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
Step 10: "Random Numbers"
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.
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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.