From Chips to Systems
The 386/486 is a powerful devise but tapping its power when
making the transition into complete system poses a number of problem. when the
286 has introduced IBM modified. the existing PC architecture to produce
the At. The modifications were fairly
small and mostly upwards commutable with the original PC design. in other words
, apart from few well understood
expectations what would work with a PC would with an AT-hardware and software
alike. The at quickly became a de factor Stranded to witch other personal
computer manufactures conformed.
with the introduction of the 386 similar modification to the
AT architecture seemed to be called
for-after all if there was a change in going from 16 to 32 bits .IBM took this
apparent need for changed as a way of
trying to gain control of the PC market. Instate of basing their new design on
the existing AT the chose to produce something quite new and different - the PC/2.
Wail the PS/2 range is closely related to the at design in
that Nelly all software that will run on an At will run on a PS/2 it is
incomparable at the hard wear level. the PS/2 range uses a MCA (MICRO CHANEL
ARCHITECTURE) to connect additional hardware instead of the ISA (INDUSTRY
STANDARD ARCHIVE in the AT .MCA has lot of performance advantages but IBM chose
to make it a proprietary technology and so most
other manufactures were deterred from using it.
the alterative
approach to the PS/2 was to simply use a
3869 in place of a 286 in a traditional AT design , modified as little as
possible. Initially the problem was that different manufacturers made the
necessary modifications in slightly different ways. In the main this didn't
matter to much and only affected the way that additional memory was added. As
memory technology progressed even this difficultly tended to vanish and a 386
design that is from the users point of view, identical to an AT evolved.
This at / 386 design has all the advantages of being
hardware commutable with the original AT but has the disadvantage of not begins
able to deal with high speed add-on
cards. in other words ISA is not as fast or as flexible as MCA. Whether this is
a real problem or not depend very much on the application your interested in.
there is an argument that ISA is a used
to its full capacity in most system anyway! Even so this Christmas resulted in
a group of manufacturers defining a new improved version of ISA-EISA (EXTENDED
INDUSTRY STANDARD ARCHITECTURE).EISA offers many of the advantages of MCA but
it is upward comparable with ISA and so you can use existing AT style hardware with it.
Much of the fuss concerning which type mo machine to buy is
irrelevant. The key fact to remember is that the effect of ISA, MCA or EISA
depend on what add-on you require and in ,many cases it isn't critical see
chapter 3.For example , if you are plugging in a card to provide an additional
printer port then from a performance
point of view it doesn't matter which bus your are using because they are all
fast enough to cope with the date rate of printer. On the other hand if you
want to contact a high speed disk drive then you might need MCA or EISA to
achieve the performance you desire.
The situation isn't quite as clear cut as if you need high
speed disk you need MCA or EISA'.In
practice the speed disk using ISA can be improved by selecting IDE , SCSI or
ESDI drives (Chapter 6) and this might be all that is necessary for the
application. Another confusing factor is that one of the main task for which MCA was designed that connecting additional
memory that will work at the full speed of the processor has largely become
redundant. part of the reason has been
the introduction of spacial add-on brad with high speed connection to the processor just for
memory extinction but the main reason is
that the amount of memory that can be
fitted to the main system bored has increased to the point where memory extensions seam an unnecessary concept!
The pros and cons of the different architecture can be summaries
as.
ISA 386-AT - low cost, lots of exiting add-on cards but
limited in its maximum performance.
EISA 386 AT- Higher cost not many add-on cards as yet but
upward commutable with ISA, higher potential preference, not backed bu IBM.
MCA PS/2 - Higher
cost higher potential performance backed by IBM.
As thing have turned out the dominate architecture has
proved to be the ISA -386 The raisin is that for most application this provides
more than enough power and flexibility. in some
specialist striation the EISA 386-AT is an advantaged and this are
discussed latter in this book. the MCA PS/2 has proved to be of intrust only to
dedicated IBM coustemers It is to early to say that the MCA PS/2 detained is
unless there the reticule and unforeseen changed in the PC market the MCA bus a
and the PS/2 have to be seen as another
failed attempts at forcing a revolution on the PC using community .This isn't
to say that the EISA 386AT can be hailed as the victory of evolution over
revolution. As early day stated the plain facts is that the majority of 386-AT
designs use and need nothing more sophisticated than the ISA bus. all of these
topics are taken up in more detail in the next chapter.
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