Agnus

The name Agnus is derived from ‘Address GeNerator UnitS’ since it houses all address registers and controls memory access of the custom chips.

Agnus is the main part of the Amiga chipset in the OCS and ECS chipsets.

One of the most important functions of AGNUS can be thought of as a set of traffic lights, controlling the flow of data along 25 separate lanes (25 DMA or Direct Memory Access channels) within the computer.  It also controls all the timing within the system.

AGNUS also has contained within it several other important functions:

  1. The “Blitter”, which is capable of copying the contents of one section of Chip memory to another place in Chip memory, much faster than could be done with the 68000 CPU.
  2. The “Copper”, which is a co-processor, synchronized with the display output going to the DENISE chip (as noted above, AGNUS controls the timing within the system).

AGNUS had many revisions made to it from 1984 until it was replaced by  the Alice chip in the AGA chipset in 1992.  Mainly these revisions were to enable addressing more Chip RAM, as the initial could only address 512KB of Chip RAM.  Later this was increased to 1024KB and even 2048KB of Chip RAM.

Often these revisions of AGNUS are referred to by varying names, such as “Thin AGNUS” (512KB), “Fat AGNUS” or “Fat Lady” (1024KB) and “Super AGNUS”, “Fatter AGNUS” or “Super Fat AGNUS” (2048KB), and other variations on these, however none of these are official names, despite “Fat Lady” being printed on the mainboard of many Amiga 2000s to show where the AGNUS chip is located.

AGNUS had a part number, which is the most reliable way to determine which AGNUS chip you have:

  • 8361 and 8367 are 512KB AGNUS Chips.  These were in a 48 pin DIP socket.
  • 8370 and 8371 are 512KB AGNUS Chips.  These were in a 84-pin PLCC socket.
  • 8372, 8372A and 8375 (only 318069-16 or 318069-17) are 1024KB AGNUS Chips. These were in a 84-pin PLCC socket.
  • 8372AB, 8372B, 8375 are 2048KB AGNUS Chips. These were also in a 84-pin PLCC socket, making it easy to upgrade from a 512KB or 1024KB PLCC AGNUS).

As to which AGNUS chip came as standard with each Amiga, this was a case of continuous improvement over time, and so it was the case that even later revisions of the same model could come with a different AGNUS chip.

Agnus was replaced by the ALICE Chip in the AGA machines.