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The thing that made the Amiga personal computer line special, besides the refreshing design of the AmigaDOS operating system, were the various pieces of custom logic sprinkled about the motherboards. The amiga relied heavily on custom silicon which was designed for DMA to get good performance out of low-speed CPUs. This allowed the original Amiga computers with their approximately 7.1 MHz Motorola MC68000 processor to do things that even the earliest 486-based PC Clones could only dream of.

The most important custom chip was Fat Agnus. Agnus stands for "Address GeNerator Unites". This is the core of everything that the Amiga was. There are three Agnus chips, all of which perform the same functions; DMA, clock frequency generation in the 500 and 2000, and the Copper and Blitter circuits. The Copper is a coprocessor which is generally intended to perform some graphics hacks, and manages the Blitter. A Blitter is a circuit intended to copy blocks of memory very rapidly, which is handy for copying rectangular screen areas from place to place, or drawing sprites into the display memory from other locations in RAM. Agnus' blitter (like nearly every function of any of the Amiga's custom logic) is asynchronous, meaning that it can be copying memory while the CPU is attending to something else. On faster Amigas, some bitblit operations are faster when done by the CPU directly, provided it is not already otherwise loaded. There was a hack called cpublit which patched various functions to use the CPU, useful on 68030 and later CPUs.

  • Fat Agnus, the original, is found on the Amiga 1000, 500, and 2000. It supports only 512kb of chip ram, or memory which can be addressed by the Amiga's custom chips via DMA.
  • Fatter Agnus was shipped in some later-model 500s, and addresses 1mb of chip memory. It was also available as an aftermarket upgrade for the A500 and A2000.
  • Super Fat Agnus was standard equipment on the Amiga 3000, the last of the OCS Amigas, and also available as an upgrade for the Amiga 500 and 2000. This chip addresses up to 2mb of chip ram.

Denise, for "Display Encoder", performs basically all of the video output functions; It generates video clocks, contains the RAMDAC, and so on. There are three denise chips, which primarily offer little more than enhanced resolutions. The 1000 called it "Daphne". The 500 and 2000 came with "Denise" as normal. The 3000 got a version called "Fat Denise", which added a 640x480 mode, and 8bpp graphics. (Older Amigas have a 12bpp HAM mode which takes advantage of certain qualities of the chipset, but is not straightforward to use.) Finally, ECS Amigas got Super Denise, capable of still higher resolutions and bit depths.

Paula (Port Audio UART and Logic) controls the serial ports, floppy drive, mouse, and keyboard. In the A1000 this was referred to as "Portia", which they must have felt was too contrived.

Gary runs the bus. The A3000 got an upgraded version called Fat Gary, and the Amiga 4000's version includes an IDE Host Adapter.

CIA stands for "Complex Interface Adaptor", and it operates the parallel port, manages the serial ports, and runs the joystick ports, which are the same as the Atari 2600 standard, much like the Commodore 64.

Alice succeeded Fat Agnus in the ECS chipset. Both Agnus and Alice use a 16 bit data bus, but Alice has 23 bits of addressing. (The 68000 CPU, the original in the Amiga, has 24 bits of address space.)

Lisa replaces Denise in the AGA Amigas.

Gayle supersedes Gary in the Amiga 600 and 1200, Commodore's "bargain" Amiga computers. It manages chip ram, memory expansion, and contains the IDE host adapter for these models.

Amber is a deinterlacer, or "flicker fixer" as it is known in Amiga parlance. This is on the board in the A3000, and was also found on the A2320 flicker fixer. Amber is named after the designer's daughter.

Akiko is found only in the Amiga CD32, the later-model CD-based Amiga game system intended to compete with systems like the 3DO and Philips' CD-i. (There was an earlier, OCS (Old ChipSet) design called the CDTV.) This chip allows you to use "chunky" pixels via a "chunky 2 planar" conversion process, and is used for psuedo-3d games like Doom. It also controls the CDROM and replaces some functions of Gary and Gayle. Akiko was unfortunately used on very few games, since most CD32 games were nothing more than quick and dirty ports from other Amiga systems.

Buster is the bus controller found in the Amiga 3000 and 4000. It comes in two major flavors, Level I (Up to revision 7) and Level II (Revisions 9 and 11.) The second edition of the chip is significantly different and in fact has about double the gate count. Level I Busters do not do Zorro III bus masters. In addition, revision 9 busters have a couple of bugs which cause problems with some devices. The Commodore 4091 SCSI adapter is one of the cards which will not work with it; The highly popular Spectrum graphics card is another.


  1. Haynie, Dave and Ponting, Shane, (Super) Buster / The Buster - according to Dave Haynie. (http://www.amiga-hardware.com/buster.html)
  2. Buster. Planeta Amiga, June 26, 1996. (http://www.upv.es/amiga/18.htm)

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