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.