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The Amiga's Custom Chips - OCS

The Original Chip Set (OCS), not to be confused with the Initial Chip Set which is also often called the Original Chip Set, was used for all of the second generation machines, i.e the A500, B2000, A2500, A1500 and CDTV. It consisted of three main chips, Paula, Agnus and Denise although the Agnus chip underwent a number of upgrades of its life, and the graphics potential available from the Amiga depends on which chip has been fitted.

Paula The custom chip responsible for all of the sounds generated from all OCS Amigas. It provides four voices of sound output configured as two stereo channels with nine octaves over a range of complex waveforms, and uses both amplitude and frequency modulation. It also handles many of the input/ output tasks such as disk and interrupt control, the mouse, joystick and serial ports.
Agnus The custom chip used to control and move data to be displayed on the screen for all OCS Amigas. It is also responsible for timing the machine and even directs animation. Agnus has been upgraded a number of times, and the capabilites of the one installed in your machine dictates just what characteristics your machine has.
  • Agnus (8361) .5Mb Chip RAM, 5 bit colour
  • Fat Agnus (8370) .5Mb Chip RAM, Extra Halfbright (6 bit colour)
  • Fat Agnus (8371) .5Mb Chip RAM
  • Fatter Agnus (8372A) 1Mb Chip RAM, PAL/NTSC modes
Denise The custom chip that is responsible for the video output and is used to maintain the screen display for all OCS Amigas. It controls the resolution, colours, sprites, text lines, etc.

Copyright 2005 Amiga Auckland Inc. All rights reserved.
Revised: September 25, 2005.