The Amiga did most of its work away from its processor. Three custom chips sat beside the Motorola 68000: Agnus fetched memory for everything and contained the copper and the blitter, Denise turned bitplanes and sprites into a picture, and Paula played four channels of sampled sound and ran the disk, the serial port and the interrupts. Commodore’s Hardware Reference Manual introduces them in one sentence: “three custom chips named Paula, Agnus, and Denise which provide for superior color graphics, digital audio, and high-performance interrupt and I/O handling.”
They were designed at Amiga, the small Californian company Commodore bought in 1984, by a team led by Jay Miner, who, as Amiga Computing told the story in 1988, “had designed the chips in the Atari 800 and the VCS”. When Amiga first showed the machine, at the Consumer Electronics Show in January 1984, the chips did not exist yet. Each was a stack of “large printed circuit boards”, so fragile that it “took up a seat to itself on the plane”. Real chips followed in time for the summer show in Chicago that June, and the Amiga 1000 launched with them in July 1985. Miner was also one of the three authors of Commodore’s first hardware manual for the machine in 1985.
Who does what
| Chip | Main jobs |
|---|---|
| Agnus | Generates the address for all 25 DMA channels and decides who gets each memory cycle; contains the copper and the blitter; generates the system clocks; holds the beam position counter |
| Denise | Combines up to six bitplanes and eight sprites into a colour for each pixel; playfield and sprite priority; collision detection; mouse counters |
| Paula | Four DMA sound channels, two to each side of the stereo output; disk data; serial port; interrupts |
Only Agnus addresses memory. Denise and Paula receive their data from it: when Agnus runs a bus cycle, the A500 service manual explains, “it outputs a code on the Register Address Bus telling the other 2 chips the nature of the bus cycle”. Everything the chips work on must therefore sit in Chip memory, the part of RAM Agnus can reach. Memory beyond that, Fast memory, belongs to the processor alone, and “the custom chips cannot steal cycles when the 680x0 is using ROM or external memory”.
The chips and the processor share Chip memory by taking turns. Disk, audio, display and sprite DMA come first, then the copper, then the blitter, and the 68000 last. Most of the time the processor does not notice, because it uses only every other memory cycle anyway. A wide, deep display is when it does: four high-resolution bitplanes shut the 68000, the blitter and the copper out of Chip memory for the whole visible line. The Agnus entry has the details.
What games and demos did with it
Each feature of the chips came with a catch, and the games of the time show both sides.
- Hardware scrolling, line by line. Dave Jones, writing in Amiga Format in 1990, explained that the Amiga “can change the scroll value every line if required: take a look at Shadow Of The Beast for some impressive use of the hardware scroll.” The copper can load a new value as the beam reaches each line.
- Two playfields. Denise can show two independent scrolling layers, at the cost of colour: eight per layer, one of them transparent. Jones’s own Menace was built that way, with the player’s ship as a hardware sprite on top.
- Bobs as well as sprites. Eight 16-pixel sprites only go so far, so the aliens in Menace were drawn into the bitplanes by the blitter. For Blood Money, Jones said, he switched to a software scroll, with the blitter copying the whole screen each frame, because of “the number and size of the aliens”.
- Sampled sound. Four DMA channels of 8-bit samples meant recorded instruments and voices from the first model, and made the Amiga the home of the tracker; see Paula.
- Thousands of colours at once. Hold-and-modify mode let a still picture use all 4,096 colours. It fringed at sharp edges, and its six bitplanes took half the memory cycles the processor could otherwise have used during the display. The One reported in 1988 that Pioneer Plague was the first game to make “full use” of it.
Three generations
The chips changed twice, and the Amiga model decides which set a program meets.
| Chip set | Agnus | Denise | Paula | Machines |
|---|---|---|---|---|
| Original (OCS) | 8361 (NTSC), 8367 (PAL); later Fat Agnus 8370, 8371 | 8362 | 8364 | A1000, early A500 and A2000 |
| Enhanced (ECS) | 8372A (1MB Chip memory), 8372B and 8375 (2MB) | 8373 | 8364 | A3000 (1990), A500 Plus (1991), A600 (1992); later A500s and A2000s have the ECS Agnus, and A500s pair it with the original Denise |
| AGA | Alice (8374) | Lisa (4203) | 8364 | A1200 and A4000 (1992), CD32 (1993) |
The original chip set could reach 512K of Chip memory. The Fat Agnus kept that limit, and added 512K of “slow” memory that only the processor could use.
The Enhanced Chip Set raised the Chip memory limit to 1MB and then 2MB, allowed much larger blits, and gave Denise new modes: SuperHires, 1,280 pixels across in four colours, and a 640 by 480 Productivity mode for multisync monitors. The A1000 was left out: “There are no provisions for installing the ECS in the original A1000,” the manual notes. Paula did not change. The A600’s schematics describe the 8364 as “functionally unchanged”.
AGA replaced Agnus and Denise with Alice and Lisa, but kept Paula. At the A1200’s London launch in October 1992, Amiga Format listed what the new chips offered: a palette of 16,777,216 colours, 256 on screen instead of 32, and a new HAM mode with 262,144. The magazine also reported that “the speed of the blitter … has been vastly improved”. Programmers found otherwise. Shaun Southern of Magnetic Fields told The One in 1994 that apart from the processor “the only thing which is any faster … is the screen fetch. If you want more colours then you have to put more data down, but the blitter’s no faster at all.”
Common misstatements
Short summaries of the chip set often get these wrong:
- The copper has three instructions, not two. The manual says it “has only three instructions”:
MOVE,WAITandSKIP. - The blitter is not limited to AND, OR and XOR. It “can combine the data from the three source DMA channels in up to 256 different ways”, one for each value of an eight-bit control byte.
- HAM does show up to 4,096 colours at once, as the manual says, but not by free choice. Each pixel either picks one of 16 registers or copies its left-hand neighbour and changes one of its three components.
- Denise is not a memory user alongside Agnus. Neither Denise nor Paula addresses memory; Agnus does all the fetching.
Sources
- AmigaWorld, volume 1 number 2, November/December 1985, p. 6: the Amiga’s debut on 23 July 1985.
- Amiga Hardware Reference Manual, 3rd edition (Commodore-Amiga, Addison-Wesley, 1991): the three chips (p. 1); display modes, HAM and the copper (pp. 2-3); audio channels to the stereo outputs and disk DMA (p. 3); Chip and Fast memory, the Fat Agnus and ECS (pp. 3-5); the copper’s three instructions (p. 20); HAM (p. 86); stereo channel assignment (p. 137); the blitter’s 256 functions (p. 174); DMA priorities and bitplane costs (pp. 194-197); ECS features and the A1000 (p. 295); ECS SuperHires (pp. 299-300).
- Amiga Hardware Manual (Commodore-Amiga, 1985): “The text of this manual was written by Robert Peck, Susan Deyl, and Jay Miner.”
- Amiga 1000 Service Manual (Commodore): parts list, 8361 Agnus, 8362 Denise, 8364 Paula. A500 Service Manual, PN 314981-04 (Commodore, October 1990): the register address bus (p. 2-7). A3000 Service Manual (Commodore, March 1990), A500 Plus Service Manual (Commodore, October 1991), A600 System Schematics (Commodore, April 1992) and A4000 Service Manual (Commodore): chip part numbers for each machine.
- Amiga Computing, December 1988, pp. 24-26: the Amiga story as R.J. Mical told it, including Jay Miner’s Atari work, the CES prototype boards and the first real chips.
- Amiga Format, issue 8, March 1990, pp. 63-64: Dave Jones on hardware scrolling, Shadow of the Beast, Menace and Blood Money. Issue 12, July 1990, pp. 156-157: Menace’s hardware-sprite ship. Issue 41, December 1992, pp. 22-23: the A1200 launch and the AGA chip set.
- The One, issue 3, December 1988: Pioneer Plague and HAM. Issue 72, September 1994: interview with Magnetic Fields, Shaun Southern on the A1200.
See also
- Agnus, Denise and Paula: each chip in detail.
- Copper and Blitter: the co-processors inside Agnus.
- Commodore Amiga: the machines and their history.
- Jay Miner and Dave Jones.
- Bob graphics, parallax scrolling and sprite multiplexing.
- Commodore.