Buses

The organization of Fig. 1-5 was used on minicomputers for years and also on the original IBM PC. However, as processors and memories got faster, the ability of a single bus (and certainly the IBM PC bus) to handle all the traffic was strained to the breaking point. Something had to give. As a result, additional buses were added, both for faster I/O devices and for CPU to memory traffic. As a consequence of this evolution, a large Pentium system currently looks something like Fig. 1-11.

This system has eight buses (cache, local, memory, PCI, SCSI, USB, IDE, and ISA), each with a different transfer rate and function. The operating system must be aware of all of them for configuration and management. The two main buses are the original IBM PC ISA (Industry Standard Architecture) bus and its successor, the PCI (Peripheral Component Interconnect) bus. The ISA bus, which was originally the IBM PC/AT bus, runs at 8.33 MHz and can transfer 2 bytes at once, for a maximum speed of 16.67 MB/sec. it is included for backward compatibility with old and slow I/O cards. The PCI bus was invented by Intel as a successor to the ISA bus. It can run at 66 MHz and transfer 8 bytes at a time, for a data rate of 528 MB/sec. Most high-speed I/O devices use the PCI bus now. Even some non-Intel computers use the PCI bus due to the large number of I/O cards available for it.

In this configuration, the CPU talks to the PCI bridge chip over the local bus, and the PCI bridge chip talks to the memory over a dedicated memory bus, often running at 100 MHz. Pentium systems have a level-1 cache on chip and a much larger level-2 cache off chip, connected to the CPU by the cache bus.

In addition, this system contains three specialized buses: IDE, USB, and SCSI. The IDE bus is for attaching peripheral devices such as disks and CD-ROMs to the system. The IDE bus is an outgrowth of the disk controller interface on the PC/AT and is now standard on nearly all Pentium-based systems for the hard disk and often the CD-ROM.

Figure 1-11. The structure of a large Pentium system

The USB (Universal Serial Bus) was invented to attach all the slow I/O devices, such as the keyboard and mouse, to the computer. It uses a small four-wire connector, two of which supply electrical power to the USB devices. USB is a centralized bus in which a root device polls the I/O devices every 1 msec to see if they have any traffic. It can handle an aggregate load of 1.5 MB/sec. All the USB devices share a single USB device driver, making it unnecessary to install a new driver for each new USB device. Consequently, USB devices can be added to the computer without the need to reboot.

The SCSI (Small Computer System Interface) bus is a high-performance bus intended for fast disks, scanners, and other devices needing considerable bandwidth. It can run at up to 160 MB/sec. It has been present on Macintosh systems since they were invented and is also popular on UNIX and some Intel-based systems.

Yet another bus (not shown in Fig. 1-11) is IEEE 1394. Sometimes it is called FireWire, although strictly speaking, FireWire is the name Apple uses for its implementation of 1394. Like USB, IEEE 1394 is bit serial but is designed for packet transfers at speeds up to 50 MB/sec, making it useful for connecting digital camcorders and similar multimedia devices to a computer. Unlike USB, IEEE 1394 does not have a central controller. SCSI and IEEE 1394 face competition from a faster version of USB being developed.

To work in an environment such as that of Fig. 1-11, the operating system has to know what is out there and configure it. This requirement led Intel and Microsoft to design a PC system called plug and play, based on a similar concept first implemented in the Apple Macintosh. Before plug and play, each I/O card had a fixed interrupt request level and fixed addresses for its I/O registers. For example, the keyboard was interrupt 1 and used I/O addresses 0x60 to 0x64, the floppy disk controller was interrupt 6 and used I/O addresses 0x3F0 to 0x3F7, and the printer was interrupt 7 and used I/O addresses 0x378 to 0x37A, and so on.

So far, so good. The trouble came when the user bought a sound card and a modem card and both happened to use, say, interrupt 4. They would conflict and would not work together. The solution was to include DIP switches or jumpers on every I/O card and instruct the user to please set them to select an interrupt level and I/O device addresses that did not conflict with any others in the user’s system. Teenagers who devoted their lives to the intricacies of the PC hardware could sometimes do this without making errors. Unfortunately, nobody else could, leading to chaos.

What plug and play does is have the system automatically collect information about the I/O devices, centrally assign interrupt levels and I/O addresses, and then tell each card what its numbers are. Very briefly, that works as follows on the Pentium. Every Pentium contains a parentboard (formerly called a motherboard before political correctness hit the computer industry). On the parentboard is a program called the system BIOS (Basic Input Output System). The BIOS contains low-level I/O software, including procedures to read the keyboard, write to the screen, and do disk I/O, among other things. Nowadays, it is held in a flash RAM, which is nonvolatile but which can be updated by the operating system when bugs are found in the BIOS.

When the computer is booted, the BIOS is started. It first checks to see how much RAM is installed and whether the keyboard and other basic devices are installed and responding correctly. It starts out by scanning the ISA and PCI buses to detect all the devices attached to them. Some of these devices are typically legacy (i.e., designed before plug and play was invented) and have fixed interrupt levels and I/O addresses (possibly set by switches or jumpers on the I/O card, but not modifiable by the operating system). These devices are recorded. The plug and play devices are also recorded. If the devices present are different from when the system was last booted, the new devices are configured.

The BIOS then determines the boot device by trying a list of devices stored in the CMOS memory. The user can change this list by entering a BIOS configuration program just after booting. Typically, an attempt is made to boot from the floppy disk. If that fails the CD-ROM is tried. If neither a floppy nor a CD-ROM is present the system is booted from the hard disk. The first sector from the boot device is read into memory and executed. This sector contains a program that normally examines the partition table at the end of the boot sector to determine which partition is active. Then a secondary boot loader is read in from that partition. This loader reads in the operating system from the active partition and starts it.

The operating system then queries the BIOS to get the configuration information. For each device, it checks to see if it has the device driver. If not, it asks the user to insert a floppy disk or CD-ROM containing the driver (supplied by the device’s manufacturer). Once it has all the device drivers, the operating system loads them into the kernel. Then it initializes its tables, creates whatever background processes are needed, and starts up a login program or GUI on each terminal. At least, this is the way it is supposed to work. In real life, plug and play is frequently so unreliable that many people call it plug and pray