Mainboard or expansion board ?


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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