3 Generations of Game Machine Architecture

1
3 Generations of Game Machine Architecture

• Joe Decuir
• jdecuir_at_nwlink.com
• alumnus of Atari Amiga

2
Agenda

• My background
• Atari Stella system (2600)
• Atari Colleen system (400/800, 5200, etc)
• Amiga Lorraine system (1000, 2000, etc)
• looking ahead

3
My background, pre-Atari

• Clearly an engineer from my first Erector set
  (age 6)
• BS MS degrees from Berkeley, in EECS and
  pre-med, including Intel 4004 design
• A couple years spent in medical instrument design
  and in medical research that leveraged those
  instruments.
• Bought my one 6502 at Wescon in 1975.

4
Stella Architecture

• Product requirements
• Design choices
• Hardware/firmware tradeoffs
• Results
• Lessons

5
Stella Requirements

• Atari management had a clear vision for the
  product provide a means to bring successful
  Arcade games home.
• We had to hit a 200 max retail price for the
  console, for Christmas of 1977.
• Expected product life 3 years (e.g. to 1979)
• Non-goals be an expandable personal computer.

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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

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Stella Implementation

• From coin-op games, there were two obvious ways
  to architect the system
• non-programmable random logic
• programmable (uP) with screen bit-map
• Fatal flaws in both
• random logic would be slow development, and not
  re-usable
• bit maps were expensive

8
Bit maps and games

• Games traditionally contain a background of some
  kind (playfield) and foreground active objects
  (players, sprites, etc).
• If only a bit map is used for everything
• sprite graphics and motion resolution are the
  same as the playfield resolution, forcing
• a hard choice between high cost or clunkiness

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The breakthrough soft video

• Mayer and Milner proposed using a fast enough
  processor to reload video on a line-by-line
  basis. The processor replaces logic.
• They had to wait for the right chip
• Intel 8080 was expensive, needed 3 supplies.
• Motorola 6800 was too slow
• Fairchild F8 was even slower
• MOS 6502 was perfect fast and cheap

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Why was the 6502 so good?

• Process speed depletion load pullup transistors
  were much faster and smaller than enhancement
  pullups (e.g. 6800).
• Architecture speed
• little-endian addresses pipelined instructions
• indexed-indirect and indirect-indexed
  instructions allowed use of zero page as an array
  of fast memory pointers.

11
Design decisions

• Our target coin op games were two player action
  (Tank - Combat), sports (Basketball) and paddle
  (Pong -Video Olympics) .
• We decided that we needed
• 2 8-bit motion objects (P0, P1)
• 3 1-bit motion objects (M0, M1, Ball)
• 20 or 40 bits of low resolution playfield

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Hardware Software tradeoffsMotion control

• The easy way to make these motion objects would
  require a binary horizontal counter, and 5 8-bit
  position registers and comparators. We thought
  this would be huge.
• The cheap way was to use dynamic polynomial
  counters, running in parallel. Motion in
  implemented with resets and motion vectors.

13
Motion control, continued

• To appease the programmers, I generated a
  Compute Horizontal Reset CHRST utility.
• Called with object index in X, position in A
• computes a loop count (15 clocks)
• computes a residual motion vector (/-7)
• waits for sync, loops, resets and writes motion
• For the programmers, this was good enough

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Motion control, epilog

• An alternative that we considered too late
• keep the polynomial horizontal counter
• replace the separate object counters and motion
  registers with simple position latches and
  comparators
• use a 160-byte look up table in cartridge ROM to
  map binary horizontal positions to polynomial
  counter values.

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Other TIA chip features

• 4 7-bit palette registers
• 15 collision detection latches
• 2 channel sound system
• variable prescaler
• 45 bit polynomial counters
• volume registers
• trigger and potentiometer input ports
• trigger input could be used for light pens or
  light guns.

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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.

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Example Stella Game structure

• In Vertical Blank
• detect collisions and control inputs
• decide new game conditions
• computer new game graphics pointers
• In Display, for each line or two
• step graphics pointers
• fetch graphics
• wait for horizontal blank, and write graphics

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Atari 2600 market history

• 1st shown at June 1977 CES show
• 250,000 sold in 1977, with 6 cartridges
• 550,000 sold in 1978, but big unsold inventory
  (800,000 made that year).
• Sales continued to double annually until the
  market collapse in 1983.
• Est. sales over 50 million by late 1980s.

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Atari 2600 Lessons

• Make the hardware a flexible platform for clever
  developers. (Bill Joys theorem)
• Good hardware-software tradeoffs make the product
  economically viable.
• Real marketing is essential. This means that
  somebody has a clear vision of
• who will buy the product
• why they buy the product at all
• why they buy the product from us

20
Colleen System Architecture (Atari 400/800, 5200,
65XE, etc)

• Product requirements
• Design choices
• Hardware/firmware tradeoffs
• Results
• Lessons

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Colleen requirements

• Split requirements
• support 1978 vintage arcade games. We knew we
  would need to leapfrog the 2600 before somebody
  else did.
• support home computer character and bit map
  graphics. We saw the Apple II, Commodore and
  Radio Shack appliance machines coming.
• Result
• Atari 800 was full computer
• Atari 400 was a game machine w/flat keyboard

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Colleen computer requirements

• Keyboard
• Character graphics
• Enough memory to run large programs.
• Means to load programs from tape or disk.
• Means for memory expansion
• Means for peripheral expansion

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Colleen game Requirements

• Hardware drives the graphics, so that the CPU has
  time to do other things.
• More moving objects.
• Multi-colored character maps, so that complex
  playfields can be generated without large amounts
  of memory.
• Playfield motion vertical and horizontal
  scrolling.

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Colleen System Architecture

• 5 LSI parts
• 6502 at 1.8 MHz, all address and interrupts
• ANTIC Video address generator, etc
• CTIA/GTIA video output
• POKEY pots, keyboard, audio, serial bus
• 6520 PIA simple 16 bits of parallel I/O
• at least 4KB of DRAM (1st shipped w/8K)
• 1-2 ROM cartridge slots
• 4 game controller ports

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Colleen Video architecture

• 5 motion object generators
• Display list processor generates playfield or
  character displays, on line-by-line basis.
• Both graphics generators get DMA access, so they
  share memory with the 6502.
• Maximum resolutions 320x240x1 in monochrome or
  160x240x2 in color
• 8-bit palette registers 4 color, 4 luminance

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ANTIC Display list modes

• 8-bit instructions
• 4-bit display mode
• 2-bits scroll control
• 1-bit interrupt request
• 1-bit load new address
• bit map modes 40x8x2 to 320x1x1
• text modes 20x(8x8)x2 and 40x(8x8)x1
• color characters 40x(4x8)x2

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ANTIC player graphics

• DMA from 5 fixed strips of 120 or 240 bytes of
  memory.
• Horizontal motion by writing a binary register
  (the way the programmers wanted Stella to work).
• Vertical motion by erasing and rewriting the
  picture.
• Vertical reuse possible use Display list
  interrupts to rewrite horizontal positions.

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More about playfield motion

• Complex games may have a huge virtual playfield,
  windowed on the screen.
• For each display instruction with horizontal
  scrolling enabled, ANTIC will load 192 pixels
  worth of data, and clip, controlled by the
  horizontal scrolling register.
• For each display instruction with vertical
  scrolling enabled, ANTIC will clip that line,
  controlled by the vertical scrolling register.

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Non-video I/O

• Pokey supported 4 audio channels
• Pokey supported 8 potentiometer inputs, sampled
  in hardware
• Pokey scanned a keyboards, using commodity CMOS
  external muxes.

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Colleen system issues

• The most direct competitor was the Apple II
• The Apple II was not bundled with a TV RF
  modulator, so it was not regulated by FCC.
• It could have slots! Peripherals were easy.
• The 800 was cased in cast aluminum
• The 800 had internal memory slots.
• The 800 used a serial bus for external expansion,
  19,200 b/s. The peripherals had intelligence, so
  they were expensive.

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Colleen in the market

• First debut in 1979. Sold respectably but not
  near as well as the Apple.
• FCC changed the rules in 1979 Class B
• Commodore C64 jumped in with lower cost and more
  memory.
• Atari answered with 800XL, etc, but trailed Apple
  and Commodore (and Radio Shack)
• Repackaged as pure game machine, 5200, competed
  with wealth of 2600 games.

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Colleen Lessons

• A personal computer has to have easy expansion,
  with lots of room for 3rd parties to add value
  (e.g. Apple II and IBM PC/AT family)
• It appears to confuse things to sell a hybrid
  game console and crippled PC (e.g. Atari 400).
• It was bad luck that the FCC didnt change the
  rules until after the Atari 800 shipped.
  Something like the 1200XL might have flown.

33
Amiga Lorraine System

• Company founded by Dave Morse (Mattel marketing)
  and Jay Miner. I was badge 3.
• Original product conception generate
  cartoon-quality video games.
• Also a computer, with a built-in floppy driver
  for distributing games.
• Commodore turned it into a color multi-media
  MAC in 1984, shipped in 1986.

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Amiga System Architecture (Amiga 1000, Amiga
2000, etc)

• Product requirements
• Design choices
• Hardware/firmware tradeoffs
• Results
• Lessons

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Amiga computer requirements

• Use a modern processor 68000 (w/o memory
  management unit)
• Built in floppy drive, not built in hard drive.
• 40 or more characters/line on TV, 80 or better on
  a monitor
• GUI OS requirement added after Commodore purchase
  in 1984.

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Amiga gaming requirements

• Hardware assist for
• common graphics operations
• changing the hardware synchronous with the video
  beam.
• Foreground and background bit maps
• Multiple motion objects
• Multi-channel DMA audio
• Loading games from floppy drive.
• Interact with external video (disk, VCR, etc)

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Amiga system design

• 4 LSI chips
• 68000 CPU, at 7.2 MHz
• AGNUS address generator/DMA engine
• DENISE video output chip
• PAULA I/O chip
• 256K DRAM minimum, 512K common
• 2 game ports
• Floppy drive

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Amiga video design

• Primary display is a set of bit plane engines.
• Bit planes can be used as a set of 4, for
  16-color displays, or in two sets of 3 and 3, for
  an 8-color foreground (or motion objects) and an
  8-color background.
• 8 identical sprite engines.
• Palette registers are 12 bit 4 each RGB, or 4
  each color, saturation and luminance

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Amiga bit blitter detail

• Automates common graphics operations
• line draw
• area fill
• splicing and manipulating images
• Bit blitter
• 3 input images, 1 output image
• 2 input images can be barrel shifted
  independently
• any arbitrary operation can be performed,
  selected by 8-bit mask.

40
Amiga sprite engines

• Each engine reads a string of 16-bit words in
  memory.
• The first words specify top left corner and size.
• Subsequent words are data (16 x 2 bits/line)
• Sprites can be reused to the end of the screen.

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Amiga Audio

• 4 DMA channels
• 8 bits/sample
• adjustable sample rate
• sample table in memory can be from 2 bytes
  (square wave) to 8192 bytes.
• Separate pointers each chunk of audio can come
  from anywhere in memory.

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Amiga System Issues

• The first version would have looked like the
  Amiga 500, with 128K of DRAM and no GUI OS, and
  shipped in 1984.
• Amiga ran out of money, which slowed development.
• Commodore bought it in June of 1984.
• Commodore repositioned it as a color multimedia
  MAC-like PC

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Amiga in the market

• The Amiga 1000, loaded with computer options,
  shipped in 1986. Competitors
• IBM PC/AT and clones
• Mac, with color
• Atari ST
• It was the best multi-media computer of its time,
  but it had a distinct market only for video
  production (with the Video Toaster)
• It became a cult favorite, but a market failure.

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Amiga lessons

• Again, clear product focus matters a lot
  competent marketing people working with engineers
  who listen.
• Bad luck matters the collapse of the video game
  business in 1984 poisoned the well financially
  and prospectively. Otherwise, we would play
  Amiga consoles instead of the NES in the
  mid-1980s.