Yes, one of our engineers has written the following white paper.
By Paul Wolf
Power Computing Corporation
Introduction
In the ever-changing world of the personal computer industry, Apple's adoption and introduction of the PCI standard into the Macintosh OS marketplace marked a very important evolution in their long term product development and strategy. Until Apple's introduction of the PowerMacintosh 9500 and subsequent introduction of the remaining PowerSurge Macintoshes in August, 1995, MacOS users had been strictly using systems that solely contained either NuBus or PDS (processor direct slot) expansion slots.
Today, with the introduction of the PCI standard, users will be able to get even more performance from their MacOS system. However, many users have invested a significant amount of money, time and energy into NuBus technology and will find it difficult to make the transition to PCI. And like all substantial product transitions, the move to PCI has not been seamless - marked by transitional software issues, installed base questions and a lagging third-party hardware development sector.
Adoption of the PCI standard can be a daunting proposition both for end-users and developers alike, who necessarily view the transition as an expensive proposition either in terms of NRE's, existing investments in NuBus hardware and finally, by the lack of availability of PCI-solutions in specifically targeted, vertical markets including digital/audio, digital prepress, digital video and the engineering/scientific market.
Expecting these issues and wishing to help ease this monumental transition, Power Computing with Apple's assistance, set out in late 1994 to begin development of what has become Stargate - the code name for a proprietary ASIC developed by Power Computing. The Stargate ASIC has been designed on a riser card that plugs into a connector on the new Power Computing PowerWave 604/150, PowerWave 604/132 and PowerWave 604/120 computer motherboard, that allows users to have both PCI and NuBus slots in their systems.
This White Paper will examine the following areas:
- What is PCI and why is it beneficial?
- What is Stargate and how will it benefit you?
- How does it work and what's its performance?
About PCI
PCI is an acronym which stands for Peripheral Component Interconnect. Originally designed by Intel in the early 1990's, PCI is now governed by a consortium of industry partners including Apple Computer. This group, which includes software companies, component suppliers, and peripheral vendors, has a goal of ensuring the compatibility and interoperability across the PCI standard and to continue to increase the throughput of PCI even further.
There are several key reasons that PCI in the MacOS market is preferable over NuBus technology. First, PCI is a widely adopted industry standard in the personal computer market. This allows for a commonality to be promoted with other platforms. Second, since it is industry standard, MacOS computer makers can attract a more widely varied pool of expansion cards for different solutions. Third, the addition of PCI to the MacOS market gives the platform a potentially tremendous boost in performance over the NuBus standard.
Power Computing's PowerWave 604/150, PowerWave 604/132, and PowerWave 604/120 products include the PCI standard. However, because there is a significant market of those users who still need and want NuBus, the company has developed Stargate in order to allow users a combination of PCI and NuBus slots in their MacOS systems. Essentially offering the best of both worlds, Stargate allows users more control and flexibility in managing their own transition to PCI.
What is Stargate?
Stargate is a proprietary ASIC (Application Specific Integrated Circuit) developed by Power Computing to allow both PCI and NuBus expansion slots to coexist in the same computer. To explain more about how Stargate works, looking at the architecture of the PCI Power Macintoshes versus the NuBus Macintoshes will be helpful.
A simplified way of thinking about the architecture of the PCI Power Macintoshes is that they have a processor bus (where communication between the processor, memory and level 2 cache occurs) and a PCI bus (which is used for communication with the on-board I/O as well as with expansion cards). The PCI Power Macintoshes use a processor-bus-to-PCI-bus bridge to allow the processor access to devices on the PCI bus. The NuBus Power Macintoshes are similar, except that they instead have a processor-bus-to-NuBus bridge.
One possible architecture for our PowerWave 604/150, PowerWave 604/132 and PowerWave 601/120 systems with both PCI and NuBus slots would be to have one of each of these bridges (processor to PCI and processor to NuBus). This theoretically gives the processor the best performance when accessing either PCI or NuBus devices. However, because it is best to absolutely minimize the number of devices present on the processor bus in order to keep the processor's access to RAM and level 2 cache very fast, having both bridges on the processor bus is not recommended.
Another way to implement a PCI plus NuBus computer would be to start with a NuBus system and add a NuBus-to-PCI bridge, requiring that all processor to PCI communication proceed across NuBus first. There are many problems, however, with this implementation. Because PCI has a bandwidth nearly three times that of NuBus, trying to squeeze PCI traffic onto NuBus would lead to a reduction in PCI performance down to, or probably even lower than NuBus performance. From a software standpoint this is a difficult option as well since the version of Mac OS System Software that was designed for the NuBus Power Macintoshes was not designed with PCI in mind. Adding a new expansion mechanism to an older architecture is nearly an impossibility.
There is a third option, which is to start with a PCI system and add a PCI-to-NuBus bridge. This works well for a variety of reasons. From a performance standpoint, because PCI is more than three times faster than NuBus, there is no performance hit for either the NuBus cards nor for the PCI cards when the NuBus traffic is routed across the PCI bus. From a software standpoint, the newer PCI Power Macintosh ROMs and System Software are not only aware of how to deal with the new PCI cards but still retain the NuBus support as well, making it easy to incorporate the older architecture into the new. And, Power Computing's systems use PCI Power Macintosh ROMs.
How We Did It
It is for these reasons that Power Computing chose the latter option. Our PCI-to-NuBus bridge chip, Stargate, is found on our PCI-plus-NuBus expansion riser cards, along with one or two PCI and one or two NuBus connectors. Stargate itself is a single Application Specific Integrated Circuit (ASIC) that contains all of the logic required to implement a PCI-to-NuBus bridge.
In addition to the Stargate ASIC, there are several pieces of software that are necessary to reconfigure various aspects of the operating system to allow it to recognize the presence of the NuBus slots, and to make the differences between the PCI-based NuBus and the standard Macintosh NuBus transparent to the system as well as to the NuBus cards themselves. This software is stored on a ROM on the same riser card that contains the Stargate ASIC and expansion connectors so the user never needs to worry about loading special control panels or extensions. Using a NuBus card on these machines is exactly like using one on a Apple Power Macintosh - plug it in, boot up
and it's there.
Performance
The Power Computing ASIC designers responsible for Stargate were able to create a very high performance architecture, in many cases significantly exceeding the NuBus performance of the Power Macintosh 8100. In this way, a potential customer debating whether to upgrade from an 8100 to the new Power Computing machines, not only can be assured that their NuBus cards will work, but can also be confident the performance will be there as well, and likely even improved. Similarly, the performance of the PCI expansion slots on the riser card are comparable to Apple's new PCI Power Macintosh systems.
Machines Affected: PowerWave