Commodore, developer of the popular PET computer (1977), one of the first microcomputers with bitmapped graphics, acquired Amiga Corporation in 1984.
The Commodore Amiga was a low-cost landmark machine when it launched in 1985. It had high color graphics and displayed 4,096 colors simultaneously (using the Amiga Hold-And-Modify (HAM) display mode). In comparison, the Apple Macintosh, introduced a year before, was only black and white, and IBM’s EGA, also released in 1984, only had sixteen colors. An Amiga sold for $1,295 at the time, whereas an IBM PC and Apple Mac sold for $2,145 and $2,495, respectively.
The Amiga had stereo sound, whereas most other machines had no sound or mono sound output. It had a multitasking operating system (which Sun Microsystems tried to buy); other machines could only do one thing at a time. The engineering marvel that it was, Commodore had difficulty marketing the machine’s many advancements to consumers. However, there were few applications for the devices, and the consumer market was not quite ready.
The Commodore Amiga block diagram (Source: Jon Peddie Research)
The Amiga had four main chips: the central processor, a Motorola 68000, a general-purpose control chip named Angus that managed the system’s RAM, and contained a BitBlt engine (a blitter that could do fast transfers of data in memory without involving the central processor), and the Copper video-synchronizing coprocessor. The audio chip was called Paula, which had four independent 8-bit pulse-code modulation (PCM) sound channels.
Blitter OBjects (known as BOBs) are like sprites drawn by the blitter into the screen bitmap. Unlike sprites, they were not independent of the screen colors and resolution but could use as many colors as the chosen screen mode allowed. Figure 1 shows a general block diagram of the Amiga.
The graphics controller was named Denise, and it would be the first of three generations. Denise was a contraction of Display Enabler.
The Denise could be programmed to get video or image data from 1 to 5-bit planes and translate that into a color LUT entry. The number of bit planes was arbitrary, and it could use 2, 4, 8, or sixteen bits instead. The number of bit planes (and the resolution) could be changed on the fly, usually by the Copper video coprocessor. That made very economical use of RAM and helped balance CPU processing speed and graphic quality. There was also be a sixth-bit plane used in one of three graphics modes:
The Commodore Amiga’s graphics chipset could combine three source bitmaps using 256 Boolean functions with three inputs. It was quite advanced for the day, especially for such a low-cost device.
Dual-playfield. In this mode, each (of the three) bit planes got drawn on top of each other. Each plane got independently scrolled while the background color of the top playfield came through. That proved helpful for video overlay of things like scrolling text on images.
Extra-HalfBrite. This mode checked if a pixel got set on the sixth-bit plane and then cut the pixel’s brightness in half.
HAM. The most popular display mode was HAM. Each 6-bit pixel had two control bits and four data bits. The control bits could modify red, green, or blue. The four data bits used a 16-color display lookup. That allowed all 4,096 colors to be displayed on the screen simultaneously. It also provided a form of lossy image compression in hardware. Figure 2 shows an example of the image quality of the Amiga.
The chip also supported two horizontal graphics resolutions: low-res (using 140 ns pixels) and hi-res (using 70 ns pixels at 320 or 640 horizontal pixels wide without using overscan).
The pixel output was regulated by the main system clock and based on the NTSC colorburst timing. The Denise chip also supported overscan and could provide data for up to 800 lines. However, it was only helpful for scrolling and special effects that involved a partial display of large graphics. Nonetheless, it fully supported 736×482 (NTSC) and 736×580 (PAL) TV resolutions.
The Amiga was ideal for presenting graphics in a TV mode, given its timing structure to scanlines and its DMA resource allocations. However, the Denise chip did not support a dedicated text mode.
The Amiga Denise chip was the first to blend video, graphics, and audio in one chip.
The Denise chip could also composite up to eight 16-pixel-wide sprites per scan line. The sprites had three visible colors and one transparent color. Using the CPUs and the Copper registers, each sprite could be reused multiple times in a single frame and increase the total sprites per frame. One of the first Amiga games to utilize sprite repositioning was Hybris, released in 1988.
The Amiga and its Denise chip was the first multimedia device to blend video, graphics, and audio. As a result of providing open access to all its registers, it got used in some fantastic video production projects.
Lorain Begat Denise
Amiga’s original chipset codename was Lorraine. The development work on the Lorraine project in 1983 used an 8 MHz Sage IV, Motorola 68000-based microcomputer nicknamed Agony.
Amiga funded the Lorraine development by selling game controllers it had built and securing a loan from Atari. The chipset was designed for video game machines. But the company had terrible timing. Amiga created Lorraine just as the first home video game boom was ending. In 1983, there was a recession in the console gaming market in the U.S. called the video game crash of 1983–1985. The crash was due to market saturation of game consoles, games available, and fading interest in console games as gamers moved to PCs.
In July 1984, Jack Tramiel, recently resigned CEO and founder of Commodore, bought Atari. In 1985, he launched the Amiga based on Atari chips.
One of the most significant features of the Amiga was how its unique chips could be programmed to produce exceptional graphics for the time. The Amiga was well known for that capability and became hobbyists’ and experimenters’ favorite machine, even after the company had gone out of business.
However, despite its advanced technical features and capabilities, the poorly marketed Amiga 1000 was unsuccessful.
4 Amiga Epilogue
The Enhanced Chip Set (ECS) was the second generation of the Amiga Computers chipset and offered minor improvements over the original design. ECS was also in the Amiga 3000 introduced in 1990. Another version was developed, but by April 1994, Commodore International filed for bankruptcy. The Amiga was a highly advanced system, but Commodore was already facing financial difficulties, and the Amiga could not save it.
The Atari chipset pushed multimedia development into the design of graphics controllers. It influenced Nvidia and Yamaha’s decisions to make their first chips multimedia devices. The Atari chipset also inspired engineers at AT&T who developed the Targa board.
Commodore, developer of the popular PET computer (1977), one of the first microcomputers with bitmapped graphics, acquired Amiga Corporation in 1984.
