These three chips — Paula, Agnus, and Denise — were the beating heart of the Amiga. Each had a specific task, and together they built a computer decades ahead of its time. These chips are the forefathers of the graphics cards and professional sound cards of the following decade. Read about the Agnus chip in the exclusive AmigaStore article.

Paula; The Sound Magician

The Paula chip contains logic circuits for audio playback, floppy disk drive control, serial port I/O, and mouse/joystick buttons. Its functionality remained identical across all Amiga models that Commodore released. Paula was designed by Glenn Keller. The name is a fabricated acronym for Ports, Audio, UART and Logic — and coincidentally the name of the chip designer's girlfriend!

What made Paula revolutionary was that it could play back sampled sound from memory without needing the CPU to do most of the work. A comparison helps here: on the Apple Macintosh of that time, to play a sound, the processor had to load the next bit of sample data into the sound register over 22,000 times per second, interrupting whatever it was doing at that moment. A processor can only do one thing at a time — effectively slowing it down dramatically.

On the Amiga, the processor tells Paula where the sample is stored in chip RAM, how long to play it, and at what speed. Paula can then play the sound completely without processor intervention — unless the sound needs to be cut short for some reason.

A simple example clarifies this difference. Imagine you're baking a cake and want to listen to music at the same time. You are the processor. On the Amiga and Macintosh, you work at roughly the same speed — not exactly true in real life, but close enough for this example. The CD is the sound you want to hear, loaded in RAM, and the CD player is the sound producer — like the audio output circuit on the Macintosh or the Paula chip on the Amiga.

Now you put a CD in the player and press play. If the Amiga does this, the CD plays from start to finish without interrupting your cake baking. If you leave it to the Macintosh, you have to stop everything every second during baking, go to the CD player, and press play again. With the Macintosh, your cake baking would take days!

This capability was truly revolutionary at the time, and this is why Amiga computers always performed better than Macintosh or Atari ST despite having the same 68000 processor.

Agnus; The Data Traffic Manager

The name Agnus derives from "Address Generator UnitS" — because it contains all address/pointer registers and controls memory access for the custom chips. Agnus is the central part of the Original Chip Sets — OCS — and Enhanced Chip Sets — ECS — of the Amiga.

The way AGNUS works is like a set of traffic lights. AGNUS similarly controls data flow along 25 separate lines — 25 DMA channels — within the chipset, and also handles video timing.

Agnus also contains the Blitter and Copper coprocessors. The Blitter is dedicated to copying the contents of one part of chip memory to another location in chip memory, much faster — about twice as fast — than the 68000 processor. Besides simple copying, it can also provide more manipulations: logical combination of up to 3 sources, precise pixel shifting, line drawing, and simple area filling.

The COPPER stays synchronized with video beam position counters with direct access without needing to move chip memory data. It can wait until a specific position is reached and then move a value to a chipset hardware register. Copper's third instruction — SKIP — can help build simple IF-THEN clauses. Despite its apparent simplicity, Copper is very practical for many activities — from simple register refreshing and reloading to fully controlled color and resolution mid-screen, or even starting brightness movement in sync with the beam.

Agnus underwent many changes from 1984 until the Alice chip in the AGA chipset replaced it in 1992. The Agnus hierarchy has two main branches: the "thin" Agnus and the "fat" Agnus. The thin Agnus is the original 48-pin DIP package used in the Amiga 1000 and early "German" Amiga 2000. Only two types of thin Agnus exist: 8361 with NTSC timing and 8367 with PAL timing. The thin Agnus has eighteen-bit DMA pointer registers and can address 512KB of chip RAM. It also has a small "bug" that was fixed in the "fat" versions.

The "fat" Agnus is the more famous 84-pin PLCC package version introduced in the Amiga 500, also used in Amiga 2000-B, Amiga 3000, and CDTV systems. The term "fat" is because this chip now includes a simple clock generator producing all necessary system clocks from the main 28MHz oscillator, and a DRAM controller for slow RAM inside the fat Agnus.

In the AGA chipset, Agnus's embodiment is the Alice chip — both have similar hardware and tasks.

Denise; The Eye of Amiga

DENISE is a fabricated acronym for Display ENabler. In the early days of the Amiga 1000 and Amiga 500, it was often called "Daphne." There is no functional difference, except the version used in the early Amiga 1000 — 8362 R5 in a ceramic package — which cannot display EHB graphics mode.

In OCS and ECS machines, Denise uses up to 6 "bitplane" data registers to receive display data that Agnus retrieves from chip RAM through its respective DMA channels. The bitplane data in relation to the currently active graphics mode results in a constant stream of 12-bit digital RGB output values — 4 bits for each red, green, and blue component — synchronized with the display mode.

The digital data then enters a resistor network known as the Video Hybrid — or VIDIOT — producing the final analog RGB signal. Synchronization with the display mode continues and the image is ready for display on TV or monitor.

Denise is responsible for reading axis information from both mouse ports in multiplexed fashion, and also for displaying any sprites in use — with up to 8 sprite data registers fed at the beginning of each scanline through Agnus's respective sprite DMA channels. Additionally, it controls relative priorities between the display background and sprites and can detect specific sprite/playfield collisions — a great help for game design.

Denise was later replaced by the "Lisa" chip in the AGA chipset — the only truly 32-bit component of the AGA chipset. It received a fully pin-compatible update called 8373 R4 — known as "Super Denise" — found by default in Amiga 3000, Amiga 500 Plus, some late Amiga 2000 versions, and Amiga 600. Together with ECS Agnus, it is part of the ECS chipset.

Later Denise versions could support a wider range of screen resolutions and scan rates, but also required an enhanced Agnus to provide the necessary timing for the new display modes. Therefore, converting any old Amiga 500 or 2000 to ECS specifications is entirely possible — exceptions being the A1000 and early "German" A2000 which use the 48-pin "DIP Agnus" instead of the "fat Agnus."

The new display modes differed from the original modes only by NTSC/PAL timing — 60/50Hz, 15.7kHz — and could use newer VGA-style displays, but since default modes remained the original ones, this capability was practically only available to owners of dual-scan or true multi-scan displays like the Commodore 1940/1942 models. Access to such monitors was always limited since only ancient CGA and outdated EGA modes used frequencies below 31kHz in the PC world. By 1991, VGA was practically standard and there was little reason for manufacturers to support building such monitors.

This problem was partially solved by internal/external scandoublers/flickerfixers that allowed the original modes to display on such monitors. Amiga 3000 models already shipped with a Scandoubler/Flickerfixer. The same hardware — based on the Amber chip — was sold by Commodore as a separate video memory card A2320, but since it supports 12-bit color depth, it was only suitable for the Amiga 2000.

Several Scandoubler/Flickerfixer devices were built by other manufacturers for AGA-based Amigas, but some support 18-bit depth — 262,144 colors — instead of full 24-bit — 16,777,216 colors — of AGA and should be tested with a real AGA.