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Let’s Make a Game!

Step 2: Timers

By Darrell Spice, Jr. (adapted by Duane Alan Hahn, a.k.a. Random Terrain)

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Original Blog Entry

 

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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 therelook 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

Be sure to check out the other assembly language tutorials and the general programming pages on this web site.

 

 

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Table of Contents for Let’s Make a Game!

Introduction

Step 1: Generate a Stable Display

Step 2: Timers

Step 3: Score and Timer Display

Step 4: 2 Line Kernel

Step 5: Automate Vertical Delay

Step 6: Spec Change

Step 7: Draw the Playfield

Step 8: Select and Reset Support

Step 9: Game Variations

Step 10: “Random Numbers”

Step 11: Add the Ball Object

Step 12: Add the Missile Objects

Step 13: Add Sound Effects

Step 14: Add Animation

 

 

 

 

Useful Links

Easy 6502 by Nick Morgan

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 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.

 

 

6502 Instruction Set with Examples

A useful page from Assembly Language Programming for the Atari Computers.

 

 

6502.org

Continually strives to remain the largest and most complete source for 6502-related information in the world.

 

 

Guide to 6502 Assembly Language Programming by Andrew Jacobs

Below are direct links to the most important pages.

 

 

Stella Programmer's Guide

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 Specifications

Atari 2600 programming specs (HTML version).

 

 

Atari 2600 Programming Page (AtariAge)

Links to useful information, tools, source code, and documentation.

 

 

MiniDig

Atari 2600 programming site based on Garon's "The Dig," which is now dead.

 

 

TIA Color Charts and Tools

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.

 

 

Game Standards and Procedures

A guide and a check list for finished carts.

 

 

Stella

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.

 

 

JAVATARI

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.

 

 

batari Basic Commands

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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In Case You Didn’t Know

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