Early 386
machines depended on the use of expansion boards to hold the majority of their
RAM. This was mainly due to the low capacity of memory chips available at the
time and hence the large number needed to produce any reasonable sized memory.
The only problem with expansion boards in the type of connector/bus used to plug
them intc the mainboard. For the 386/486
the familiar ISA bus used on the 286/AT
is grossly inadequate for memory
expansion, being limited to 16
bits. In the case of the 16-bit 386SX, however, memory expansion via the ISA
bus is the possibility. To overcome the
shortcoming of the ISA bus most manufactures
have opted for proprietary 32-bit bus connectors. In the case of the
386SX there have even been proprietary 16 bit bus connectors. These are fast enough to support memory expansion but they have the disadvantage of not being standardized. If your machine
uses a 32-bit proprietary memory
expansion bus then you have to buy plug-in cards that are made specially for your machine and usually by the same
manufacturer. This single source
situation is only a problem in that is restricts your shopping around, keeping price high, and you run a small risk of not being
able to obtain expansion cards if buy any chance the
manufacture should go out of business.
However, some machines , notably those made by
IBM and Compaq , have sold in sufficiently large numbers for third
party manufacturers to offer
lower cost alternatives.
The only way
of providing standard 32-bit memory expansion cards is to use either the 32-bit MCA or EISA bus. Both these
standards have been extensively
described in chapter 3. Briefly , they are both can be out –performed in terms of
simplicity, cost and speed by
memory installed directly on the main-board .
Indeed, this
last comment could be applied to most memory expansion adapters. That most
economical and efficient form of memory
is that installed on the main board of your machine, and the upper limit on the amount of memory
installable without expansion adapters
is and important consideration in choosing a machine. If
your machine does need memory expansion
adapters then you can generally buy them fully populated. i.e complete with chips, or with 0
Kbytes installed i.e without chips. If you want a trouble-free life then buy expansion
boards fully populated . but if you want
to save money then buy boards and chips
separately. The process of installing them on the mainboard. In many cases the
cost of a memory expansion card is close
to that of a replacement mainboard, complete with the amount of memory you require, and upgrading in this way is often a sensible alternative.
Finally,
don’t fail into the trap of confusing expanded memory, EMS or LIM memory
boards with the sort of memory boards.
Used to extent the memory in 386/486 based systems. Expanded memory is
described in detail in chapter 10 but
briefly 386/486 systems use extended memory as
opposed to expanded memory hardware. Any expanded memory that
may be required can be produced by using software that converts extended memory to expanded
memory. If this important topic
is worrying you then turn to chapter 10.
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