case You Didn't Know


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Check out these videos:

If You Got the COVID Shot and Aren't Injured, This May Be Why

Thought Experiment: What Happens After the Jab?

The Truth About Polio and Vaccines

What Is Causing the Mysterious Self-Assembling Non-Organic Clots and Sudden Deaths?

Full Video of Tennessee House of Representatives Health Subcommittee Hearing Room 2 (The Doctors Start Talking at 33:28)



H Word and I Word = Good

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Negative Ions = Good

Negative ions are good for us. You might want to avoid positive ion generators and ozone generators. A plain old air cleaner is better than nothing, but one that produces negative ions makes the air in a room fresher and easier for me to breathe. It also helps to brighten my mood.



Litterbugs = Bad

Never litter. Toss it in the trash or take it home. Do not throw it on the ground. Also remember that good people clean up after themselves at home, out in public, at a campsite and so on. Leave it better than you found it.



Climate Change Cash Grab = Bad

Seems like more people than ever finally care about water, land, and air pollution, but the climate change cash grab scam is designed to put more of your money into the bank accounts of greedy politicians. Those power-hungry schemers try to trick us with bad data and lies about overpopulation while pretending to be caring do-gooders. Trying to eliminate pollution is a good thing, but the carbon footprint of the average law-abiding human right now is actually making the planet greener instead of killing it.


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Watch these two videos for more information:

CO2 is Greening The Earth

The Climate Agenda



How to Wake Up Normies

Charlie Robinson had some good advice about waking up normies (see the link to the video below). He said instead of verbally unloading or being nasty or acting like a bully, ask the person a question. Being nice and asking a question will help the person actually think about the subject.


Interesting videos:

Charlie Robinson Talks About the Best Way to Wake Up Normies

Georgia Guidestones Explained

The Men Who Own Everything


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