PC Processors - Computer Busses - page 493

A

PC Processors

A.1 Introduction

Intel marketed the first microprocessor, named the 4004 . This device caused a revolution in
the electronics industry because previous electronic systems had a fixed functionality. With
this processor the functionality could be programmed by software. It could handle four bits
of data at a time (a nibble), contained 2000 transistors, had 46 instructions and allowed 4 KB
of program code and 1 KB of data. The PC has since evolved using Intel microprocessors
(Intel is a contraction of Int egrated El ectronics).
The second generation of Intel microprocessors began in 1974. These could handle 8 bits
(a byte) of data at a time and were named the 8008, 8080 and the 8085. They were much
more powerful than the previous 4-bit devices and were used in many early microcomputers
and applications such as electronic instruments and printers. The 8008 has a 14-bit address
bus and can thus address up to 16 kB of memory (the 8080 has a 16-bit address bus giving it
a 64 kB limit).
The third generation of microprocessors began with the launch of the 16-bit processors.
Intel released the 8086 microprocessor which was mainly an extension to the original 8080
processor and thus retained a degree of software compatibility. IBM's designers realised the
power of the 8086 and used it in the original IBM PC and IBM XT (eXtended Technology).
It has a 16-bit data bus and a 20-bit address bus, and thus has a maximum addressable capac-
ity of 1 MB.
A stripped-down 8-bit external data bus version called the 8088 is also available. This
stripped down processor allowed designers to produce less complex (and cheaper) computer
systems. The 8086 could handle either 8 or 16 bits of data at a time (although in a messy
way). An improved architecture version, called the 80286 , was launched in 1982, and was
used in the IBM AT (Advanced Technology).
In 1985, Intel introduced its first 32-bit microprocessor, the 80386DX. This device was
compatible with the previous 8088/8086/80286 (80X86) processors and gave excellent per-
formance handling 8, 16 or 32 bits at a time. It has full 32-bit data and address buses and can
thus address up to 4 GB of physical memory. A stripped-down 16-bit external data bus and
24-bit address bus version called the 80386SX was released in 1988. This processor can thus
only access up to 16 MB of physical memory.
In 1989, Intel introduced the 80486DX which is basically an improved 80386DX with a
memory cache and math co-processor integrated onto the chip. It had an improved internal
structure making it around 50% faster than a comparable 80386. The 80486SX was also in-
troduced, which is merely a 80486DX with the link to the math co-processor broken. Clock
doubler/ trebler 80486 processors were also released. In these devices the processor runs at a
higher speed than the system clock. Typically, systems with clock doubler processors are
around 75% faster than the comparable non-doubled processors. Typical clock doubler proc-
essors are DX2-66 and DX2-50 which run from 33 MHz and 25 MHz clocks, respectively.
Intel also produced a new range of microprocessors which run at three or four times the sys-