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Budget Risc: Silicon Graphics' Iris Indigo

CADCAM, October 1991

(Historical Article)

Cover Comment

[Magazine Cover] Silicon Graphics made its name with powerful 3D workstations offering high performance graphics in silicon, targeted on animation, visualization and molecular design. In an effort to develop a share of the volume market, Silicon Graphics has penetrated down into the general purpose Cadcam workstation market, so that 35 per cent of its sales are into Cadcam and Cae. Products such as the Personal Iris 4D/20 introduced in 1988 and the Irisvision PC graphics subsystem launched earlier this year, along with the new Iris Indigo, are intended to extend SG's products into the general scientific and engineering community.

The Iris Indigo from Silicon Graphics is a remarkable new workstation which offers an impressive combination of price and performance. Based on a Mips R3000A/33 processor delivering 26 Specmarks or 4.2Mflops with 24-bit colour on an 8-bit display, a basic Cad configuration of 8MB memory, 236MB hard disk and a 16in monitor costs 8490 UKP.

[The Iris Indigo] This sort of pricing not only undercuts SG's workstation competitors (including Sun's IPX), but for the first time makes available standalone Risc-based computing that outperforms a top-end 486 PC on price and performance.

SG has always produced 3D workstations, a fact which has weighed against the company when selling to companies with a 2D traditional cad need which have viewed the 3D capability as superfluous funcitonality and wasted money. With the Indigo, SG has a competitively priced 2D workstation which can do competent 3D work when required.

Cad software houses interested by the combination of budget 2D and 3D graphics and which are porting their products to the Indigo include Alias, Wavefront, MCS, Cimlink, SDRC, Cadcentre and Matra Datavision, with Autodesk also porting to the Indigo.

The Indigo is an early example of the sort of budget Risc PCs that we can expect to see from the Ace consortium and its rivals, and SG is quick to stress that the Indigo is the first of a family of Risc PCs.

Buyers of Unix workstations are living in exciting times with price/performance ratios improving almost monthly. Sun's IPX is also an attractive entry-level Cad machine and HP and IBM are preparing to launch budget workstations.

Purple Processing

The Iris Indigo immediately strikes you as different from most PCs and workstations. For starters it really is indigo and when you turn it on it plays a polite one-octave chord rather than the strident single beep of most PCs or the ostentatious power-on rhapsodies of some Macs. Once I got past the more obvious novelties and hype, however, I really felt the Indigo could be a prototype successor to the PC. It is designed for high-volume mass production at an affordable price, has sufficient power to run real 3D colour applications plus a clear upgrade path, and it conforms to all the open computing standards. What is more, it is the first Unix machine I have tested that feels as easy to use as a PC.

The Indigo has been very carefully specified to give excellent graphics at 1024 x 768 resolution, and to be physically light and compact and easy to use, upgrade and maintain. It is intended to slot in below the existing Personal Iris range and, interestingly, it out performs by a factor of nearly 50 per cent the recently discontinued Personal Iris 4D/25 which Cadcam has been using as a workstation benchmark.

Quick View:

One of the most promising product launches in a long time, with a proven processor and a potentially large software base. A 1024 x 768 resolution Unix box does not sound all that exciting, but the Indigo offers such an excellent balance of features, including graphics capability, performance, cost and general usability that it deserves to be a huge success.

Iris Indigo - Rating ****** (max 6)

Plus points:

Minus points:

Ease of installation: Outstanding
Ease of use:          Very Good
Functionality:        Very Good
Documentation:        Very Good
Value for money:      Outstanding
Supplier:             Silicon Graphics
Price:                Entry-level 8MB/diskless system
                      is 6,790 UKP; 16MB/540MB disk
                      system is 11,240 UKP (19in monitor
                      1,870 UKP extra)

Virtual 24

One of the main innovations in the Indigo design is 'Virtual 24', which enables it to run 24-bit colour applications without modification, by taking the 24-bit output and dithering it dynamically using an optimized 8-bit palette. Dithering gives the illusion of having more colours in the palette than are actually available.

[The Iris Indigo]

Internal design

The Indigo is based round a Mips chipset comprising an R3000A CPU with an R3010A floating-point processor, clocked at 33MHz; it produces 30mips or 26 Specmarks with this combination and is fully Ace compatible. The design will allow use of the R4000 processor in the future. Memory options are from 8 to 96MB in 8 or 32MB increments, using 2, 4 or 8MB Simms (in multiples of four) to populate the 24 available slots. The review machine came with four 4MB modules installed - 8MB is not really enough for running high-powered packages such as Anvil, especially as system memory is also used by the graphics engine.

The hardware is housed in a mini-tower case measuring 36.5cm high x 24cm wide x 28cm deep, and is formed from simple sheet-metal pressings under a purple plastic skin: these keep weight and cost down while providing a very adequate casing. The single cooling fan is of a fairly large diameter, and thus runs particularly quietly - in fact, the Indigo is quite inaudible over the noise of the tower PC on which I am writing this.

Display quality is very clear, with a Sony 16in Trinitron screen showing the 1024 x 768 pixels at what I feel is the optimum size for this output - large enough to make good use of the resolution, yet not too bulky or heavy. A 19in display is also available.

It takes about 30 seconds to remove both processor and graphics boards from the Indigo - you just push two buttons and off comes the front panel, undo one screw and unclip the boards which occupy the left side of the case. Despite being a pre-production machine there were no patch wires, and the general construction and board quality was excellent. Two huge Silicon Graphics Asics dominate the processor module, dwarfing the Mips processor and floating point chip - these are the same as those in the Iris 4D/35.

The graphics module, although the same size as the processor board, is sparsely populated but is fitted with one further Asic (connecting the graphics module to the system bus), and expansion connectors and spacing posts which are used for fitting the optional video card.

The rest of the machine contains the power supply and the Scsi bus, with three half-height 3.5in drive bays in the right-hand side of the case (the Scsi controller, however, is on the processor board). The system drive of the review machine was a Seagate ST1480N: 426MB with a 14ms access time, but production models supplied in the UK will be fitted with Conner drives (of 236MB or 540MB) to increase the European content of the machines.


Internally mounted peripherals can be installed or removed in seconds, with automatic connection to power supplies and the Scsi bus on the backplane. This combination of easy peripheral access and modular construction of the processor makes for simple user upgrading and fault-finding. In contrast to the optical mice used with the rest of the Personal Iris range, the Indigo is equipped with a mechanical mouse, which seems very heavy going after a Logitech high resolution mouse. Five audio connectors are fitted (all 3.5mm stereo jacks which would be considered totally unacceptable by any hi-fi enthusiast) for microphone (mono only), line-level analogue input and output, headphones and digital input/output, which can be at any recognized sampling rate from 3.675 to 48kHz, with accuracy up to 24-bits per sample.

One novel audio application (not supplied) is a telephone auto-dialler - hold your (touchtone) handset within 2 inches of the built-in speaker and the Indigo plays the dialling sequence.

The two serial connectors are Mac-compatible, allowing the connection of relatively cheap Mac peripherals, which should have appeal for the multimedia market. Other peripherals which can be fitted internally include a second hard disk, 3.5in floppy disk drive and 1.3 or 2GB 4mm Dat drive (in addition to fulfilling the backup function, a Dat drive can be used with the audio features of the Indigo). There is no room to fit a CDROM drive internally, but an external drive is available.

To secure the system, a lock will be provided which will pass from front to back of the cabinet to prevent its removal, although how this would stop anyone stripping out all drives and circuit boards is less clear.

Operating system

Indigo systems come with the operating system and environment already installed. Irix 4.0 is based on Unix System V Release 3 with Berkeley enhancements, and conforms to all the acronyms you can think of including Posix, Fips and X/Open standards. Use of raw Unix is, thankfully, on the wane for the ordinary user and this machine comes with an amazing mix and match (or nearly match!) collection of software based around 4Dwm, an OSF/Motif-compliant window manager.

On this open systems basis, SG first adds Iris Workspace, which provides an iconbased graphical interface to the file system. A rich set of icons is provided, and some SG applications come with entertaining animated icons. Others, such as the car buried in the ground representing a core dump, are humorous and always easily understood. The Workspace can be tailored to show the parts of the file system you are normally interested in, together with icons for your commonly used applications.

Other useful window-based tools include the System Manager, which enables quick, easy configuration of system devices and administration of users, and the Transfer Manager, which is used for input and output from local and networked devices. This makes tape use simpler, but you can only read tapes created on other SG systems using the transfer manager (to read tapes created on non-SG machines, you have to use a different device driver so as not to swap the byte order when reading; this can only be done using tar from a shell).


Most graphics operations are performed in software, including Z-buffer calculations. As the performance figures show, the Indigo is still faster than a 4D/25 by a considerable margin - Silicon Graphics claims that it is 'nearly equivalent' to the 4D/25 Turbo Graphics. Font rendering is performed by the CPU using Display Postscript giving up to 180K Cps on text display, while graphics are output at up to 200K 3D vectors/s.

Although colour dithering has been employed before to display 24-bit images using an 8-bit frame buffer, in general this involves picking an optimized 256-colour palette for the file to be displayed, and sticking to it. With Virtual 24, this is not the case; the 256-colour palette is chosen dynamically for each displayed frame. There is a hardware limitation on this in that only four colour maps are maintained, but in practice this should not cause problems. The great advantage of this approach is that all operations except display are carried out at full 24-bit colour resolution, so output to printer or slide is not compromised in any way.

As all these graphics operations move data along the main system bus, this needs to be fairly fast, and in fact operates at a respectable 33MHz - which gives peak transfer rates for block operations of 133MB/S, enabling the Rex graphics chip to squirt data onto the screen at up to 40 million pixels/s.

Networking support is more than adequate, including support for TCP/IP, NFS, SNA and Decnet protocols, with the option of Netvisualizer, a monitor which gives a visual display of traffic on the network.

[The Iris Indigo Motherboard]


If the computer industry has its way, we will all be using multimedia tomorrow. To this end the Iris Indigo Video Option board enables most video operations to be performed on-screen, including the display of live video signals in a window with over/underlaying of graphics, plus output of 24-bit RGB. The option board is equipped with three inputs and an output, all software is switchable between Pal and NTSC standards. and can also carry out image processing on the signal.

Many colour printers are supported by the Indigo in addition to generic Postscript devices, but although the serial ports can operate at up to 38.5KB/S, the Printer Manager still only allows 9600bps as the maximum.


As this was a beta test machine, only a second beta version of the Iris Indigo Owner's Guide was available. This was up to the very high standard of other Silicon Graphics documentation, somehow managing to combine sufficient technical detail without becoming over-complex. The standard set of manuals includes the Iris Workspace User's Guide, systems administration, utilities and software installation guides.

The Indigo is like any other workstation in that it is only as useful as its available applications, which in Cad terms in the near future includes Alias, Wavefront, MCS, Cimlink and SDRC, with Autodesk porting Autocad to SGI for the first time. Once the Indigo and other Ace machines start running Windows/NT as well as Unix the range of applications available should become enormous.

The review machine came equipped with Iris Showcase, a rather interesting presentation and drawing tool, which allows documents to include 3D models and raster images. These documents can then be set to play as a slide show and such documents may also be distributed by email.

Wingz, the spreadsheet from Informix, was also installed and worked very well - it is a nice change to see software beginning to appear on Unix machines with the sort of graphics quality PC and Mac users are accustomed to. Other applications which will be available include Autocad, the Framemaker DTP package, Wordperfect and Adobe Illustrator, in addition to the 1800-odd existing Iris applications.


Anvil 5000 release 2.1 was supplied by MCS for benchmarking. The Indigo has been designed to give efficient windowing performance, and window redrawing is swift. Although the Cadlab tests show that the Indigo is about 50 per cent faster than a 4D/25, in practice it seems quite a bit faster for general use. Although it is considerably slower than the 4D/35 (reviewed Cadcam, March 1991), for most general Cad work the performance is adequate. The dithered 24-bit output also works much better than other similar systems I have seen, and provides a good low-cost entry to 3D colour work.

The Indigo offers an impressive combination of price and performance. It is not so much a workstation as one of the first arrivals of a future generation of Rise PCs.

Martin Gordon is a computer consultant and technical adviser to Cadcam.

Tech Spec

Silicon Graphics Iris Indigo:

CPU:      33MHz Mips R3000A processor with R3010A FPU,
          32KB instruction cache, 32KB data cache
RAM:      16MB Ram (8-96MB available)
Storage:  426MB Scsi fixed disk (see text)
          150MB external tape drive
Display:  Virtual 24 graphics system
          Sony 16in Trinitron monitor
          1024 x 768 resolution, 60Hz refresh rate
Input:    AT-style keyboard, three-button mechanical mouse
Ports:    Scsi-2, 2 RS232 serial, Centronics bidirectional
          parallel, Ethernet, Dat-quality audio I/O via five
          3.5mm audio ports, 13w3 and D-sub video connectors,
          2 expansion slots
Software: Irix system R4.0; 4Dwm OSF/Motif-compliant window
          manager; Iris Workspace desktop; Anvil 5000 R2.1

CAD LABS Test results

                        4D/25G    Indigo      Relative
                                            Performance (*)

1. Load screwdriver     60.00     46.47         1.29
   (Wire frame)

2. Zoomviewpoint        80.00     63.81         1.25

3. Load PART2           14.00     13.00         1.08
   (FEM + colour map)

4. Load PART3          115.00     43.15         2.67

5. Select all surfaces 108.00    100.08         1.08

6. Write to disk        30.00     16.85         1.78

7. Hardware shade -    155.00    122.46         1.27
   Bench 1

8. Hardware shade -    163.00    130.53         1.25
   Bench 3

Average 1.46

All timings are in seconds.

* Relative performance figures are computed by comparing
the Indigo configuration against a Silicon Graphics 4D/25,
with the 4D/25's performance taken as having a value of 1.
See text for additional details.

Ian's SGI Depot: FOR SALE! SGI Systems, Parts, Spares and Upgrades

(check my current auctions!)
[WhatsNew] [P.I.] [Indigo] [Indy] [O2] [Indigo2] [Crimson] [Challenge] [Onyx] [Octane] [Origin] [Onyx2]
[Future Technology Research Index] [SGI Tech/Advice Index] [Nintendo64 Tech Info Index]