By Andrew Davie (adapted by Duane Alan Hahn, a.k.a. Random Terrain)
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Page Table of Contents
Time is tight. Really tight! The general approach has been to think of the TV frame as the limiting factor for the capabilities of the machine. Whatever you can do in 'one frame' (i.e, nominally @60Hz on NTSC or @50Hz on PAL) . . . that's IT. So in fact you can work out exactly how much time you have to do stuff. As we've seen in earlier tutorials, the '2600 programmer has to pump data out to the TIA in sync with the TV as it's drawing scanlines. You need to feed the TV scanlines to draw a proper picture. There are 76 cycles per scanline, and 262 scanlines per standard TV frame (312 for PAL). So 76 * 262 = 19912 cycles per frame. Multiply that by the NTSC frame rate (actually 59.94Hz) and you get . . . 1193525.28 (i.e., there's our 1.19MHz CPU clock speed). It all makes sense.
So, just 262 lines. The visible screen is smaller than that, of course (usually 192 scanlines of actual graphics)—so we only need to pump data to the screen for a smaller number of lines. The rest is black, nothing to see. Below is a good visual diagram of where the time goes.
So, during those blank lines, the CPU doesn't have to pump data to the screen. In fact these two major areas of 'blackness' (that is, the vertical blank, and the overscan) account for 37 scanlines (*76 = 2812 cycles) and 30 scanlines (*76 = 2280 cycles). Now that's not exactly swimming in available CPU capacity but it's better than nothing. So the general usage of these blank areas has been to whack in 'stuff' that takes a fair bit of time to do.
The problem is, you can't whack in too MUCH stuff. Because when those 37 scanlines of time have elapsed, you MUST be writing to the TIA again to make sure the next frame is displaying properly. Same for the 30 lines of overscan. There's no getting around it; you take too much time, and you stuff up the timing, and consequently the TV picture will roll, judderjitter and basically look horrible. The hard and fast rule has been to simply stay within the limitation, or to reduce the number of visible scanlines to give more processing time for doing more complex STUFF. Each scanline of visible data you sacrificed, you got 76 scanlines of available time to do your stuff. A compromise.
Fortunately, we have the timer registers. These are single countdown registers that will regularly decrement a value written to them. I only use TIM64T—this one counts 64 cycle blocks. If I write 10 to it, then I would expect it to reach 0 some 640 cycles later. So, the usage has been to calculate the amount of time before the screen drawing has to (re)commence, divide by 64, and put that value in TIM64T. By reading INTIM and waiting until that reaches 0, you effectively wait the right number of cycles. You can do your (variable time) 'stuff' and not really care about how long it takes (as long as it doesn't take TOO long), and after it's finish you enter a tight loop just reading INTIM and waiting for it to go to 0. When it goes to 0, fire off a WSYNC and then begin the TV frame drawing once again.
That's how it's BEEN done, but that's not how I did it in Boulder Dash!
The INTIM register effectively tells you not only if you're out of time, but also exactly how MUCH time you have remaining (in blocks of 64 cycles if you're using TIM64T). So, if you think about it, you can actually make decisions about if you should call a subroutine based on this value. For example, say you had a small routine which you know takes (say) 1000 cycles to run. That's 1000/64 units (= 15.625). So, if INTIM was reading 16 or greater you KNOW you can call that subroutine and not run out of time! This gets rather nice. Given a guaranteed maximum run-time for any subroutine (and you get this by cycle-counting the subroutine very very carefully), you can use this knowledge to determine if/when it's appropriate to call that subroutine. Furthermore, after you HAVE called the subroutine, you can repeat the process—look at INTIM and determine if there's enough time to run OTHER subroutines.
So the whole concept of '2600 programming basically changes here. Now we have an asynchronous system, where you have a queue of 'tasks' that you have to do. These tasks in Boulder Dash are generally creature logic (process a boulder, the amoeba, etc). Each of these tasks are cycle-counted so we know exactly how long the worst-case is. And each of these tasks is only run if there's available time. If not, then they simply return and in the next chunk of available time, they will be called again.
So, this is how the timeslicing engine works! Every part of the game logic is broken down into as small (quick) units of code as practicable. Rather than have the whole processing for an object in a single huge and costly block of code, where possible these are broken down into even smaller 'sub-tasks'. And those tasks are effectively placed in a queue which is processed by the task manager. The task manager is a tight loop which pulls a task off the task stack, vectors to the appropriate handler for the task, and repeats. The tasks themselves are responsible for deciding if there's enough time for them to do their own stuff (i.e., fairly object-oriented in that regard). If a task doesn't think there's enough time (again, by simply reading INTIM and comparing with it's own timing equate), it simply returns. If it has enough time to do its stuff, it does so and makes sure that it's no longer on the task queue. Tasks can even add other tasks to the queue, for later processing!
The upshot of all this is that a game doesn't have to be able to handle the very worst case most expensive thing ever in a single frame. The tasks split across multiple frames, if needed. In other words, there's now a separation between game logic (running over multiple frames if required) and the frame display (running exactly at the TV frame rate). Yes, Virginia, '2600 games can slow down. Now for most situations this isn't ideal—but in reality it doesn't really matter. Most of the gameplay for the '2600 Boulder Dash just never slows down. But occasionally, very occasionally (say, when an amoeba turns into 200 boulders and they all start falling at the same time)—well, the system can handle it. Because although it may only have enough processing power to handle (say) 20 boulders in a single frame, that's OK, because the other boulders are effectively stacked and processed the next frame. And the queue may be really big for a few game loops, and the game will lag... probably not very noticeably... but when the queue is empty again, everything is back to running full speed.
So the above is the secret to making much more complex games than have heretofore been produced on the machine. You CAN keep the TV display going full speed (60Hz) while doing processing-intensive game logic. And you CAN do very very very complex game logic taking absolutely heaps of processing time. The trick, as noted, is to separate out the two so they are not synchronous—and to divide the complex logic into discrete, very quick, sub-components.
Divide and conquer!
Other Assembly Language Tutorials
Session 25: Advanced Timeslicing
This book was written in English, not computerese. It's written for Atari users, not for professional programmers (though they might find it useful).
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).
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.
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.
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.
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.
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.
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).
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.
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.
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.
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 #ad Jarrow B-Right for many years. It makes me much easier to live with.
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.
If you are overweight, have type II diabetes, or are worried about the condition of your heart, check out the videos by 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: #ad Undoctored, #ad Wheat Belly, and #ad Eat Rich, Live Long.
Negative ions are good for us. You might want to avoid positive ion generators and ozone generators. Whenever I need a new air cleaner (with negative ion generator), I buy it from surroundair.com. 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.
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.
Watch these two YouTube videos for more information:
Hydrofracking is bad for you, your family, your friends, and the environment.
Although some people with certain conditions may not be able to take it, hydroxychloroquine is a cheap drug that has been prescribed by doctors since the 1950s and it seems to be helping many people who have COVID-19 when administered early enough. (Hydroxychloroquine is also supposedly safe and tolerable as an anti-cancer therapy.) Seems like most news sources are going out of their way to make it sound like hydroxychloroquine is the most dangerous drug in the world, but they also make it sound like it’s the greatest drug in the world for lupus and rheumatoid arthritis patients. They basically say that using hydroxychloroquine for COVID-19 patients would be taking that great and wonderful drug away from the other patients who need it. So which is it? Is hydroxychloroquine deadly or divine?
If you believe that a couple of Trump supporters took the medicine hydroxychloroquine and it’s President Trumps fault that the husband died, you’ve been duped. Watch this video. The wife was a prolific Democratic donor, it seems she hated her husband, she used fish tank cleaner (not the medicine hydroxychloroquine), and now she is the subject of a homicide investigation.
Some people claim that the reason so many news sources want to keep doctors from using hydroxychloroquine for COVID-19 is that they are desperate to keep everyone afraid to leave their homes since mail-in voting will make voter fraud much easier (the only way they could beat Trump). Others claim that the rabid anti-hydroxychloroquine campaign was to make way for the expensive new drug called remdesivir. Drug companies can’t make much money with old generic drugs, so new drugs must be pushed. Both claims could be true since remdesivir supposedly isn’t as good as hydroxychloroquine.
According to Dr. Shiva Ayyadurai, hydroxychloroquine does four things: (1) stops viral entry, (2) stops viral RNA replication, (3) stops viral particle assembly, and (4) stops cytokine storm. Remdesivir only stops viral RNA replication. Did you get that? Hydroxychloroquine does four things and remdesivir only does one. The doctor also said that nearly 70 percent of the people who took remdesivir had some type of adverse effect. If all of that is true and the more anemic medicine ends up being used by most doctors thanks to the smear campaign against hydroxychloroquine, the average American will beg to vote from home.
In case you didn’t know, Patrick Howley reported that one of the authors of the ‘study’ saying that hydroxychloroquine doesn’t work at VA hospitals got a research grant from Gilead (the company that makes remdesivir). Does that seem a little fishy to you?
Bryan Fischer said in an article that Dr. Fauci has known since 2005 that chloroquine is an effective inhibitor of coronaviruses. You might also want to check out the following three links:
“The Disruptive Physician” had this to say at Twitter: “Meanwhile, regular doctors like me are using HCQ + Azithromycin and Zinc to good effect. One nursing home in NE Ohio had 30 cases - started everyone on HCQ, no deaths. Quick recovery. Why would the MSM hide this? Why would twitter block people who question the WHO?” You might also want to check out Dr. Stephen Smith, Dr. Ramin Oskoui and Dr. Yvette Lozano.
In case you’re interested, here are a few COVID-19 patients who appear to claim that hydroxychloroquine saved their lives: elderly couple Louis Amen and Dolores Amen, Daniel Dae Kim, Rio Giardinieri, John McConnell, Margaret Novins, Jim Santilli, Billy Saracino, and Karen Whitsett (Democratic member of the Michigan House of Representatives).
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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.