Upgrading and Repairing PCs (17th Edition)
| PCs can be broken down into many categories. I like to break them down in two waysby the type of software they can run and by the motherboard host bus, or processor bus design and width. Because this book concentrates mainly on hardware, let's look at that first. When a processor reads data, the data moves into the processor via the processor's external data bus connection. The processor's data bus is directly connected to the processor host bus on the motherboard. The processor data bus or host bus is also sometimes referred to as the local bus because it is local to the processor that is connected directly to it. Any other devices connected to the host bus essentially appear as if they are directly connected to the processor as well. If the processor has a 32-bit data bus, the motherboard must be wired to have a 32-bit processor host bus. This means the system can move 32 bits of data into or out of the processor in a single cycle.
Different processors have different data bus widths, and the motherboards designed to accept them require a processor host bus with a matching width. Table 2.2 lists all the Intel and major Intel-compatible processors, their data bus widths, and their internal register sizes.
A common misconception arises in discussions of processor widths. Although the Pentium and newer processors all have 64-bit data bus widths, most include internal registers that are only 32 bits wide, and they process 32-bit commands and instructions. The Intel Itanium, AMD Athlon 64, and Pentium 4 with EM64T are the first Intel-compatible processors to have 64-bit internal registers. Thus, from a software point of view, all chips from the 386 to the Athlon/Duron and Celeron/Pentium 4 have 32-bit registers and execute 32-bit instructions. From the electronic or physical perspective, these 32-bit, software-capable processors have been available in physical forms with 16-bit (386SX), 32-bit (386DX and 486), and 64-bit (Pentium and beyond) data bus widths. The data bus width is the major factor in motherboard and memory system design because it dictates how many bits move in and out of the chip in one cycle.
The Itanium processor has an Intel architecture 64-bit (IA-64) instruction set, but it can also process the same 32-bit instructions as processors ranging from the 386 through the Pentium 4. The Athlon 64 has an x86-compatible 64-bit architecture but is designed to use 32-bit instructions written for normal Intel or compatible x86 processors as efficiently as a normal Athlon XP or comparable processor would.
Referring to Table 2.2, you can see that all Pentium and newer systems have a 64-bit processor bus. Pentium processors, whether they are the original Pentium, Pentium MMX, Pentium Pro, or even the Pentium II/III or 4, all have 64-bit data buses, as do comparable processors from AMD (K6 family, Athlon, Duron, Athlon XP, and Athlon 64). As you can see from Table 2.2, systems can be broken down into the following hardware categories:
What is interesting is that besides the bus width, the 16-through 64-bit systems are remarkably similar in basic design and architecture. The older 8-bit systems are very different, however. This gives us two basic system types, or classes, of hardware:
In this verbiage, PC stands for personal computer; XT stands for an extended PC; and AT stands for an advanced-technology PC. The terms PC, XT, and AT, as they are used here, are taken from the original IBM systems of those names. The XT was a PC system that included a hard disk for storage in addition to the floppy drives found in the basic PC system. These systems had an 8-bit 8088 processor and an 8-bit Industry Standard Architecture (ISA) bus for system expansion. The bus is the name given to expansion slots in which additional plug-in circuit boards can be installed. The 8-bit designation comes from the fact that the ISA bus found in the PC/XT class systems can send and receive only 8 bits of data in a single cycle. The data in an 8-bit bus is sent along eight wires simultaneously, in parallel.
16-bit and greater systems are said to be AT-class, which indicates that they follow certain standards and that they follow the basic design first set forth in the original IBM AT system. AT is the designation IBM applied to systems that first included more advanced 16-bit (and later, 32- and 64-bit) processors and expansion slots. AT-class systems must have a processor that is compatible with Intel 286 or higher processors (including the 386, 486, Pentium, Pentium Pro, Pentium II, Pentium III, Pentium 4, and Pentium M processors), and they must have a 16-bit or greater system bus. The system bus architecture is central to the AT system design, along with the basic memory architecture, interrupt request (IRQ), direct memory access (DMA), and I/O port address design. All AT-class systems are similar in the way these resources are allocated and how they function. The first AT-class systems had a 16-bit version of the ISA bus, which is an extension of the original 8-bit ISA bus found in the PC/XT-class systems. Eventually, several expansion slot or bus designs were developed for AT-class systems, including the following:
A system with any of these types of expansion slots is by definition an AT-class system, regardless of the actual Intel or Intel-compatible processor that is used. AT-type systems with 386 or higher processors have special capabilities not found in the first generation of 286-based ATs. These distinct capabilities are in the areas of memory addressing, memory management, and possible 32- or 64-bit wide access to data. Most systems with 386DX or higher chips also have 32-bit bus architectures to take full advantage of the 32-bit data transfer capabilities of the processor. For a time, PC systems continued to incorporate a 16-bit ISA slot for backward-compatibility and lower-function adapters. However, virtually all motherboards eventually eliminated ISA slots in favor of PCI slots along with an AGP slot for high-performance graphics. However, just as ISA support in motherboards has faded from the landscape, so too will PCI and AGP in favor of the PCI Express standard. Chapter 4, "Motherboards and Buses," contains in-depth information on these and other PC system buses, including technical information such as pinouts, performance specifications, and bus operation and theory. Table 2.3 summarizes the primary differences between the older 8-bit (PC/XT) systems and modern AT systems. This information distinguishes between these systems and includes all IBM and compatible models.
The easiest way to identify a PC/XT (8-bit) system is by the 8-bit ISA expansion slots. No matter which processor or other features the system has, if all the slots are 8-bit ISA, the system is a PC/XT. AT (16-bit plus) systems can be similarly identifiedthey have 16-bit or greater slots of any type. These can be ISA, EISA, MCA, PC Card (formerly PCMCIA), CardBus, VL-Bus, or PCI. Any system using the new high-speed serial buses such as PCI Express or ExpressCard also qualifies as an AT-class system. Using this information, you can properly categorize virtually any system as a PC/XT type or an AT type. No PC/XT type (8-bit) systems have been manufactured for many years. Unless you are in a computer museum, virtually every system you encounter today is based on the AT-type design. |
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