The "Express" graphics family is usually associated with the Indigo system family, but actually it was used in other machines as well. The following list contains the system families and the types of express graphics available (see Summary Table for differences):

XS/XS8 | XS8Z | XS24 | XS24Z | XZ | Elan
XS | XS24 | XS24Z | Elan | Extreme ("Ultra")
Personal Iris
Indigo 2
XZ (2GE) | XZ (4GE)

Because of the technical differences between various machines Express graphics was used on, different form factors of the board have been built. Directly integrated into the host system are the common Indigo and Indigo 2 versions as well as the Indy version. All these systems do support GIO32 boards. The Crimson and the Personal Iris do not have such an interface, so the Express boardset (Indigo type boards) is always used with some additional interface logic.


General Features

Screen Resolution: 1280 x 1024 pixels 
Screen Refresh Rate: 60Hz / 72Hz
8 (XS8) or 24 (XS24, XZ, Elan) bitplanes
24 bit Z-Buffer (XS24Z, XZ, Elan)
4 stencil bitplanes (XS24Z, XZ, Elan)
4 overlay and 4 window clipping ID planes
Genlock capability
Stereo viewer control Advanced Graphics Functionality Accumulation buffer
Arbitrary clipping planes
Multiple lighting source effects:
Spot lighting
Eight light sources
Two-sided lighting
Ambient, diffused, and specular
Fog and haze effects
Alpha blending
Anti-aliased RGB lines and points
Fullscene antialiasing
Texture mapping
Depth cueing, dithering, stenciling
Pan and zoom
Soft shadow and depth of field
Sphere rendering
Soft shadow and depth of field
Sub-pixel positioning
Stereo graphics
X11 pixel operations


Indigo*)XS / XS8GR2-XS11no8bit
Indigo*)XS / XS8ZGR2-XS with Z-Buffer 1 1 yes 8bit
Indigo*)XS24Z GR2-XS24 with Z-Buffer 1 1 yes 24bit
Indigo*) XZ GR2-XZ 2 1 yes 24bit
Indigo*) **) XSM GR2-XSM 4 1 no 24bit
Indigo*) Elan GR2-Elan 4 1 yes 24bit
Indy XZ GR3-XZ 4 1 yes 24bit
Indigo 2 XZ GR3-XZ 2 1 yes 24bit
Indigo 2 XZ GR3-Elan 4 1 yes 24bit

*) "Indigo" in this table means "Indigo-Style" which includes the Boards used in Crimson and Personal Iris systems - although it is quite possible that these two machines have never been sold with all the variants that were available for the Indigo.

**) Common version, but might have never been sold.



This is a functional overview of the Express graphics options:

  • The operations begin with a command token and data being written into the FIFO.
  • The Command Parser reads these and feeds the information into the Geometry Engines for execution.
  • The Geometry Engines perform per vertex and slope calculations on the data and write the results into an output FIFO.
  • From that FIFO the data is read by the Raster Engine which includes edge and span processing and converts primitives into pixels.
  • The Raster Engine does also per-pixel operations and writes pixel data to the framebuffer.

Functional Components

Basic processing units used on the various "Express" graphics options are:

HQ2 Command Engine
On Indy and Indigo 2 systems gfxinfo reports a HQ2.1 command engine.
GE7 Geometry Engine
A Geometry Engine is a floating point processor, that is optimized for geometry calculations and is capable of up to 32 MFlops (each!).
RE3 Raster Engine
On Indy and Indigo 2 systems gfxinfo reports a HQ3.1 Raster Engine.
VC1, XMAP5 (later: XMAP7) and ZRB4
processors are used in the further output creation process or Z-buffering



Express/Ultra graphics is available in 3 basic variants for a couple of different Silicon Graphics systems:

  • Express for Indigo, Personal Iris and Crimson (GR2 Express)
  • Express for Indigo 2 (GR3 Express)
  • Express for Indy (GR4 Express)

Additionally there seems to be a VME implementation of the original Express graphics called VGR2.

GR2 Express for Indigo (Personal Iris and Crimson)

"Express" graphics on the original Indigo is actually a boardset consisting of the following individual boards:

This board has 4 sockets for GE7 Geometry Engines, it also houses the HQ2 command engine and has sockets for 3 VM2, 1 VB1.1 and 1 ZB4. The external and backplane connectors are on this board too.
VB1.1 or VB1.2
RAMDAC among other things. The VB1.2 has a connector which is used for some video options. This is not available on VB1.1 boards.
Video Memory - 1 module required for 8bit, 3 for 24bit
This is the optional hardware Z-buffer module.

As additional features such as Z-Buffer or 24bit color are implemented in extra hardware that is plugged onto the basic GR2 board - which made upgrading easy. A common misconception is that upgrading the GEs was done by adding additional processors to the GR2 board - the usual way was to replace the whole GR2 with all GEs on them if a customer wanted to upgrade from XS24Z to Elan for example.

Although it is possible to build at least 12 variants with the options color (8bit/24bit), Z-buffer (yes/no) and number of Geometry Engines (1/2/4), but it is not known if all are really supported because not all of them have a name and only the ones mentioned so far have been available directly from Silicon Graphics. For a list of known variants see Summary Table at the End of this Section.

GR3 Express for Indigo 2

The "Express" boards used in the Indigo 2 are different from the other ones - basically they are identified as GR3 instead of GR2 by the hinv command and have a different board format. The board set of a 2 or 4 GE system ("XZ") consist of the following boards:

This board houses the HQ2 command engine, 2 GE7 geomtry engines (on one chip) and a RE3 Raster Engine. The connection to the backplane is on this board.
Aside from a video I/O slot this board also contains the RAMDAC. All external connectors are on this board.

GR4 Express

Last but not least the "Express" boards on the Indy are also different from the basic type used on the Indigo. The individual boards are:

Aside from the connector to the Indy mainboard the RAMDAC is placed on this board. The external 13W3 connector is here as well.
Second board of the XZ boardset including the Geometry Engines.



The Indigo system does have a specific slot that is intended for the graphics boardset. The Express boardset must be installed into this slot, other configurations are phyically impossible.

Indigo 2

The boardset for Indigo 2 consists of two fullsize boards that are connected to each other. Only one of these boards connect to the GIO64 slots in the midplane of the Indigo 2. In general it does not matter in which slots the boards are installed. It is worthy to note though, that each of the logical GIO64 buses may be used by only one device. This is important to consider when other GIO64 options are already installed in the system.


The Indy does have an onboard GIO32 slot in which the Express 2-board set is installed.


The boardset for the Crimson consists of the normal graphics option as it is used in the Indigo systems plus one additional additional item: The MG1 adapter allows to mount the boardset into a 9U VME sized frame and includes a board that interfaces the graphics option to the Crimson backplane.

Personal Iris

The "Express" boardset used in the Personal Iris systems is the same GR2 based set that is also used in the Indigo or Crimson systems. To attach it to the CPU board two additional adapters/connectors are required: CN1 and CB1. "Express" graphics is only supported on 4D/3x machines and requires IP12+ / IP14+ boards.



Elan Graphics for Crimson and PowerSeries.

Crimson Elan frontpanel.

Crimson Elan cardcage.


Indigo Elan Graphics

Indigo Elan Graphics

Indigo 2

XZ graphics, VB2 board.

XZ graphics, GR4 board.

XZ boardset.

XZ boardset, back side.



  • Elan graphics Technical Report
  • Graphics Rendering Architecture for a High Performance Desktop Workstation
    Chandlee B. Harrell and Farhad Fouladi [Published by ACM in 1993]