This information should help all who want to understand more about the Geometry
Engines (GE) and related chips which give Silicon Graphics workstations their 
graphical "magic".

This document covers SGI's Indigo2 IMPACT and OCTANE systems.  The engines in 
both of these systems are essentially the same - their bus architectures are 
different - EISA for the Indigo2 and PCI for the OCTANE.
(Editor's note - That actually should be GIO-64 for Indigo 2 and XIO
for Octane - G. Douglas, Oct. 31, 2000).  SGI benchmarks show 
that in spite of these bus differences, their graphical performance is almost 
identical - assuming the same number and type of "engines".

All systems have a Command Processor which distributes graphic primitives to 
the appropriate graphics engines.  There are three principle classes of graphics
engines.  The Geometry Engine (GE) transforms 3D geometry, performs convolutions
and other 2D imaging operations.  Each Geometry Engine (GE11 in an IMPACT or 
OCTANE) can do 480 Million Floating Point Operations per Second (MFLOPS).  If a
system has two Geometry Engines, its MFLOPS capability is typically doubled - 
in the case of an Maximum IMPACT or OCTANE SSI or MXI - to 960 MFLOPS.

The next engine is the Raster Engine (RE).  Its job is to do high performace 
pixel fills.  The RE's in the IMPACT and OCTANE are able to support a fill rate 
of 120 Million pixels/second.  If there are two RE's in a system, its fill rate 
is doubled to 240 Million pixels/second.

Lastly, there is the Texture Engine and Memory which interpolates and maps 
image and texture data.  On the IMPACT and OCTANE, texture memory is dedicated 
(i.e. not part of the system RAM - as it is on the O2).  The more texture memory
the better - and the maximum for these systems is 4MB. As you can see from the 
chart below, both systems can be configured with or without a Texture Engine.

So what system has what engines?  It's not easy to know.  In fact if you read 
Brad Reger's information on the Indigo series, you could probably guess that SGI
didn't alway follow their marketing literature perfectly.  However, as far as I 
can tell, SGI always gave you more than their literature stated when they chose 
to make a change!  The table below indicates the marketing name and number of 
engines of each class for that system.  The existance of a "High-AA"  version 
of the High Impact is never mentioned in SGI marketing literature, but they do 
exist!  Note that the High-AA IMPACT runs at speeds typical for a standard High
IMPACT, but can execute some operations, such as AntiAliasing (AA), at the same
speed as a Maximum IMPACT.

Note:  The (2) GE board of a High-AA IMPACT CANNOT be used as part of the three 
board (GD2, RA, RB) Maximum IMPACT set!  The "hinv" and "gfxinfo" commands will 
say you have a Maximum IMPACT, but you do not!  You will get slightly better 
than HIGH IMPACT performance, but much less than MAXIMUM IMPACT performance.  
Apparently, the "hinv" and "gfxinfo" commands just count the number of GEs and 
REs to define a graphic board set, and in this case, it's not enough!


Product                     Geometry     Raster     Texture     Texture RAM

Indigo2 Solid IMPACT           1           1           N             -
Indigo2 High IMPACT            1           1           Y         1MB or 4MB
Indigo2 High IMPACT (High-AA)  2           1           Y         1MB or 4MB
Indigo2 Maximum IMPACT         2           2           Y         1MB or 4MB
OCTANE/SI                      1           1           N             -
OCTANE/SI w/Texture            1           1           Y            4MB
OCTANE/SSI                     2           2           N             -
OCTANE/MXI                     2           2           Y            4MB


Lyle Bickley
Bickley Consulting West Inc.
lbickley@bickleywest.com