One of the most controversial topics concerning the SGI 320 is that of upgrading the CPU(s). Whether you are upgrading your workstation's single or dual processors, the question is always the same: how fast can I go!

The 320's upgradeability is dependant on a number of factors, namely the PROM version, Voltage Regulator Module (VRM) revision and motherboard revision. The last item is the real determining factor when upgrading the 320 as the earlier motherboards may not support a 'maxed out' dual CPU setup.

There are two options available when upgrading CPU(s): single or dual processor. The maximum speeds attainable in either configuration are listed below:

Maximum single CPU upgrade:
1 x 1.4Ghz Celeron (100Mhz FSB)

Maximum dual CPU upgrade:
2 x PIII @ 1Ghz (100Mhz FSB)

The 320's Front Side Bus (FSB) is fixed at 100Mhz, therefore the later P3s utilising a 'Tualatin' core cannot be used, as they require a faster FSB (eg. 133). Furthermore, the 320’s CPU sockets are Slot1 (SECC2) so any upgrades should either use this variant of the CPU or an adaptor (slocket) for the FCPGA configuration.

Dual Configuration
The maximum dual configuration setup reported so far is a 320 running dual 1Ghz P3s (100Mhz FSB). The CPUs use SECC2 connectors and an appropriate VRM is necessary (see below). The stepping values of both chips must also be identical. This setup will give the best performance on a 320 when running multi-threaded applications, although it is also the most expensive option.

One thing to note is the space required to install two such processors. Because of the space taken up by the CPU's fan/heatsink, it is recommended that boxed Intel 1Ghz P3s (Intel Part #: BX80526H1000256) be purchased. Their lower profile will comfortably fit the confined space on the 320 motherboard as opposed to the cheaper, generic fans/heatsinks.

Single Configuration
When upgrading a single CPU, a Celeron running at 1.4Ghz becomes an option as it uses a 100Mhz FSB. Processors at this speed almost always come in FCPGA configurations, which means that a slot adaptor (slocket) is required to use them with the 320. For a complete solution see Powerleap's iP3T (link). Unfortunately, because the Celeron is a 'budget' version of it's Pentium counterpart, it does not support SMP, hence can only be used in uniprocessor setups.

A single processor configuration, however, gives better performance for non-threaded applications. Even though a Celeron is slower than it's Pentium counterpart at the same clock speed, the 1.4Ghz version uses a Tualatin core and other optimisations to outperform the fastest P3 that can be used in a 320 (1.1Ghz). This is also a cheaper option than the dual setup.

A few things to bear in mind when going for the Powerleap Celeron solution:

• Set JP2 / JP3 jumpers (FrontSide Bus) to AutoDetect (should be default setting).
• Set JP5 CPU (Core Voltage) to 1.550V-1.575V (Default)
  
  PowerLeap has stated that it is possible to 'overclock' the CPU by setting the jumper higher to 1.700V-1.725V, but the correct operation of the CPU is not guaranteed.
  
  
• Disable the CPU's L2 ECC in the PROM System Settings. This is optional if the CPU upgrade is causing 'display artifacts', random crashes during bootup, disappearing icons and applications refusing to run.

PROM

There have been 7 PROM versions for the SGI 320 released so far. For a list of the Release Notes for each version, follow the (link).

It is generally accepted that for maximum upgradeability these workstations should be flashed with the latest version, 1.1005 (link).

Another advantage of updating the PROM to the latest version is to negate the need to adjust the four speed jumpers. There is no need to fiddle with jumpers anymore as they become redundant. Instead, the 320 senses the processor speed by reading its VID pins (Voltage Identification) and then matching this with the MU table (microcode) stored in the PROM. Upgrading the PROM installs microcode for the latest P3s being produced at the time the PROM was written.

Voltage Regulator Module (VRM)

The SGI 320 requires a VRM when upgrading to dual processors. There has been much talk in the past concerning what VRM is required for upgrading the 320 to dual 1Ghz PIIIs. For an explanation on VRM conformity and to track down the revision appropriate to your workstation, follow the (link).

Motherboard Revision

When upgrading a 320's CPU(s), the motherboard revision is the ultimate determining factor. The first batch of 320s released by SGI were only ever supposed to support P2 or early P3 processors (Katmai). SGI's official line on the upgradeability of the 320 can be found under the PROM Release Notes.

Below is a list of all known motherboard revisions. For more detailed information pertaining to the various motherboard revisions, follow this link.

• 006 A/D/H/J/K
• 007 /A/B
• 117A
• 118

Hacking older motherboards

Thanks to Bob Skiles, the following tutorial (link) should give owners of older 320s (i.e. those with mobo versions earlier than about 006H) the ability to upgrade their workstations to the 'maxed out' dual CPU configuration without having to purchase a new motherboard.