Thanks to Matt Jernigan for compiling and contributing this page.
(Adapted by Duane Alan Hahn, a.k.a. Random Terrain.)
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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.
| Address | Name | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | Function |
| 20 | HMP0 | 1 | 1 | 1 | 1 | . | . | . | . | Horizontal Motion Player 0 |
| 21 | HMP1 | 1 | 1 | 1 | 1 | . | . | . | . | Horizontal Motion Player 1 |
| 22 | HMM0 | 1 | 1 | 1 | 1 | . | . | . | . | Horizontal Motion Missile 0 |
| 23 | HMM1 | 1 | 1 | 1 | 1 | . | . | . | . | Horizontal Motion Missile 1 |
| 24 | HMBL | 1 | 1 | 1 | 1 | . | . | . | . | Horizontal Motion Ball |
Horizontal motion allows the programmer to move any of the 5 graphics objects relative to their current horizontal position. Each object has a 4 bit horizontal motion register (HMP0, HMP1, HMM0, HMM1, HMBL) that can be loaded with a value in the range of +7 to -8 (negative values are expressed in two's complement from). This motion is not executed until the HMOVE register is written to, at which time all motion registers move their respective objects. Objects can be moved repeatedly by simply executing HMOVE. Any object that is not to move must have a 0 in its motion register. With the horizontal positioning command confined to positioning objects at 15 color clock intervals, the motion registers fills in the gaps by moving objects +7 to -8 color clocks. Objects can not be placed at any color clock position across the screen. All 5 motion registers can be set to zero simultaneously by writing to the horizontal motion clear register (HMCLR).
There are timing constraints for the HMOVE command. The HMOVE command must immediately follow a WSYNC (Wait for SYNC) to insure the HMOVE operation occurs during horizontal blanking. This is to allow sufficient time for the motion registers to do their thing before the electron beam starts drawing the next scan line. Also, for mysterious internal hardware considerations, the motion registers should not be modified for at least 24 machine cycles after an HMOVE command.
These addresses write data (horizontal motion values) into the horizontal motion registers. These registers will cause horizontal motion only when commanded to do so by the horiz. move command HMOVE. The motion values are coded as shown below:
| D7 | D6 | D5 | D4 | Clocks | Effect |
| 0 | 1 | 1 | 1 | +7 | Move left indicated number of clocks |
| 0 | 1 | 1 | 0 | +6 | |
| 0 | 1 | 0 | 1 | +5 | |
| 0 | 1 | 0 | 0 | +4 | |
| 0 | 0 | 1 | 1 | +3 | |
| 0 | 0 | 1 | 0 | +2 | |
| 0 | 0 | 0 | 1 | +1 | |
| 0 | 0 | 0 | 0 | 0 | No Motion |
| 1 | 1 | 1 | 1 | -1 | Move right indicated number of clocks |
| 1 | 1 | 1 | 0 | -2 | |
| 1 | 1 | 0 | 1 | -3 | |
| 1 | 1 | 0 | 0 | -4 | |
| 1 | 0 | 1 | 1 | -5 | |
| 1 | 0 | 1 | 0 | -6 | |
| 1 | 0 | 0 | 1 | -7 | |
| 1 | 0 | 0 | 0 | -8 |
Warning: These motion registers should not be modified during the 24 computer cycles immediately following an HMOVE command. Unpredictable motion values may result.
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
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
...
SetPosX SUBROUTINE
; calculates the values and positions objects:
; INPUT:
; A = x-position
; X = object to position (0=P0, 1=P1, 2=M0, 3=M1, 4=BL)
JSR CalcPosX ; 6 $FAEF
SetPosX2:
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
.waitPos:
DEY ; 2
BPL .waitPos ; 2/3 5-cycle loop = 15 TIA color clocks (pixels)
STA RESP0,X ; 4
RTS ; 6
...
CalcPosX SUBROUTINE
; 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
;
; INPUT:
; A = x-position
; RETURN:
; 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)
.skipIny:
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).
Mult16:
ASL ; 2 A's lower 4 bits shifted to upper 4
ASL ; 2
ASL ; 2
ASL ; 2
Wait12:
RTS ; 6
...
DoHMove:
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: Atari 2600 Programming (#ad)
Amazon: 6502 Assembly Language Programming (#ad)
Atari 2600 Programming for Newbies (#ad)
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Session 2: Television Display Basics
Sessions 3 & 6: The TIA and the 6502
Session 5: Memory Architecture
Session 7: The TV and our Kernel
Session 9: 6502 and DASM - Assembling the 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 20: Asymmetrical Playfields (Part 3)
Session 22: Sprites, Horizontal Positioning (Part 1)
Session 22: Sprites, Horizontal Positioning (Part 2)
Session 23: Moving Sprites Vertically
Session 25: Advanced Timeslicing
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 6502 Instruction Set broken down into 6 groups.
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.
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.
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.
Gives a summary of whole instruction set.
Describes each of the 6502 memory addressing modes.
Describes the complete instruction set in detail.
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 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.
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.
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.
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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Did you know that Trump's rushed Operation Warp Speed rona jab has less than one percent overall benefit? Some people call it the depopulation jab and it has many possible horrible side effects (depending on the lot number, concentration, and if it was kept cold). Remember when many Democrats were against Trump's Operation Warp Speed depopulation jab, then they quickly changed their minds when Biden flip-flopped and started pushing it?
Some brainwashed rona jab cultists claim that there are no victims of the jab, but person after person will post what the jab did to them, a friend, or a family member on web sites such as Facebook and they'll be lucky if they don't get banned soon after. Posting the truth is “misinformation” don't you know. Awakened sheep might turn into lions, so powerful people will do just about anything to keep the sheep from waking up.
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?
Take a look at my page about the famous demonized medicines called The H Word and Beyond. You might also want to look at my page called Zinc and Quercetin. My sister and I have been taking zinc and quercetin since the summer of 2020 in the hopes that they would scare away the flu and other viruses (or at least make them less severe). Here's one more page to check out: My Sister's Experiences With COVID-19.
Some people appear to have a mental illness because they have a vitamin B deficiency. For example, the wife of a guy I used to chat with online had severe mood swings which seemed to be caused by food allergies or intolerances. She would became irrational, obnoxious, throw tantrums, and generally act like she had a mental illness. The horrid behavior stopped after she started taking a vitamin B complex. I've been taking Jarrow B-Right (#ad) for many years. It makes me much easier to live with. I wonder how many people with schizophrenia and other mental mental illnesses could be helped by taking a B complex once or twice a day with meals (depending on their weight)?
Unfermented soy is bad! “When she stopped eating soy, the mental problems went away.” Fermented soy doesn't bother me, but the various versions of unfermented soy (soy flour, soybean oil, and so on) that are used in all kinds of products these days causes a negative mental health reaction in me that a vitamin B complex can't tame. The sinister encroachment of soy has made the careful reading of ingredients a necessity.
I wouldn't be surprised to find out that unfermented soy is the main reason why so many soy-sucking Democrats in the USA seem to be constantly angry and have a tendency to be violent when hearing words or reading signs that they don't agree with. If I unknowingly eat something with unfermented soy in it, I get irritable, angry, and feel like breaking things, so it's not the placebo effect. Scientists in the future will probably find out that unfermented soy can make people angry. We already know that food sensitivities cause mood changes. It took me over a decade to figure out that unfermented soy was affecting my mood. What if millions of people are having a similar reaction to soy and don't even know it? Some people eat it and drink it every day.
I started taking AyaLife (99% Pure CBD oil) as needed in April of 2020. So far it's the only thing that helps my mood when I've mistakenly eaten something that contains soy. AyaLife is THC-free (non-psychoactive) and is made in the USA. I also put a couple dropper fulls under my tongue before leaving the house or if I just need to calm down.
It's supposedly common knowledge that constantly angry Antifa-types basically live on soy products. What would happen if they stopped eating and drinking soy sludge and also took a B complex every day? Would a significant number of them become less angry? Would AyaLife CBD oil also help?
If you are overweight, have type II diabetes, or are worried about the condition of your heart, check out the videos by Ken D Berry, William Davis, and Ivor Cummins. It seems that most people should avoid wheat, not just those who have a wheat allergy or celiac disease. Check out these books: Undoctored (#ad), Wheat Belly (#ad), and Eat Rich, Live Long (#ad).
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.
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.
Eliminating farms and ranches, eating bugs, getting locked down in 15-minute cities, owning nothing, using digital currency (with expiration dates) that is tied to your social credit score, and paying higher taxes will not make things better and “save the Earth.” All that stuff is part of an agenda that has nothing to do with making the world a better place for the average person. It's all about control, depopulation, and making things better for the ultra-rich. They just want enough peasants left alive to keep things running smoothly.
Watch these two videos for more information:
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
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