PC Microprocessor Developments and Features
PC microprocessor design grows more complex with each
generation, and CPU packaging keeps changing to provide room for
additional features and operating requirements. Microprocessors
have evolved from the 4004 described earlier into today's
high-speed Pentiums. Each new processor has brought higher
performance and spawned new technology. Six basic elements are
customarily used to gauge the performance and capability of a CPU
- Speed: The maximum number of clock cycles measured in
megahertz. The higher the speed, the quicker a command will be
- Number of transistors: More switches, more computing
- Registers: The size (in bits) of the internal registers. The
larger the registers, the more complicated the commands that can be
processed in one step.
- External data bus: As data bus size increases, so does the
amount and complexity of code (information) that can be transferred
between all devices in the computer.
- Address bus: The size of the address bus determines the maximum
amount of memory that can be addressed by the CPU.
- Internal cache: The internal cache is high-speed memory built
into the processor. This is a place to store frequently used data
instead of sending it to slower devices (speed is relative in
computers) such as RAM and hard disk drives. It is built into the
processor and has a dramatic effect on speed. We cover cache in
more detail later in this lesson.
Intel has held most of the PC CPU market share since the
original IBM PC was introduced. Closely following each new Intel
launch, rivals such as Advanced Microdevices (AMD) and Cyrix have
offered alternative chips that are generally compatible with the
Intel models. This development, in turn, drives prices down and
spurs a new round of CPU design. Another player is Motorola, a firm
that manufactures the microprocessors used in the Apple family of
computers, among others.
Intel's 8086 and 8088: The Birth of the PC
We have already introduced the "pre-PC" CPUs. Now we take a look
at the models that have powered one of the most dramatic
developments of the modern world: the inexpensive, general-purpose
On June 6, 1978, Intel introduced its first 16-bit
microprocessor, known as the 8086. It had 29,000 transistors,
16-bit registers, a 16-bit external data bus, and a 20-bit address
bus to allow it to access 1 MB of memory. When IBM entered the
computer business, the 8086 was too powerful (and expensive) to
meet its requirements.
Intel then released the 8088 processor, which was identical to
the 8086 except for an 8-bit external data bus, and a slower top
clock rate. This meant that 8-bit components (more common at the
time) could be used for the construction of PCs, and 8-bit
applications written for earlier machines could be converted for PC
use. The following table compares the 8088 and 8086 chips.
||Number of Transistors
||CPU Speed (MHz)
||External Data Bus
The early 8088 processors ran at 4.77 MHz, while later versions
ran at 8 MHz. The 8086 and 8088 processors came as a 40-pin DIP
(dual inline package) containing approximately 29,000 transistors.
The DIP is so named because of the two rows of pins on either side
of the processor, as shown in Figure 4.8. These fit into a set of
slots on a raised socket on the motherboard. The small u-shaped
notch at one end of a DIP-style CPU denotes the end that has pin 1.
During installation, you well need to be sure to line it up
correctly, or you might have to repeat the process.
Figure 4.8 DIPP (Dual Inline Package Processor) used
for 8086, 8088, and 80286 CPUs
The 8088 and 8086 are software-compatible-they can run exactly the
same programs (assuming the PCs that use them don't have other
complicating factors). The benefit of using an 8086 is its 16-bit
external data bus. This allows an 8086-based computer to execute
the same software faster than an 8088 computer with the same clock
The early IBM personal computers based on the 8086 and 8088
- 16 KB of memory.
- Cassette tape recorder or floppy disk drive for program and
- Nongraphics monochrome monitor and monochrome display adapter
Soon, a new industry was born as third-party vendors started
manufacturing add-ons and improved models of the basic design.
Graphics cards with color and better resolution, clocks, additional
memory, and peripherals, such as printers, extended the features of
the new appliance. "Clones" offered some of these extras at very
competitive prices, as a way to attract buyers who wanted a lower
price and did not need the comfort of purchasing from a big name
A clone is a computer that contains the same microprocessor and
runs the same programs as a better-known, more prestigious, and
often more expensive machine.
Most of the 8088 and 8086-based PCs used some variation of
MS-DOS. The variations limited the growth of the software market
because of the compatibility issues they presented between versions
of MS-DOS. Buyers had to be sure that a program would run on their
specific version of MS-DOS.
As users found more ways to take advantage of the PC's power,
developers and owners alike soon felt the limitations of the
original IBM PC design. The engineers who created it never
envisioned the need for more than 16 K of RAM. "Who would ever need
more than that?" one is quoted as saying. The cassette drive was
never a big seller; most buyers opted for one or two 5.25-inch
floppy disk drives, and many soon craved color graphics and the
space of the "massive" 5- and 10-MB hard disk drives.
To meet that growing demand, IBM introduced a more robust PC,
the XT (eXtended Technology), that could take advantage of a hard
disk drive and came with either a monochrome or four-color display
and more RAM. Clone makers soon followed suit.
The 80286 and the IBM PC AT
In February, 1982, Intel introduced the 80286 6-MHz
microprocessor (later pushing the clock speeds to 10 and 12.5 MHz),
commonly called the 286, with a 24-bit address path. In 1983, IBM
unveiled its PC AT (Advanced Technology) computer, based on the
286. It had a larger, boxier design, came with a standard hard
drive, and a new expansion slot format, rendering older add-on
The AT could run the same applications as the PC XT (8088), but
run them faster. The use of a 24-bit address path allowed the 286
to access up to 16 MB of memory. The clone-makers soon followed
suit, taking advantage of third-party versions of the 286. Chip
makers Harris and AMD produced versions of the 286 that could run
at up to 20 MHz.
Computers based on the 80286 chip featured:
- Two memory modes (real and protected).
- 16 MB of addressable memory.
- Clock speeds up to 20 MHz.
- Reduced command set (fewer program commands to do more
- Multitasking abilities.
- Virtual memory support.