The O2 was an entry-level Unix workstation introduced in 1996 by Silicon Graphics, Inc. (SGI) to replace their earlier Indy series. Like the Indy, the O2 used a single MIPS microprocessor and was intended to be used mainly for multimedia. Its larger counterpart was the SGI Octane. The O2 was SGI's last attempt at a low-end workstation. Hardware System architecture Originally known as the "Moosehead" project, the O2 architecture featured a proprietary high-bandwidth Unified Memory Architecture (UMA) to connect system components. A PCI bus is bridged onto the UMA with one slot available. It had a designer case and an internal modular construction. Two SCSI drives could be mounted on special caddies (1 in the later R10000/R12000 models due to heat constraints) and an optional video capture / sound cassette mounted on the far left side. CPU The O2 comes in two distinct CPU flavours; the low-end MIPS 180 to 350 MHz R5000- or RM7000-based units and the higher-end 150 to 400 MHz R10000- or R12000-based units. The 200 MHz R5000 CPUs with 1 MB L2-cache are generally noticeably faster than the 180 MHz R5000s with 512 KB cache. There is a hobbyist project that has successfully retrofitted a 600 MHz RM7xxx MIPS processor into the O2. Memory There are eight DIMM slots on the motherboard and memory, and all O2s are expandable to 1 GB using proprietary 239-pin SDRAM DIMMs. The Memory & Rendering Engine (MRE) ASIC contains the memory controller. Memory is accessed via a 133 MHz 144-bit bus, of which 128 bits are for data and the remaining for ECC. This bus is interfaced by a set of buffers to the 66 MHz 256-bit memory system. I/O I/O functionality is provided by the IO Engine ASIC. The ASIC provides a 64-bit PCI bus, an ISA bus, two PS/2 ports for keyboard and mouse, and a 10/100 Base-T Ethernet port. The PCI bus has one 64-bit slot, but the ISA bus is present solely for attaching a Super I/O chip to provide serial and parallel ports. Disks The O2 carries an UltraWide SCSI drive subsystem (Adaptec 7880). Older O2's generally have 4x speed Toshiba CD-ROMs, but any Toshiba SCSI CD-ROM can be used (as well as from other manufacturers, the bezel replacement however is designed to fit Toshiba design and also IRIX cannot utilize CD-DA mode other than Toshiba). Later units have Toshiba DVD-ROMs. The R5000/RM7000 units have