Let’s Make a Game!
Step 10: “Random Numbers”
By Darrell Spice, Jr. (adapted by Duane Alan Hahn, a.k.a. Random Terrain)
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Original Blog Entry
And yes, those are air quotes. 
Inside the Atari (and computers in general), there's no such thing as a Random Number. We can, however, simulate random numbers by using a Linear Feedback Shift Register. The LFSR we're going to use was posted by batari, it's a rather slick bit of code in that it can be either an 8-bit or 16-bit LFSR.
Random:
lda Rand8
lsr
ifconst Rand16
rol Rand16 ; this command is only used if Rand16 has been defined
endif
bcc noeor
eor #$B4
noeor
sta Rand8
ifconst Rand16
eor Rand16 ; this command is only used if Rand16 has been defined
endif
rts
LFSR's create what appear to be a random sequence of numbers, but they're not. A properly constructed 8-bit LFSR will start repeating values after 255 values have been obtained. All you need to do in order to use the above routine as an 8-bit LFSR is allocate a variable called Rand8:
; used by Random for an 8 bit random number Rand8: ds 1 ; stored in $AB
A 16-bit LFSR will repeat after 65535 values have been obtained—that's a lot better than 255; however, it requires you to allocate another variable. With the Atari's limited 128 bytes of RAM, games often didn't have RAM to spare so they'd just use an 8-bit LFSR. Collect is a rather simple game though, so we have RAM to spare and will allocate the second variable Rand16.
; optionally define space for Rand16 for 16 bit random number Rand16: ds 1 ; stored in $AC
The random number generator does have potential problem to be aware of—if you're using an 8-bit LFSR and Rand8 has the value of 0 then the LFSR will always return 0. Likewise for the 16-bit LFSR if both Rand8 and Rand16 are 0 it will always return 0. So we need to initialize the LFSR as CLEAN_START set all the RAM variables to 0.
InitSystem:
CLEAN_START
...
; seed the random number generator
lda INTIM ; unknown value
sta Rand8 ; use as seed
eor #$FF ; both seed values cannot be 0, so flip the bits
sta Rand16 ; just in case INTIM was 0
One thing that helps when using an LFSR is to keep reading values at a regular rate, even if you don't need the value. What this does is impose an element outside of the Atari's control—namely the time it takes the human to things: hit the RESET switch to start a game, collect the next box, etc. I've added this to VerticalBlank:
VerticalBlank:
jsr Random
...
Now that we have a function for random numbers we need to use them. First up is new routine that will randomly position any object. To use this function we just need to call it with X register holding a value that denotes which object to position. The values are the same ones used for the PosObject function, namely 0=player0, 1=player1, 2=missile0, 3=missile1 and 4=ball.
RandomLocation:
jsr Random ; get a random value between 0-255
and #127 ; limit range to 0-127
sta Temp ; save it
jsr Random ; get a random value between 0-255
and #15 ; limit range to 0-15
clc ; must clear carry for add
adc Temp ; add in random # from 0-127 for range of 0-142
adc #5 ; add 5 for range of 5-147
sta ObjectX,x ; save the random X position
jsr Random ; get a random value between 0-255
and #127 ; limit range to 0-127
sta Temp ; save it
jsr Random ; get a random value between 0-255
and #15 ; limit range to 0-15
clc ; must clear carry for add
adc Temp ; add in random # from 0-127 for range of 0-142
adc #26 ; add 26 for range of 26-168
sta ObjectY,x ; save the random Y position
rts
I've renamed InitPos to NewGame and modified it to call RandomLocation. It runs through a loop setting all the box objects. Starting X value will be 1 or 2 based on the number of players in the selected game variation.
NewGame:
...
; Randomly position the boxes for the new game. Set X to 1 for a 1 player
; game or 2 for a 2 player game so that the appropriate objects will be
; randomly placed in the Arena.
lda Variation
and #1 ; value of 0=1 player game, 1=2 player game
tax ; transfer to X
inx ; start with 1 for a 1 player game, or 2 for a 2 player game
IPloop:
jsr RandomLocation ; randomly position object specified by X
inx ; increase X for next object
cpx #5 ; check if we hit 5
bne IPloop ; branch back if we haven't
...
rts
I also modified OverScan so that during a 1-player variation a collision between player0 and player1 will be detected. If so, it calls another new function, CollectBox.
OverScan:
...
bit Players ; test how many players are in this game variation
bmi RightPlayer ; test Right Player collisions if its a 2 player game
bit CXPPMM ; else see if left player collected box drawn by player1
bpl OSwait ; player0 did not collide wth player1
ldx #1 ; which box was collected
jsr CollectBox ; update score and reposition box
jmp OSwait ; 1 player game, so skip Right Player test
CollectBox will increase the player's score and call RandomLocation again to move it to a new position. When CollectBox is called, Y must hold which player (0 for left, 1 for right) and X must hold the object that was collected.
CollectBox:
SED ; SEt Decimal flag
clc ; CLear Carry bit
lda #1 ; 1 point per box
adc Score,y ; add to player's current score
sta Score,y ; and save it
CLD ; CLear Decimal flag
jsr RandomLocation ; move box to new location
rts
CollectBox means that the game is now playable for 1-player games! I managed to score 26 on game variation 1, how well can you do?
One thing you might notice in that screenshot is that the right player's score is not visible. Since we're done using the score for diagnostics, I've made a few changes to the digit graphics:
The first change is the left 0 image was blanked out—this provides leading zero suppression:
The second change is the A image is now blanked out—NewGame uses this to blank out the right player's score in 1 player games.
NewGame:
...
; reset scores
ldx #0
stx Score
bit Players ; check # of players
bpl BlankRightScore
stx Score+1
rts
BlankRightScore:
lda #$AA ; AA defines a "space" character
sta Score+1
rts
Lastly, the graphics for B through F have been removed to save ROM space.
The ROM and the source are at the bottom of my blog entry.
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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Table of Contents for Let’s Make a Game!
Step 1: Generate a Stable Display
Step 3: Score and Timer Display
Step 5: Automate Vertical Delay
Step 8: Select and Reset Support
Step 10: "Random Numbers"
Step 12: Add the Missile Objects
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.
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, a friend, 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.
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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.
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: 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 YouTube 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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