The I/O Bus

Computers have a number of physical paths, called buses, for moving data. An important one, the system bus, was discussed earlier in this chapter. Buses may not be very exciting, but as we try to find ways to make our computers faster, they're a critical part of the equation. Naval officers observe that the speed of a convoy is that of its slowest ship. Similarly, if a bus can't handle all the traffic that converges on it, the whole computer will have to slow down.

As noted, the first microcomputers had just one internal I/O bus to which everything—CPU, memory, video, disk I/O—was connected. Soon, however, this bus was saturated as the various parts contended for access to memory. The memory bus was then split off, but problems resurfaced almost immediately. Now the issue was the video system. As we'll discuss in Chapter 3, third generation video systems were much more demanding of memory than their predecessors. Vendors of graphics cards responded by taking their boards off the I/O bus and connecting them directly to the memory bus. This approach quickly challenged the standardization that had made the PC such a success. Video card vendors then addressed the emerging Tower of Babel by creating a new standard for these "local bus" connections—the VESA (Vide Electronics Standards Association, later known as the VL-bus) standard. While intended to handle more than video connections, VESA's new design was not especially flexible and didn't offer a clear path to future enhancement, so it too was replaced.

PCI

Intel, fearing that the booming PC market would lose momentum without standards, came to the rescue by essentially dictating a new technology, known as PCI for Peripheral Component Interconnect. Wisely, Intel established the specifications, but then immediately made it an "open" standard by creating an industry body to manage it. The PCI bus, with a capacity of 132 MB vs. 32 MB for ISA, took off quickly.

PCI doesn't connect directly to the memory bus; a controller chip provides an interface. As designed, it is 32 bits wide and operates at 33 MHz. Despite the fact that PCI was specified by Intel, its detached architecture allows it to be used with any CPU. In fact, PCI is now employed by Apple for its PowerPC-based Macs and by Compaq/DEC for its Alpha-based workstations.

The increasing demands on hardware systems are now overtaking the PCI bus, and a superfast PCI-X (133 MHz/64 bits) is expected to be the standard for server systems. There is some thought that the FireWire technology (see the next section) will replace PCI for desktop machines.

External I/O

Until recently, computers were connected to external devices like printers and modems with serial and parallel cables. These are now rapidly becoming obsolete.

The leader of the replacement generation, known as USB (for universal serial bus), offers significant advantages. One benefit is simplicity. A single cable can connect lots of different devices, including printers, modems, monitors, mice, and disk drives. Another advantage of USB is "hot swapping." With older connections, you had to restart the computer every time something new was attached to it. USB not only allows connections to be made and broken without restarting the system, devices can be "daisy-chained" to one another rather than all connected to a central point. All new computers support USB.

Although USB has sufficient capacity to support even high bandwidth devices such as disk drives, there is a widespread belief that it will not be fast enough to handle very high speed connections such as those that are required for video cameras and DVD drives. The alternative for this high bandwidth market is known as FireWire (also called IEEE 1394). FireWire is very similar to USB. It supports lots of different kinds of devices, is hot swappable, and can be daisy-chained. The big differences are that it has much higher capacity (400 Mbps vs. 12 Mbps) and allows daisy-chained devices to communicate directly with each other (peer to peer connections). While many believe that FireWire will quickly replace USB, experience suggests that this may not be the case. It often happens that while a more capable technology waits in the wings to take over the lead, the older approach, with the momentum provided by a significant installed base, improves just enough to retain its position. In fact, USBv2.0, boasting speeds of 480 Mbps, is ready to challenge FireWire as this is written.

One by One External buses are moving from parallel to serial technologies. Instead of one wire for each of 8 or 16 bits, there is now a single data wire; bits follow each other in sequence. While this might seem less efficient, it is in fact much better. Parallel cables are expensive to make (to deal with skew and for other reasons). In addition, their bulky shielding makes them heavy and inflexible. Serial cables, on the other hand, need only a small amount of shielding to protect a very high speed wire.

Plug and Pray? A part of the chipset is something called the BIOS, which stands for Basic Input Output System. Since this is really software encased in silicon, we'll talk about it in Chapter 6. Note, though, that the BIOS is in part responsible for managing the connection of peripheral devices to the computer. In the old days, users of Microsoft software had to change BIOS settings whenever a new card was put into a machine (they also often had to change settings on the boards themselves). With the advent of the PCI bus, a new type of BIOS was developed that allowed automatic installation—"plug and play." After a rocky start (occasioning the "plug and pray" comment), this technology seems to work fairly well. As is often the case, though, we should acknowledge that Apple's Macintosh had successfully implemented something similar many years earlier.