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System Types
PCs can be broken down into many categories. I like to break them down in two ways—by 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.
→ See "Data I/O Bus," p. 48.
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 processors and their data bus widths.
Table 2.2 Processors and Their Data Bus Widths
|
Processor |
Data Bus Width |
|
8088 |
8-bit |
|
8086 |
16-bit |
|
286 |
16-bit |
|
386SX |
16-bit |
|
386DX |
32-bit |
|
486/AMD-5x86 |
32-bit |
|
Pentium/AMD-K6 |
64-bit |
|
Pentium Pro/Celeron/II/III |
64-bit |
|
AMD Duron/Athlon |
64-bit |
|
Pentium 4 |
64-bit |
|
Itanium |
64-bit |
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 identified—they have 16-bit or greater slots of any type. These can be ISA, EISA, MCA, PC-Card (formerly PCMCIA), Cardbus, VL-Bus, or PCI. 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.