Fat Agnus refers to the 8372A or 8375 revision of the Agnus custom chip found in later OCS-based Amiga systems, including certain Amiga 500 and Amiga 2000 models. Unlike the earlier "Thin Agnus" (8370/8371) which supported only 512KB of Chip RAM, Fat Agnus could address up to 1MB of Chip RAM, enabling more memory for graphics, audio, and custom chip operations. Physically distinguished by its square PLCC (Plastic Leaded Chip Carrier) package compared to the earlier DIP package, this chip functions as the DMA controller and address generator, managing memory access for the CPU, chipset, and peripheral devices.