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Sun's Move from SBus to PCI Bus How Will it Affect You?

article by Mark Sandman at Aurora Technologies

GOOD NEWS FOR SUN USERS

Sun's decision to incorporate the PCI I/O bus into its newest generation of SPARC workstations and servers should be received as good news by Sun users. The adoption of the PCI bus standard offers many benefits and few disadvantages. The higher throughput offered by the PCI I/O bus will provide improved performance on high-bandwidth applications such as video conferencing. Sun SPARC users will also benefit from a broader choice of peripherals and, most likely, at more attractive prices than SBus add-ons. There is a substantially larger and more competitive market for PCI-bus peripherals. Installation and set-up of PCI-bus peripherals is essentially the same as for SBus products. The only real disadvantage resulting from Sun's migration to the PCI I/O bus is that old SBus peripherals will not work on the new PCI I/O-based workstations and servers. However, Sun is expected to introduce some systems capable of supporting both SBus and PCI bus.

ABOUT SBUS

SBus is an I/O interconnect bus developed by Sun and defined by IEEE Standard 1496. Sun introduced its proprietary general-purpose I/O interconnect bus on the Sun SPARCstation 1 in 1989. Sun developed a proprietary I/O bus because alternatives available at that time simply did not offer the bandwidth capable of keeping pace with the throughput of the SPARC CPU. The original 32-bit implementation of SBus offers a peak bandwidth up to 100 MBps (Megabytes per second) compared to ISA (Industry Standard Architecture) which provides a maximum theoretical bandwidth of 16 MBps.

A PROLIFERATION OF I/O ARCHITECTURES

In the 1980s, a number of other desktop I/O architectures were introduced offering higher throughput than the relatively low bandwidth ISA standard. Compaq Computer, among others, championed EISA (Extended Industry Standard Architecture) with a maximum theoretical throughput of 33 MBps, IBM introduced their proprietary MCA bus (Microchannel Architecture) with a maximum theoretical throughput of 32 MBps, and Apple introduced their NuBus. However, none of these technologies were able to gain cross-platform acceptance.

PCI - A DE FACTO INDUSTRY STANDARD I/O BUS

PCI was originally proposed by Intel Corporation as a next generation bus to replace the increasing number of incompatible bus architectures being developed. Shortly after, the PCI Special Interest Group (PCI SIG) was convened for the purpose of monitoring and enhancing the development of the PCI architecture. PCI SIG consists of over 300 member companies representing all facets of the computer industry. The PCI bus is a high-performance bus that provides a processor-independent data path between the CPU and high-speed peripherals. PCI was designed specifically to accommodate multiple high performance cards for graphics, full motion video, SCSI, and high speed LANs. PCI provides very fast throughput, however it supports only a few devices without the addition of a PCI bridge chip. Today, personal computers typically include both a PCI I/O bus, as well as a more general expansion bus, typically ISA, for other devices, such as modems, that do not require such fast data throughput. However, it is expected that soon most PC manufacturers will provide only a PCI bus.

PCI REVISION 1.0 AND REVISION 2.1

There are two versions of the PCI bus. PCI Revision 1.0, released in 1992, is currently in widespread use by both Intel-based PC manufacturers and providers of PCI I/O peripherals. It runs at 33 MHz allowing it to transfer up to 132 MBps over a 32-bit bus. PCI Revision 2.1 (which supersedes Revision 2.0) incorporates refinements to the original 33 MHz/32-bit specification and extends the specification to optionally support a 66 MHz implementation employing a 64-bit bus. The 66 MHz/64-bit implementation provides a maximum theoretical throughput of 528 MBps. The higher throughput is particularly important for high-speed video and other real-time applications. Sun has incorporated the PCI Revision 2.1 I/O bus in its new Ultra 30 series workstations.

BENEFITS OF THE PCI I/O BUS ARCHITECTURE

PCI offers significant benefits compared to the I/O technologies it replaces:

• High throughput resulting from faster clock speeds and wider bandwidth, and a more direct path to the CPU and memory.

• Easy installation. Plug and play support enables the host system to automatically configure newly installed devices without requiring user set-up.

• Processor independence. The same card can be used on many different platforms (providing the appropriate software driver is available). In addition to Sun, the PCI I/O bus is also used in Intel Pentium, Apple PowerPC, Digital Alpha and other computer systems.

• Low power consumption for workstations, PCs and laptops (the PCI specification defines both a 5 Volt and 3 Volt implementation).

• Multiple bus support makes it easier for both hardware developers and end users to increase their computer's expansion capabilities.

• Bus mastering enables intelligent devices, such as a NIC, to gain control of the bus to access memory independent of the CPU.

• Improved system performance. Wider bandwidth, requiring less CPU processing cycle time and bus mastering capabilities, frees the host processor for other tasks.

PCI BUS VERSUS SBUS

PCI Revision 2.1 offers a maximum theoretical throughput of 528 MBps. By comparison, the original 32-bit implementation of SBus offers throughput up to a maximum of 100 MBps. The 64-bit implementation of SBus offers throughput up to of 200 MBps.

Note: Actual usable bandwidth is typically one-third to two-thirds of the theoretical maximum.

As previously stated, most PCI peripherals offered today are based on the 33 MHz/32-bit architecture, supporting a maximum theoretical throughput of 132MBps. So, for Sun Solaris SPARC users, the real performance benefits of incorporating the PCI and the Revision 2.1 extensions will come as more peripherals become available that use the 66 MHz/64-bit implementation. Performance benefits will be in the range of a 2- to 5-times improvement. For Sun Solaris x86 users (Intel implementation of Solaris), the PCI I/O bus is a major improvement over the technologies it replaces. PCI bus is easier to install, processor independent, and offers significantly higher throughput. The 32-bit/33 MHz PCI implementation provides a maximum theoretical throughput more than 8-times greater than ISA. The 66 MHz/64-bit implementation will provide a maximum theoretical throughput more than 30-times greater than ISA.

THE BOTTOM LINE

Both Solaris SPARC and Solaris x86 users will not see any difference in the performance of lower bandwidth peripherals such as modems, tape drives, data entry terminals, and printers regardless of what I/O bus they previously used. For Sun Solaris users moving to systems employing the PCI bus, the real difference will be in the performance of high-bandwidth tasks. This includes almost anything to do with multimedia, full motion video, real-time data feeds, and high-speed networking. Sun Solaris SPARC users should also benefit from a broader choice of peripherals at more attractive prices.

AN IMPORTANT CAVEAT - THE KEY TO PERFORMANCE IS THE DEVICE DRIVER

The PCI-bus specification assures hardware compatibility across platforms. However, this does not eliminate the need for platform-specific software device drivers. Sun, for example, has been working closely with its strategic third-party vendors for more than two years to ensure quality PCI add-on products would be available when they announced their first PCI I/O-based systems. In the July 15, 1997 press release announcing the Ultra 30 PCI-I/O based workstations, Charles Andres, group manager for I/O technologies at Sun, noted,

"We've selected quality-focused Solaris-proficient hardware vendors for this (Sun PCI Vendor) program to ensure that customers who choose Sun's PCI I/O-based systems will have a wide range of add-on PCI products from which to choose."

Implicit in this statement is that, although PCI is platform independent, it is important for suppliers to have an in-depth understanding of the specific operating system environment in order to deliver products that provide optimum performance on the target platform.

Aurora has been developing multiport serial controllers for the Sun Solaris environment since 1989. Over the years, we've benchmarked a number of competitor products offering comparable hardware features to our products, but providing significantly inferior performance. In almost every case, the flaw was not in the hardware design but in the implementation of the device driver.

For companies developing PCI-bus peripherals, it makes good business sense to provide device drivers for as many hardware platforms as possible. However, hardware that performs well on one platform can perform poorly on another depending upon the quality of the device driver. As a user, it makes good sense to put your trust in companies with proven experience delivering solutions for the Solaris SPARC platform.

ABOUT THE AUTHOR

Mark Sandman is Marketing Manager for Aurora Technologies, based in Waltham, Massachusetts, USA. The company designs and manufactures a wide range of PCI-bus and SBus compatible multiport serial communications controllers and servers for Sun Solaris and Windows NT systems. Aurora multiport serial communications products are used to interconnect modems, terminals, instrumentation, and printers directly to a host system. They also provide a cost-effective solution for remote LAN and WAN access. Typical applications include office automation, remote access, Internet access, console management, manufacturing automation, telecommunications, financial services, and point-of-sale retailing.

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