Sierra was the code name for a 16-bit/32-bit personal computer designed by the Sunnyvale Research Lab (SRL) of Atari, Inc. starting around 1983. The design was one of several new 16-bit computer systems proposing to use a new chipset from Atari Corporate Research.
The graphics portion consisted of a two chip system called "Silver and Gold", Gold generated the video output while Silver was a sprite processor that fed data to Gold. The chipset was collectively known as Rainbow, and the system is sometimes referred to by this name. The audio portion of the chipset consisted of a powerful sound synthesizer known as AMY. The CPU had not been chosen, but the Motorola 68000, National Semiconductor 32016 and Intel 286 were being considered. Several proposed operating systems were considered including VisiCorps Visi On and Atari's own internal OS code-named "Snowcap".
Sierra was bogged down since its inception through a committee process that never could come to a consensus on the design specifications. A second project, Atari Gaza, ran in parallel, designing an upscale workstation machine running either BSD Unix or CP/M-68k. Atari management concluded they had no way to sell into the business market, redirecting Gaza engineers to a new low-cost machine based on the Amiga chipset, "Mickey". All of these systems were still incomplete when the company was purchased by Jack Tramiel in July 1984 and the majority of the staff was laid off. Only the synthesizer caught the interest of Tramel Technology lead engineer Shiraz Shivji and the rest of the projects disappeared.
History
Earlier 8-bit designs
Atari's earlier consoles and computers generally used an off-the-shelf 8-bit central processor with custom chips to improve performance and capabilities. With most designs of the era, graphics, sound and similar tasks would normally be handled by the main CPU, and converted to output using relatively simple analog-to-digital converters. Offloading these duties to the custom chips allowed the CPU in Atari's design to spend less time on housekeeping chores. Atari referred to these chips as co-processors, sharing the main memory to communicate instructions and data. In modern terminology, these would be known as integrated graphics and sound, now a common solution for mainstream offerings.
In the Atari 2600, a single all-in-one support chip known as the TIA provided graphics and sound support to its stripped-down MOS Technology 6502-derivative, the 6507. Due to the high price of computer memory, the TIA was designed to use almost no traditional RAM. The screen was drawn from a single line in memory, which the program had to quickly change the memory on-the-fly as the television drew down the screen. This led to both a quirky design as well as surprising programming flexibility; it was some time before programmers learned the knack of "racing the beam", but when they did, 2600 games began to rapidly improve compared to early efforts.
The much more powerful Atari 8-bit family used the same basic design concept, but this time supported by three chips. The C/GTIA was a graphics chip, greatly updated compared to the TIA, sound was moved to the new POKEY which provided four-channel sound as well as handing some basic input/output tasks like keyboard handling, and finally, the software-based display system used in the 2600 was implemented in hardware in the ANTIC, which was responsible for handling background graphics (bitmaps) and character-based output. ANTIC allowed the programmer to provide a simple list of instructions which it would then convert into data to be fed to the C/GTIA, freeing the programmer of this task. This separation of duties allowed each sub-system to be more powerful than the all-in-one TIA, while their updated design also greatly reducing programming complexity compared to the 2600.
Rainbow
By the early 1980s, a new generation of CPU designs was coming to market with much greater capability than the earlier 8-bit designs. Notable among these were the Intel 8088 and Zilog Z8000, designs using 16-bit internals, which initially became available as daughtercards on S-100 bus machines and other platforms as early as the late 1970s. But even as these were coming to market, more powerful 32-bit designs were emerging, notably the Motorola 68000 (m68k) which was announced in 1979 and led a number of other companies to announce their own 32-bit designs.