The Agnus Chip

The name Agnus is derived from "Address Generator Units" because it houses all address/pointer registers and controls memory access for the custom chips. Thus, Agnus is the central component of the original and enhanced Amiga chipsets.

The way Agnus operates is similar to a set of traffic lights. Agnus controls the flow of data along 25 separate lines (25 DMA channels) within the chipset and is also responsible for video timing. In the "Fat" model, additional capabilities have been added.

Agnus also contains two coprocessors: the Blitter and the Copper. The Blitter is dedicated to copying the contents of one section of chip memory to another location, much faster than the main processor. The Copper stays synchronized with video beam position counters and can wait until a specific position is reached before transferring a value to a hardware register.

Agnus underwent many changes from 1984 until it was replaced by the Alice chip in the advanced graphics chipset. The Agnus hierarchy generally has two main branches: "Thin" Agnus and "Fat" Agnus.

The Thin Agnus is the original 48-pin package used in the Amiga 1000 and early Amiga 2000. This version could address 512KB of chip RAM. The Fat Agnus is the more famous 84-pin version introduced in the Amiga 500 and used in subsequent systems.

In the advanced Amiga chipset, the embodiment of Agnus is the Alice chip. Both share similar hardware and duties, playing a vital role in the graphics and sound architecture of the Amiga.