This article relies largely or entirely on a single source. (January 2012)
The Indigo is considered one of the most capable graphics workstations of its era, and was essentially peerless in the realm of hardware-accelerated three-dimensional graphics rendering. For use as a graphics workstation, the Indigo was equipped with a two-dimensional framebuffer or, for use as a 3D graphics workstation, with the Elan graphics subsystem including one to four Geometry Engines (GEs). SGI sold a server version with no video adapter.
The first Indigo model, code-named "Hollywood", was introduced on July 22, 1991. It is based on the IP12 processor board, which contains a 32-bit MIPS R3000A microprocessor soldered on the board and proprietary memory slots supporting up to 96 MB of RAM.
The later version (codename Blackjack) is based on the IP20 processor board, which has a removable processor module (PM1 or PM2) containing a 64-bit MIPS R4000 (100 MHz) or R4400 processor (100 MHz or 150 MHz) that implements the MIPS-III instruction set. The IP20 uses standard 72-pin SIMMs with parity, and has 12 SIMM slots for a total of 384 MB of RAM at maximum.
A Motorola 56000 DSP is used for Audio IO, giving it 4 channel 16-bit audio. Ethernet is supported onboard by the SEEQ 80c03 chipset coupled with the HPC (High-performance Peripheral Controller), which provides the DMA engine. The HPC interfaces primarily between the GIObus and the Ethernet, SCSI (wd33c93 chipset) and the 56000 DSP. The GIO bus interface is implemented by the PIC (Processor Interface Controller) on IP12 and MC (Memory Controller) on IP20.
Much of the hardware design can be traced back to the SGI IRIS 4D/3x series, which shared the same memory controller, Ethernet, SCSI, and optionally DSP as the IP12 Indigo. The 4D/30, 4D/35 and Indigo R3000 are all considered IP12 machines and run the same IRIX kernel. The Indigo R3000 is effectively a reduced cost 4D/35 without a VME bus. The PIC supports a VME expansion bus (used on the 4D/3x series) and GIO expansion slots (used on the Indigo). In all IP12, IP20, and IP22/IP24 (see SGI Indigo2) systems the HPC attached to the GIO bus.
For entry graphics, the 8-bit color frame buffer comes in three versions. One version uses the system's GIO expansion bus. Another uses the main backplane like the XS, XZ, and Elan graphics options. The final is the same, but adds a second video output, giving the computer the ability to have two "heads", or monitors.
The Indigo's XS Graphics option has a single GE7 Geometry Engine (GE), a RE3 Raster engine, a HQ2 Command engine, VC1, XMAP5. It is ideal for low-cost wireframe operations, compared to more powerful, and expensive options for textured graphics. Part of SGI's Express line of graphics, four XS graphics options were produced for the Indigo: the XS-8 offers 8-bit color, with one VM2 video RAM module; the XS-Z adds the ZB-4 Z buffer; the XS-24 adds two VM2 modules and offers 24 color bits and 32 bits including brightness; and the XS-24Z adds a Z buffer.
The XZ graphics option is also a member of SGI's Express graphics line. It is similar to the XS-24z, but it includes a second GE7 Geometry Engine ASIC, doubling its geometry performance.
The highest performance graphics option offered for the Indigo, it is a member of SGI's Express graphics line. It is like the XS-24z and XZ, but has 4 GE7 Geometry Engine ASICs, giving it twice the performance of the XZ option.
The Indigo was designed to run IRIX, SGI's version of Unix. The Indigos with R3000 processors are supported up to IRIX version 5.3, and Indigo equipped with an R4000 or R4400 processor can run up to IRIX 6.5.22.