Hardware Reference
In-Depth Information
The cost was high because the 8086 needed a 16-bit data bus rather than a less expensive
8-bitbus.Systemsavailableatthattimewere8-bit,andslowsalesofthe8086indicatedto
Intel that people weren't willing to pay for the extra performance of the full 16-bit design.
In response, Intel introduced a kind of crippled version of the 8086, called the 8088, in
June 1979.
The8088processorusedthesameinternalcoreasthe8086,hadthesame16-bitregisters,
and could address the same 1MB of memory, but the external data bus was reduced to
8 bits. This enabled support chips from the older 8-bit 8085 to be used, and far less ex-
pensive boards and systems could be made. However, because it retained the full 16-bit
internal registers andthe20-bitaddressbus,the8088ran16-bitsoftwareandwascapable
of addressing a full 1MB of RAM.
Forthesereasons,IBMselectedthe8-bit8088chip(runningat4.77MHz,taking12cycles
for the average instruction to complete) for the original IBM PC, which was introduced
in August of 1981. That event dramatically changed the fate of both Intel and Microsoft,
which provided Microsoft Disk Operating System (MS-DOS) version 1.0 for the new
computer.
This decision would affect history in several ways. The 8088 was fully software com-
patible with the 8086, so it could run 16-bit software. Also, because the instruction set
was similar to the previous 8085 and 8080, programs written for those older chips could
quickly and easily be modified to run. This enabled a large library of programs to be
quickly released for the IBM PC, thus helping it become a success. The overwhelming
blockbuster success oftheIBMPCleftinitswakethelegacy ofrequiringbackwardcom-
patibility with it. To maintain the momentum, Intel has pretty much been forced to main-
tain backward compatibility with the 8088/8086 in most of the processors it has released
since then. Since the fateful decision was made to use an Intel processor in the first PC,
subsequent PC-compatible systems have used a series of Intel or Intel-compatible pro-
cessors, with each new one capable of running the software of the processor before it.
Years later, IBM was criticized for using the 8-bit 8088 instead of the 16-bit 8086. In ret-
rospect, it was a wise decision. IBM even covered up the physical design in its ads, which
at the time indicated its new PC had a “high-speed 16-bit microprocessor.” IBM could
say that because the 8088 still ran the same powerful 16-bit software the 8086 ran, just a
little more slowly. In fact, programmers universally thought of the 8088 as a 16-bit chip
because there was virtually no way a program could distinguish an 8088 from an 8086.
This enabled IBM todeliver a PC capable ofrunninga new generation of16-bit software,
while retaining a much less expensive 8-bit design for the hardware. Because of this, the
IBM PC was actually priced less at its introduction than the most popular PC of the time,
