1. Atari 2600 Program Development
Joe Decuir
alumnus of Atari & Amiga

2. Agenda Goals and non-goals Why do this
Requirements for Program Development
Development Tools
Hardware environment
System programming model
Example Game Design (Combat)
Suggestions

3. Goals for today
Audience understands what is required to develop game software for a production Atari 2600 VCS
Some discussion about how that might apply to developing game software for successors, both hardware and emulator software.
Not goal: demonstrations – I can’t do that without a video projector.

4. Why program for the VCS today?
There is not a mass market to buy cartridges
There is an emerging base of users who enjoy simpler games:
Owners of original Atari 2600 VCS consoles
PC owners running emulators
To create games without the huge effort involved in creating modern graphics-intensive games.
Summary: fun, education, impress your friends

5. Requirements for Program Development
Target environment requirements
Specifications for the hardware, registers, etc
Program generation tools: assembler
Target environment emulator, hard or soft
Program debug tools
Program distribution:
ROM cartridges
Binary image files for use on emulators

6. Objective of Program Development
Produce a 6507 binary image:
2-4K bytes for use as a 2600 cartridge
Possibly larger when used with a software emulator, particularly if it can emulate bank-switching techniques (e.g. PCAE 2.2)

7. Development Targets There are several development targets:
Original Atari 2600 hardware
VCS Hardware clones (e.g. Coleco)
Modern hardware clones, e.g. LCD handheld models
PC emulators (e.g PCAE 2.2, etc)
The capabilities of the successive targets may exceed those of the original hardware, in memory, but they have to match some other limits to remain compatible with old games.

8. Original Production 2600 Hardware Resources
6507 at 1.2MHz ( / 3)
6502, 13 bit address space, no interrupts, RDY
128 bytes of RAM, mapped to 00XX, 01XX
Space for ROM cartridges of up to 4KB
Two joysticks, supported by TIA and 6532
TIA video and audio chip
6532 Timer

9. Original Production 2600 Hardware Limitations
NO host development capabilities
NO interrupts – single thread for everything
NO DMA, 6507 does everything
NO vertical support in hardware
Very small RAM
Small ROM, 8K address space
Note: software emulators can remove these limitations, a boon for program debugging.

10. Original 2600 Program Development Environment
6502 Cross Assembler, based on host
E.g. timesharing machine, or DEC PDP-11
Hardware Emulator, w/RAM in code space
Debug monitor, for downloading and manipulating object code
HP-1600 series symbolic logic analyzer
Capture and disassemble bus traffic
Pre-trigger or post-trigger on bus values

11. 6502 Tools 6502 is 26 years old; current tools are scarce.
Self-assemblers for 6502 code ran on Apple II, Atari 800/PCS or Commodore 64s
Windows or Macintosh PCs have plenty of resources to run cross assemblers (or compilers) for They also exist for older machines:
6502-specific hardware logic analyzers are out of production
They might be available used

12. Simple way to test game code
Make a ROM image
Burn a PROM
Mount it on a cartridge
Plug it into a production VCS
See what happens
Iterate until it works as intended

13. Easiest VCS Development Today
Start in a soft environment:
Use suitable PC Emulator (e.g. PCAE)
Generate assembly code:
PC Cross-assembler
Run the code in the emulator, with Debug tools turned on

14. PCAE 2.2 Debug Features An example Atari VCS emulator for PCs
Display and/or modify:
Program code
6507 registers
Zero page RAM
TIA registers
Current virtual beam position
Online command and TIA register reference
Breakpoints on conditions

15. Hardware environment: Testing ROMs
The developer needs only two tools:
PROM burner
PC based
Stand alone serial port devices
PROM cartridge with socket
4KB DIP package PROMs are long obsolete
Extra logic is needed to invert A12 as a chip select (e.g. an inverter)

16. Making a Hardware Debugger
Get a 6502 evaluation board with a debug monitor: KIM-1 ; JOLT (using TIM 6530 chip; Rockwell AIM-65; Synertek SYM-1; EPE
See:
Wire wrap a board with:
6507 socket, wired to the 6502 pins
Decoder logic replacing the A12 pin on the 6507
At least 4KB of RAM for Code
Serial connection to a host PC, if not on eval board
Take apart a 2600 VCS, remove the motherboard, remove the 6507 chip, connect the 6507 motherboard socket to the debug board socket.

17. Hardware test system diagram
Debug Board
W/monitor
RS-
232
Optional Hardware
logic analyzer
Host Assembler
System
Modified
Atari VCS
w/6507
socket TV
Game controllers

18. Hardware vs Software Debug environments
Software emulation wasn’t feasible until recently
PCAE runs full speed on a 486/
Software debug has many advantages:
The emulator knows the internal states of the CPU and the TIA
The emulator can freeze the action anywhere, while maintaining the game screen
Software emulation of game controls is tough
No standard hardware that matches originals

19. Atari 2600 Programming Model
System Block Diagram
Graphics
Controls
TIA registers

20. Stella System Block Diagram

21. Stella System TIA video chip (see below) 6502-based processor, “6507”:
13 bit address, no interrupts, RDY line
1.2 MHz
6532 combo
128 bytes of RAM (all mapped into zero page)
16 bits of parallel I/O (joysticks and panel)
timer (interrupt not used)
cartridge slot for 2K or 4K ROMs (24 pins)
2 game control ports

22. Stella Graphics
Fundamental pixel resolution is 1 color burst clock (280nsec, 160/line) by 1 line.
Motion objects are 1, 2, 4 or 8 clocks/bit.
Motion objects may be replicated in hardware.
Playfield is 4 clocks per bit.
Playfield bits are either repeated or reflected in hardware.

23. Other TIA chip features
4 7-bit palette registers
15 collision detection latches
2 channel sound system
variable prescaler
4+5 bit polynomial counters
volume registers
trigger and potentiometer input ports
trigger input could be used for light pens or light guns.

24. Human Input Requirements
Console controls:
Game select, and start switches
Options: handicaps, color/monochrome
Various types of game controls:
For TANK, etc: a joystick with a “fire” button
For PONG: a dual analog potentiometer
For Driving: a rotary control
For head games: a keyboard

25. HID implementation One power switch
5 bits of console parallel I/O, not scanned
5 + 5 bits of game control I/O, not scanned
2 bits in TIA, 8 bits in parallel ports
4 bits of potentiometer input, in TIA

26. TIA Register Map: 00-0A 00:0 Vertical Sync 00:1 Vertical Blank
02 Wait for Horizontal Sync
03 Reset Horizontal sync (testing)
04-05 Number and size of P0/M0, P1/M1
06-09 Color/lum registers
0A Playfield controls

27. TIA Register Map: 0B-1F 0B-0C:3 Player reflect bits
0D-0F Playfield graphics (7-4; 7-0; 7-0)
10-14 Horizontal reset, all 5 objects
15-16 Audio control
17-18 Audio frequency
19-1A Audio volume
1B-1C Player graphics (8 bits)
1D-1F Missile/ball enable (1 bit each)

28. TIA Register Map: 20-3F 20-24 Horizontal motion registers (7-4)
25-27 Vertical delay: P0, P1, Ball
28-29 Reset Missiles to Players
2A Horizontal Motion strobe
2B Horizontal motion clear
2C Clear collision latches
30-37 Collision detect latches
38-3D 4 pot inputs, 2 trigger inputs

29. Example Simple Game Design
General architecture
Display generation
Game play
Sounds

30. Combat Game Architecture
The code has three components:
Game play code
Process game control and console control inputs
Process game results (e.g. collisions)
Decide next graphics and sounds
Graphics display code
Graphics tables

31. General VCS Game timing
In Vertical Blank:
detect collisions and control inputs
decide new game conditions
computer new game graphics pointers, as inputs to the display kernel
In Display, for each line or two:
step graphics pointers
fetch graphics
wait for horizontal blank, and write graphics

32. Combat Main loop VCNTRL: generate vertical sync
GSGRCK: game select and reset
LDSTEL: load Stella (TIA) registers
CHKSW: read the joystick switches
COLIS: Detect and process object collisions
STPMPL: Move players and other objects
ROT: generate & rotate object graphics
SCROT: generate score graphics
VOUT: display the game

33. Horizontal motion For each moving object:
Given the horizontal position (0-159)
Compute a loop count for a wait loop, mod 15
Compute the horizontal motion step, -7 to +7
Wait for horizontal sync
Run the wait loop
Reset the object motion counter
Write the horizontal motion register
Write HMOVE after all registers set up

34. Combat Display Kernel For pairs of horizontal lines:
Compute indexes to playfield:
move 2.5 bytes from ROM tables
playfields are vertically reflected in software
For each object that is on, copy graphics
For 8 bit objects, copy graphics from RAM
For 1 bit objects, enable/disable
Use Wait-for-sync, and write graphics in horizontal blank

35. Concluding Suggestions
Acquire or create an emulator-based toolkit
Learn from existing code:
Download old ROM images
Disassemble them for study
Experiment by patching them and observing the results
Create original games, focused on game play:
Easy to learn
Difficult to master
Good luck monetizing your work.

36. Web Resources Salon.com: “Atari Lives”
Salon.com/tech/feature/2001/07/09/atari/index.html
PC Atari Emulators, PCAE by Dullea:
6502 Tools:
Atari Hardware Manual, scanned: