From Chips to Systems ( lesson 1.5 )

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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