Part of the custom chipset found in the Amiga. Responsible for much of the "metaprocessing" - taking care of menial tasks (Dividing the frequency of the external 28 MHz clock, handling the 25 DMA channels for I/O-intensive tasks and acting as a memory controller for chip RAM, for instance) and leaving the more intensive number-crunching to the CPU.

Fat Agnus also contains the bit blitter, capable of moving around chunks of memory as well as performing a number of operations on the memory, in a proto-MMX fashion. The blitter was, naturally, the demo-coders best friend, allowing huge scrolling texts be moved around with little effort.

Since the Amiga's internal representation of the video memory is planar, with each screen made up of a separate bitmap of the three RGB layers (rather than having the RGB values combined into one bitmap), the video memory needs to be translated into the standard single-bitmap format before it is passed to Denise, the graphics chip. This is done by the copper (short for Display Synchronized Coprocessor). The copper is programmable by placing commands in the 'copper list'. The copper list is executed every time the screen refreshes, and this was used to make some demos unscreenshottable - the copper copied the video memory just before Denise requested the video stream, and cleared the screen (or displayed a snide remark) when Denise was ready, thus only displaying the screen for about 0.02 seconds (depending on refresh rate). When the demo was paused (often performed in hardware, by simply disconnecting the clock from the CPU), it showed nothing (or abovementioned remark) in 98% of the cases.

It should also be noted that all of the custom chips could not access all of RAM .. they could only access "chip RAM"; and all the RAM access went through Fat Agnus. Initially, the Fat Agnus chip only allowed access to the "bottom" 512K of RAM; if you wanted the four oprocessors to do anything to data, it had to live in this bottom 512K.

Later on, the Fat Agnus chip was upgraded to allow 1MB of "chip RAM". This became known as Fatter Agnus. Eventually, when the pinnacle of Amiga computing, the Amiga 3000 came out, it also had a modified Fat Agnus chip, called the Fattest Agnus, which allowed it to access a full 2MB of chip RAM.

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