The Legacy Of Upward Compatibility
The Evaluation Of The PC has certainly made the transition
from the under powered mashies of the 80s ton the desktop mainframes of the 90s
but it has its disadvantages as well. The truth of the matter is that the
386/486 provides facilities that take into the realm of the work station but
for most of the time it is begins treated as faster version of the original
8088!In the other words , most of its power is simply igorned. This is Legacy
of upward compatibility. Each new
generation of processor has add features and facilities but not at the expense
of compatibility with earlier processor.
The first improvement on the 8088 can be considered to be the 8086 even though they were crate in the sane periods of time.
the 8086 is a 16 bit version of the 8-bit 8088 and , from a software writer's
points of view, it is identical but faster .As a result program that run of the
8088 will, without modification, run on the 8086 only faster. what is more
there are mor modifications that can be made that will make a program designed
for the 8088 run any faster on an 8086. Another way of looking at this is that
programe written for 8086.In of looking at this is that a program written for
the 8086 processors could be moved back down to the less powerful 8088 without
making any changes. In this sense the 8086 is also downward, or backward ,
compatibility.
In the Case of 80286,the next prossecer that Intel produced,
the story is very different. Although the 80286 is a 16-bit prosier like the
8086,it contains many advanced modes of apparition not sherd by the 8086.You
can Take an 8086 program and run without modification on the 8028 but the
program will not make any use of the 80286's new futures .As you might gussied
and programed that is written for the 80286that does make use of its special
futures cannot be moved down to and 8086 processors without extensive changes. In
other words , the 80286 upward but not dawn wards computable with the 8088 and
8086.The trouble with this situation is that very few software producer took
the tuber or risk pf producing software specially 80286 prospector and so much
of the power of this devise has remained well and truly hidden.
The same situation holds with the 80386/486.it is upward
compatible with the original 8088 and 8086 processors and that is how much
current software treats it is as a fast 8088/86.The only difference between this
and the 80286 is that now hidden power is very great indeed. If you buy small motor boat and insist on getting about by peddling it
from one place to another then people will think that your are eccentric , but
if you buy a high speed jet foil and treat it in the same way people we deiced
that your are made .the same is true of the current states of the computing.
while it might have been fine to paddle the 80286, the 80386/486 deserves to be
use fully. in latter chapters the topic of how the extra power can be released
is discussed in detail.
EmoticonEmoticon