Atari  Atari 800 (Colleen)
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Atari 800 - Coleen

The Atari 800 and the Atari 400 were both presented at the winter Consumer Electronics Show (CES) of January 1979. The first machines shipped to consumers in november of that same year.

The names of the 800 and the 400 were a referense to the amount of RAM the machines contained, 4KByte for the 400 and 8KByte for the 800. But, by the time the machines were released, the price of RAM had dropped significantly, and both machines were shipped with 8KByte of RAM. RAM could be expanded using modules on the 800, but later releases of the computers shipped with a full 16KByte for the 400, and 48KByte for the 800.

The computers had four joystick ports, but only a few games were released that used them all. Paddle controllers came as two controllers per connector, so Super Breakout could be played by 8 players.

The 400 had an inferior keyboard to the 800 but it still outsold its bigger sibling due to a better price point.

Atari 800 processor board
The processor board for the Atari 800, showing the 6502, ANTIC and CTIA chips.

The Atari 800 had a unique feature, it had two cartridge slots, the left and the right cartridge.

The idea was that with 2 cartridge slots 2 different 8K ROMs could be installed: one from memory address 0x8000 to 0x9FFF, and and one from 0xA000 to 0xBFFF. However, the left cartridge is able to occupy a full 16K, so it was the only one that was kept on later models.

Four different memory slots could add additional RAM and an additional ROM module.

Atari 8-bit Architecture

The Atari machines consist of a 6502 as the main processor, a combination of ANTIC and GTIA chips to provide graphics, and the POKEY chip to handle sound and serial input/output. These support chips are controlled via a series of registers that can be user-controlled via memory load/store instructions running on the 6502. For example, the GTIA uses a series of registers to select colors for the screen; these colors can be changed by inserting the correct values into its registers, which are mapped into the address space that is visible to the 6502. Some of the coprocessors use data stored in RAM, notably ANTIC's display buffer and Display List, as well as GTIA's Player/Missile (sprite) information.

The custom hardware features enable the computers to perform many functions directly in hardware, such as smooth background scrolling, that would need to be done in software in most other computers. Graphics and sound demos were part of Atari's earliest developer information and used as marketing materials with computers running in-store demos.

The Antic Display Processor

ANTIC is a microprocessor which processes a sequence of instructions known as a display list. An instruction adds one row of the specified graphics mode to the display. Each mode varies based on whether it represents text or a bitmap, the resolution and number of colors, and its vertical height in scan lines. An instruction also indicates if it contains an interrupt, if fine scrolling is enabled, and optionally where to fetch the display data from memory.

Since each row can be specified individually, the programmer can create displays containing different text or bitmapped graphics modes on one screen, where the data can be fetched from arbitrary, non-sequential memory addresses.

ANTIC reads this display list and the display data using DMA (Direct Memory Access), then translates the result into a pixel data stream representing the playfield text and graphics. This stream then passes to GTIA which applies the playfield colors and incorporates Player/Missile graphics (sprites) for final output to a TV or composite monitor. Once the display list is set-up, the display is generated without any CPU intervention.

There are 15 character and bitmap modes. In low-resolution modes, 2 or 4 colors per display line can be set. In high-resolution mode, one color can be set per line, but the luminance values of the foreground and background can be adjusted. High resolution bitmap mode (320x192 graphics) produces NTSC artifacts which are "tinted" depending on the color values; it was normally impossible to get color with this mode on PAL machines.

For text modes, the character set data is pointed to by a register. It defaults to an address in ROM, but if pointed to RAM then a programmer can create custom characters. Depending on the text mode, this data can be on any 1K or 512 byte boundary. Additional register controls allow flipping all characters upside down and toggling inverse video.

CTIA and GTIA chips

The Color Television Interface Adaptor (CTIA) is the graphics chip originally used in the Atari 400 and 800. It is the successor to the TIA chip of the 1977 Atari VCS. According to Joe Decuir, George McLeod designed the CTIA in 1977. It was replaced with the Graphic Television Interface Adaptor (GTIA) in later revisions of the 400 and 800 and all later 8-bit models. GTIA, also designed by McLeod, adds three new playfield graphics modes to ANTIC which allow more colors than previously available.

The CTIA/GTIA receives Playfield graphics information from ANTIC and applies colors to the pixels from a 128 or 256 color palette depending on the color interpretation mode in effect. CTIA/GTIA also controls Player/Missile Graphics (sprites) including collision detection between players, missiles, and the playfield; display priority for objects; and color/luminance control of all displayed objects. CTIA/GTIA outputs separate digital luminance and chroma signals, which are mixed to form an analog composite video signal.

CTIA/GTIA also reads the joystick triggers and the console keys Option, Select, Start, and operating the keyboard speaker in the Atari 400/800. In later computer models the audio output for the keyboard speaker is mixed with the audio out for transmission to the TV/video monitor.

The Pokey peripheral chip

The third custom support chip, named POKEY, is responsible for reading the keyboard, generating sound and serial communications (in conjunction with the PIA chip (Peripheral Interface Adapter, 6520) commands and IRQs, plus controlling the 4 joystick movements on 400/800 and later RAM banks and/or ROM(OS/BASIC/Self-test) enables for XL/XE lines). It also provides timers, a random number generator (for generating acoustic noise as well as random numbers), and maskable interrupts. POKEY has four semi-independent audio channels, each with its own frequency, noise and volume control. Each 8-bit channel has its own audio control register which select the noise content and volume. For higher sound frequency resolution (quality), two of the audio channels can be combined for more accurate sound (frequency can be defined with 16-bit value instead of usual 8-bit). The name POKEY comes from the words "POtentiometer" and "KEYboard", which are two of the I/O devices that POKEY interfaces with (the potentiometer is the mechanism used by the paddle). The POKEY chip—as well as its dual- and quad-core versions—was used in many Atari coin-op arcade machines of the 1980s, including Centipede and Millipede, Missile Command, Asteroids Deluxe, Major Havoc, and Return of the Jedi.

MOS 6502 CPU

The 6502 is an 8-bit MicroProcessor designed by MOS Technology. The team was led by Chuck Peddle and had also worked on the Motorola 6800. The 6502 is a simplified, but faster and cheaper design than the 6800.

The 6502 was introduced in 1975 and was the cheapest microprocessor on the market. Together with the Zilog Z80, the 6502 helped start the home computer revolution of the 1980s. The 6502 was used in a wide range of devices: the Atari 2600, the 8-bit Atari home computers, the Apple II, the Nintendo Entertainment System, the Commodore 64, the BBC Micro and many others. All used the 6502 or a variation of it.

The 6502 is a 1MHz design, while the 6502A is designed for 2MHz. The 6502A is 100% compatible with the original 6502.

Commodore soon bought MOS Technology, but conitnued to sell the microprocessor to competitors and licensed the design to other manufacturers.

Source: WikiPedia - MOS Technology 6502
Technical Details
Released 1978 Brand Atari Type Atari 8-bit Name Atari 800 (Colleen) CPU Class 650x CPU MOS 6502B @1.79MHz Memory RAM: 8kB
RAM max: 48kB
Sound Chip Atari POKEY (C012294) Sound 4 channels, 3.5 octaves Display Chip CTIA/GTIA & ANTIC Display 40x24 text
320x192 graphics
16 colors, 16 intensities
Best Text 40x24 Best Color 16 colors, 16 intensities Best Graphics 560x192 in 16 colors, 16 intensities Sprites Atari Player Missile Graphics (PMG) System OS Atari BASIC Storage Rom Cartridges, SIO Bus for external Tape or Disk Drive Original Price $999.95
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