Original Chip Set - Overview of Chips

Overview of Chips

The chipset which gave the Amiga its unique graphics features consists of three main "custom" chips; Agnus, Denise, and Paula. Both the original chipset and the enhanced chipset were manufactured using NMOS logic technology by Commodore's chip manufacturing subsidiary, MOS Technology. All three custom chips were originally packaged in 48-pin DIPs; later versions of Agnus, known as Fat Agnus, were packaged in an 84-pin PLCC.

Agnus is the central chip in the design. It controls all access to chip RAM from both the central 68000 processor and the other custom chips, using a complicated priority system. Agnus includes sub-components known as the blitter (fast transfer of data in memory without the intervention of the processor) and the copper (management of the monitor signal). The original Agnus can address 512 KB of chip RAM. Later revisions, dubbed 'Fat Agnus', added 512 KB pseudo-fast RAM, which for ECS was changed to 1 MB (sometimes called 'Fatter Agnus') and subsequently to 2 MB chip RAM.

Denise is the main video processor. Without using overscan, the Amiga's graphics display is 320 or 640 pixels wide by 200 (NTSC) or 256 (PAL) pixels tall. Denise also supports interlacing, which doubles the vertical resolution, at the cost of pretty bad flickering on most monitors produced during the same timeframe as the Amiga computers. Planar bitmap graphics are used, which splits the individual bits per pixel into separate areas of memory, called bitplanes. In normal operation, Denise allows between 1 and 5 bitplanes, giving 2 to 32 unique colours. These colours are selected from a palette of 4096 colours (4 bits per RGB component). A 6th bitplane is available for two special video modes: Halfbrite mode and Hold And Modify mode. Denise also supports eight sprites, sub-pixel scrolling, and a "dual playfield" mode. Denise also handles mouse and digital joystick input.

Paula is primarily the audio chip, with 4 independent hardware-mixed 8-bit PCM sound channels, each of which supports 65 volume levels (no sound to maximum volume) and sample rates from roughly 20 Hz to almost 29 kHz. Paula also handles interrupts and various I/O functions including the floppy disk drive, the serial port, and analog joysticks.

There are many similarities - both in overall functionality and in the division of functionality into the three component chips - between the OCS chipset and the much earlier and simpler chipset of the Atari 8-bit family of home computers, consisting of the ANTIC, GTIA and POKEY chips; both chipsets were conceptually designed by Jay Miner, which explains the similarity.