By Darrell Spice, Jr. (adapted by Duane Alan Hahn, a.k.a. Random Terrain)
As an Amazon Associate I earn from qualifying purchases. (I get commissions for purchases made through certain links on this page.)
Original Blog Entry
In Step 1, I used loops of sta WSYNC commands to delay the program so that Vertical Blank and OverScan would last for the proper duration. That method works fine when all we want to do is generate a static display, but as soon as we start to add game logic that won't work out so well.
The problem with the game logic is there will be so many different paths the code can take that it is nearly impossible for us to know how long the code ran, and thus we won't know how many scanlines we need to delay before the next section of code can run. As an example, if the player isn't moving the joystick then none of the "move player" logic will run. If the player is moving the joystick left and up then the "move horizontal" and "move vertical" logic will run. If the player is only holding the joystick left then only the "move horizontal" logic will run.
Fortunately for us, the Atari 2600 contains a RIOT chip. That acronym stands for RAM, Input/Output and Timer. We're interested in the Timer for this update to Collect, we'll look at RAM and I/O in a later update.
First thing I changed was OverScan. The original routine looked like this:
OverScan: sta WSYNC ; Wait for SYNC (halts CPU until end of scanline) lda #2 ; LoaD Accumulator with 2 so D1=1 sta VBLANK ; STore Accumulator to VBLANK, D1=1 turns image output off ldx #27 ; LoaD X with 27 osLoop: sta WSYNC ; Wait for SYNC (halts CPU until end of scanline) dex ; DEcrement X by 1 bne osLoop ; Branch if Not Equal to 0 rts ; ReTurn from Subroutine
So what we want to do is set a timer that will go off after 27 scanlines to pass. There's 76 cycles of time per scanline, so we need the timer to go off after 2052 cycles have passed. When we set the timer, we also select how frequently the timer will decrement in value. RIOT has options to decrement the timer every 1, 8, 64 or 1024 cycles.
The timer is set using a single byte, so it can only be set to any value from 0 to 255. As such, we know we can't use decrement every 1 cycle as 2052 is too large. So let's check if decrement every 8 cycles will work:
2052/8 = 256.5
Almost, but 256 won't fit so we're going to have to use the decrement every 64 cycles option. To figure out the initial value to set the timer to, use this equation:
(scanlines * 76) / 64
The new OverScan routine that uses the timer looks like this:
OverScan: sta WSYNC ; Wait for SYNC (halts CPU until end of scanline) lda #2 ; LoaD Accumulator with 2 so D1=1 sta VBLANK ; STore Accumulator to VBLANK, D1=1 turns image output off lda #32 ; set timer for 27 scanlines, 32 = ((27 * 76) / 64) sta TIM64T ; set timer to go off in 27 scanlines ; game logic will go here OSwait: sta WSYNC ; Wait for SYNC (halts CPU until end of scanline) lda INTIM ; Check the timer bne OSwait ; Branch if its Not Equal to 0 rts ; ReTurn from Subroutine
For Vertical Blank we're going to set up the timer a little different. There's time in the Vertical Sync we can utilize, so we'll set the timer there—look for the code using ldx and stx:
VerticalSync: lda #2 ; LoaD Accumulator with 2 so D1=1 ldx #49 ; LoaD X with 49 sta WSYNC ; Wait for SYNC (halts CPU until end of scanline) sta VSYNC ; Accumulator D1=1, turns on Vertical Sync signal stx TIM64T ; set timer to go off in 41 scanlines (49 * 64) / 76 sta WSYNC ; Wait for Sync - halts CPU until end of 1st scanline of VSYNC sta WSYNC ; wait until end of 2nd scanline of VSYNC lda #0 ; LoaD Accumulator with 0 so D1=0 sta WSYNC ; wait until end of 3rd scanline of VSYNC sta VSYNC ; Accumulator D1=0, turns off Vertical Sync signal rts ; ReTurn from Subroutine
We're also going to check the timer in the Kernel section so we can start drawing the screen as soon as it goes off:
Kernel: sta WSYNC ; Wait for SYNC (halts CPU until end of scanline) lda INTIM ; check the timer bne Kernel ; Branch if its Not Equal to 0 ; turn on the display sta VBLANK ; Accumulator D1=0, turns off Vertical Blank signal (image output on) ; draw the screen ldx #192 ; Load X with 192 KernelLoop: sta WSYNC ; Wait for SYNC (halts CPU until end of scanline) stx COLUBK ; STore X into TIA's background color register dex ; DEcrement X by 1 bne KernelLoop ; Branch if Not Equal to 0 rts ; ReTurn from Subroutine
For the moment, these changes leave Vertical Blank with nothing to do:
VerticalBlank: rts ; ReTurn from Subroutine
The ROM and the source are at the bottom of my blog entry.
Other Assembly Language Tutorials
Step 2: Timers
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.
Did you know that Trump's rushed experimental rona jab has less than one percent overall benefit? It also has many possible horrible side effects. 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 or a family member on web sites such as Facebook and Twitter 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:
Take a look at my page 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 those two supplements since summer of 2020 in the hopes that they would scare away the flu and other viruses (or at least make them less severe).
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 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: #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:
View this page and any external web sites at your own risk. I am not responsible for any possible spiritual, emotional, physical, financial or any other damage to you, your friends, family, ancestors, or descendants in the past, present, or future, living or dead, in this dimension or any other.
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.