slots (used on the Indigo). In all IP12, IP20, and IP22/IP24 (see SGI Indigo2) systems the HPC attached to the GIO bus. Graphics options Entry graphics 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. XS Graphics 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. XZ Graphics 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. Elan Graphics 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. Operating system 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. Additionally, the free Unix-like operating system NetBSD has support for both the IP12 and IP20 Indigos as part of the sgimips port. References External links IP12 - LinuxMIPS Technolust: The Indigo Page (Silicon Graphics - SGI) SGI Indigo / Silicon Graphics R4000 Architecture Indigo Power Supply basics Indigo 32-bit computers 64-bit computers