Specifications

Computer Architecture and Maintenance (G-Scheme-2014)
bit instructions using 16-bit internal registers and could address only 1 MB of
memory using 20 address lines. All original PC software was created to work with this
chip and was designed around the 16-bit instruction set and 1 MB memory model. For
example, DOS and all DOS software, Windows 1.x through 3.x, and all Windows 1.x
through 3.x applications are written using 16-bit instructions. These 16-bit OSs and
applications are designed to run on an original 8088 processor.
Later processors such as the 286 could run the same 16-bit instructions as the
original 8088, but much faster. In other words, the 286 was fully compatible with the
original 8088 and could run all 16-bit software just the same as an 8088, but, of course,
that software would run faster. The 16-bit instruction mode of the 8088 and 286
processors has become known as real mode. All software running in real mode must use
only 16-bit instructions and live within the 20-bit (1 MB) memory architecture it
supports. Software of this type is usually single-tasking—that is, only one program can
run at a time. No built-in protection exists to keep one program from overwriting
another program or even the OS in memory. Therefore, if more than one program is
running, one of them could bring the entire system to a crashing halt.
IA-32 (32-Bit) : Protected Mode
Intel 386 was the PC industry’s first 32-bit processor. This chip could run an
entirely new 32-bit instruction set. To take full advantage of the 32-bit instruction set, a
32-bit OS and a 32-bit application were required. This new 32-bit mode was referred to
as protected mode, which alludes to the fact that software programs running in that
mode are protected from overwriting one another in memory. Such protection makes
the system much more crash-proof because an errant program can’t easily damage
other programs or the OS. In addition, a crashed program can be terminated while the
rest of the system continues to run unaffected.
Knowing that new OSs and applications—which take advantage of the 32-bit
protected mode—would take some time to develop, Intel wisely built a backward-
compatible real mode into the 386. That enabled it to run unmodified 16-bit OSs and
applications. It ran them quite well—much more quickly than any previous chip. For
most people, that was enough. They did not necessarily want new 32-bit software; they
just wanted their existing 16-bit software to run more quickly. Unfortunately, that
meant the chip was never running in the 32-bit protected mode, and all the features of
that capability were being ignored.
When a 386 or later processor is running DOS (real mode), it acts like a “Turbo
8088,” which means the processor has the advantage of speed in running any 16-bit
Prepared By – Prof. Manoj.kavedia (9860174297 – 9324258878 ) (www.kavediasir.yolasite.com)
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