GIO

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GIO was Silicon Graphics’ main expansion bus in the 1990s. It was similar to other bus standards of the time, but most devices for it came from SGI, with a few third-party high-speed cards available. Later SGI moved to the XIO system, which was more like a network than a traditional bus.

Key points
- Versions and speed
- GIO32: 32-bit width, 25 or 33 MHz, about 16 MB/s for small transfers. A long-burst mode could push throughput up to 132 MB/s.
- GIO64: 64-bit width, up to 40 MHz, up to 320 MB/s. Also added support for both big-endian and little-endian addressing.
- GIO32-bis: uses GIO64 signaling and timing but keeps the GIO32 connector; designed for broad compatibility.
- How it worked
- The bus used 32-bit addressing and data on the same lines (multiplexed). Small transfers took three cycles (address, data, read/write), which was slower unless using long bursts.
- A real-time interrupt could pause long transfers when needed.
- Architecture and hardware
- Cards were small, about 6.4 by 3.4 inches, with a 96-pin connector.
- In SGI’s Indigo systems, cards were stacked vertically in the case.
- GIO64 moved to 64-bit data and could run faster, but used larger, EISA-like cards. The external connector matched EISA, though the board shape was slightly different.
- GIO64 features
- Two modes: non-pipelined (older style) and pipelined (modern, used by most boards).
- Data path included both a pipelined controller and a non-pipelined internal path to memory.
- Compatibility and evolution
- GIO32-endian behavior was SGI-style big-endian; GIO64 added endianness choice.
- GIO32-bis allowed low-throughput devices to work across GIO generations.
- Later SGI machines adopted XIO, which formats the connection more like a network than a traditional bus.

In short, GIO was SGI’s 32- and 64-bit bus for expansion cards, designed for SGI gear and mostly used for SGI graphics cards, with some high-speed third-party options. It was eventually replaced by the newer XIO system.