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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










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




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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 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.

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.




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.




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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