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Atari 2600 Programming for Newbies

Session 22: Sprites, Horizontal Positioning (Part 2)

Thanks to Matt Jernigan for compiling and contributing this page.

(Adapted by Duane Alan Hahn, a.k.a. Random Terrain.)

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Page Table of Contents

Original Session

For those looking for the missing part 2, here is Matt Jernigan's interpretation of what it was set to cover (more or less).






Fine Tuning Horizontal Position

This section is taken from the Stella Programmer's Guide by Steve Wright 12/03/79.









Undocumented Quirk

Not all objects position exactly the same. Double-wide and quad-wide player sprites shift +1 color clock. Missiles and the ball shift -1 color clock.


For example, to compensate for this, the quad-wide bridge sprite in River Raid is curiously set to an x-position of 63 (instead of 64) before its fine-positioning subroutine is called. This quirk will shift the bridge +1 color clock and draw it at the intended x-position of 64.


This quirk is discussed in more detail here:


RESPx, NUSIZx, and Player Positioning






River Raid Example Code

Below is example code from River Raid with comments from Matt Jernigan and Thomas Jentzsch:

       INX                      ; 2             x = 0! (player jet)
       LDY    playerX           ; 3             y = player's x-pos
       TYA                      ; 2             a = y
       JSR    SetPosX           ; 6             x-position player jet
       INX                      ; 2             x = 1 (enemy object)
       INX                      ; 2             x = 2 (player missile)
       LDA    missileX          ; 3             a = missile's x-pos
       JSR    SetPosX           ; 6             x-position missile
       JSR    DoHMove           ; 6             do HMOVE


; calculates the values and positions objects:
;   A = x-position
;   X = object to position (0=P0, 1=P1, 2=M0, 3=M1, 4=BL)
       JSR    CalcPosX          ; 6 $FAEF
       STA    HMP0,X            ; 4             fine position the object specified in X
       INY                      ; 2             get past the 68 pixels of horizontal blank
       INY                      ; 2
       INY                      ; 2
       STA    WSYNC             ; 3             wait for sync; wait for TIA to finish line
       DEY                      ; 2
       BPL    .waitPos          ; 2/3           5-cycle loop = 15 TIA color clocks (pixels)
       STA    RESP0,X           ; 4
       RTS                      ; 6


; calculates values for fine x-positioning:
; Basically, divides pos by 15 and stores the int result of that.
; Then, the mod of that is adjusted to equal 6 + HMP1.
;   HMPx bits 4..7: Offset value:
;     0000 ($00): No offset
;     0001 ($10): Left 1 clock
;     0010 ($20): Left 2 clocks
;     0011 ($30): Left 3 clocks
;     0100 ($40): Left 4 clocks
;     0101 ($50): Left 5 clocks
;     0110 ($60): Left 6 clocks
;     0111 ($70): Left 7 clocks
;     1000 ($80): Right 8 clocks
;     1001 ($90): Right 7 clocks
;     1010 ($A0): Right 6 clocks
;     1011 ($B0): Right 5 clocks
;     1100 ($C0): Right 4 clocks
;     1101 ($D0): Right 3 clocks
;     1110 ($E0): Right 2 clocks
;     1111 ($F0): Right 1 clock
;   A = x-position
;   Y = coarse value for delay loop (shifted to lower 4 bits), 1 loop = 5 clocks = 15 pixels
;   A = fine value for HMPx, HMMx, or HMBL
       TAY                      ; 2 $FDD8       Y = A
       INY                      ; 2             Y++           (setup for div 15; causes 15 to overflow)
       TYA                      ; 2             A = Y
       AND    #$0F              ; 2             A chopped to lower 4 bits 00001111 (fine pos value + 1)
       STA    temp2             ; 3       $ED   temp2 = A  (fine pos value + 1)
       TYA                      ; 2             A = Y
       LSR                      ; 2             A chopped to upper 4 bits and shifted to lower 4 bits (A div 16)
       LSR                      ; 2
       LSR                      ; 2
       LSR                      ; 2
       TAY                      ; 2             Y = A, backup shifted bits to Y
       CLC                      ; 2             CF = 0
       ADC    temp2             ; 3             A = A + temp2  (shifted bits added to change this from div 16 to div 15)
       CMP    #15               ; 2             is A < 15?     (look for div 15 overflow)
       BCC    .skipIny          ; 2              yes, skip to EOR  (no div 15 overflow)
       SBC    #15               ; 2              no, A = A - 15  (CF = 1 but not needed here)
       INY                      ; 2               Y++              (yes on the div 15 overflow so one-up Y)
       EOR    #$07              ; 2             A = A XOR 00000111,  7's complement, sets offset for 6 + HMPx
; NOTE: Any JSR here will use 20 cycles instead of 8
; but saves 1 byte of code overall (3 bytes instead of 4).
       ASL                      ; 2             A's lower 4 bits shifted to upper 4
       ASL                      ; 2
       ASL                      ; 2
       ASL                      ; 2
       RTS                      ; 6


       STA    WSYNC             ; 3             wait for sync; wait for TIA to finish line
       STA    HMOVE             ; 3             must follow WSYNC if horizontal motion desired
       RTS                      ; 6






Hopefully, after this, Session 24 will make more sense, which discusses other algorithms for horizontal positioning.




Other Assembly Language Tutorials

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


Amazon Stuff


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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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In Case You Didn’t Know


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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.



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The Climate Agenda



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Take a look at my page called The H Word and Beyond.


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