By Darrell Spice, Jr. (adapted by Duane Alan Hahn)
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Original Blog Entry
For this update we're adding initial support for the Select and Reset buttons. For this we're adding a new RAM variable called GameState to keep track of "Game Active" vs "Game Over".
; D7, 1=Game Active, 0=Game Over GameState: ds 1 ; stored in $A7
We're going to use D7 to denote the state as we can easily test D7 (as well as D6) by using the BIT command. You can see this in the revised Vertical Blank routine were we test GameState to determine if UpdateTimer and ProcessJoystick should be skipped over:
VerticalBlank: jsr ProcessSwitches bit GameState bpl NotActive jsr UpdateTimer jsr ProcessJoystick NotActive: jsr PositionObjects jsr SetObjectColors jsr PrepScoreForDisplay rts ; ReTurn from Subroutine
ProcessSwitches will check SWCHB to see if RESET is pressed. If so, it'll start up a new game. If not, it'll check if SELECT is pressed, and if so cancel an active game.
ProcessSwitches: lda SWCHB ; load in the state of the switches lsr ; D0 is now in C bcs NotReset ; if D0 was on, the RESET switch was not held jsr InitPos ; Prep for new game lda #%10000000 sta GameState ; set D7 on to signify Game Active rts NotReset: lsr ; D1 is now in C bcs NotSelect lda #0 sta GameState ; clear D7 to signify Game Over NotSelect: rts
In the next update ProcessSwitches will be expanded upon so that the Select routine will let you select a game variation (and if you check the source you'll see a new Arena layout is already in place for that).
In order to visually show you that the game is over, I've revised the Color routines to color cycle if the game is not active.
SetObjectColors: lda #$FF sta Temp2 ; default to color mask and ColorCycle ; color cycle bit GameState bpl SOCgameover lda #0 ; if game is active, no color cycle SOCgameover: sta Temp ldx #4 ; we're going to set 5 colors (0-4) ldy #4 ; default to the color entries in the table (0-4) lda SWCHB ; read the state of the console switches and #%00001000 ; test state of D3, the TV Type switch bne SOCloop ; if D3=1 then use color ldy #$0f sty Temp2 ; set B&W mask ldy #9 ; else use the b&w entries in the table (5-9) SOCloop: lda Colors,y ; get the color or b&w value eor Temp ; color cycle and Temp2 ; B&W mask sta COLUP0-1,x ; and set it dey ; decrease Y dex ; decrease X bne SOCloop ; Branch Not Equal to Zero lda Colors,y ; get the Arena color eor Temp ; color cycle and Temp2 ; B&W mask sta ArenaColor ; save in RAM for Kernal Usage rts ; ReTurn from Subroutine
Color cycle example:
B&W Color Cycle example:
The ROM and the source are at the bottom of my blog entry.
Goals for this tutorial.
On other systems, the video chip generates the display; on the 2600, your program generates the display.
Improve the display generation by using the built-in timer.
Using the playfield to display information.
Draw the player objects (sprites) on screen (X & Y location).
Finish the Y positioning of the player objects (sprites).
Revise our goals.
Display an arena (like the mazes in Combat).
Step 8: Select and Reset Support
Using the Game Select and Game Reset console switches.
How to implement game variations (number of players, different mazes).
How to randomize your game.
Draw the ball on screen (X & Y location).
Draw the missiles on screen (X & Y location)
Let’s make some noise!
Make the humans run instead of glide.
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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