Chapter 3: Microprocessors^57
The Evolution of the PC Microprocessor
Other manufacturers, such as AMD, VIA Cyrix, and others, make excellent processors,
but Intel is by far the leading manufacturer of PC microprocessors. Intel has consistently
set the standard by which all processors are measured.Intel 8086 and 8088
In 1978, Intel introduced the 8086 microprocessor, which had a clock speed of 4.77MHz
(megahertz). Theclock speedof a processor is the speed at which the CPU operates. Clock
speeds are rated in megahertz or millions of electronic cycles per second. A computer
rated at 5MHz has five million processing cycles per second. The more cycles per second
a computer supports, the more instructions it can execute. Remember that most instruc-
tions, because they include many processing steps, including memory transfers, ALU op-
erations, etc., require more than a single CPU cycle to complete. The 8086 was capable of
running about 0.33 MIPS (million of instructions per second). MIPS is a standard used to
measure the processing power of a processor. The 8086 processor, which included 29,000
transistors,wasa16-bitprocessor(itsdatabuswas16-itswide)andhadanaddressbusof
20-bits. It could address 1MB (megabyte) of memory, which was an incredible amount at
the time. The 8086 was not a popular choice for PCs, but it did create a baseline for all
future Intel 80x86 processors.
A year later, Intel reduced the data bus of the 8086 to eight bits and released the 8088
that still had a 20-bit address bus. In all other respects, the 8088 was a clone of the 8086,
including the number of transistors and its clock speed. IBM chose to use the 8088 for its
first personal computer, the IBM PC-XT.
As shown in Figure 3-11, the 8088 (and 8086) were packaged in a 40-pin dual inline
package (DIP) integrated circuit. Figure 3-11 also shows how each of the pins was desig-
nated to a particular value or function. This is common on microprocessors and inte-
grated circuits.
A second version of the 8088 was released later that added the Turbo feature and allowed
the processor to run at two clock rates, the regular 4.77MHz and a new Turbo at 8MHz.Intel 80286
Computer manufacturers largely skipped over the next Intel microprocessor generation,
the 80186, to adopt the Intel 80286 (see Figure 3-12). IBM chose the 80286 for its next PC
release, the PC AT. The 286 processor, as it was commonly known, was released in early
1982 with a 6MHz clock speed, 134,000 transistors, and nearly three times the power of
the8086with0.9MIPS.The286wasabletoaddress16MBofmemorywithitsaddressbus
expanded to 24 bits. Its data bus was also backward compatible to the 8086’s original 16 bits.
Later 286 versions had clock speeds of 10MHz and 12MHz.