ADD-ON MEMORIES MAKE LARGE-SCALE SYSTEMS OUT OF MINIS.

The expansion of a minicomputer-based system almost always requires corresponding expansion of the memory capacity. There are two approaches to providing add-on main memory in minicomputers: the internal plug-in method and the external plug-in method. Both functionally plug into the computer's bus. Add-ons of the internal plug-in variety use memories that are designed to be almost direct copies of the memories supplied by the computer manufacturer. This method offers two advantages. It is a very low cost method of getting additional main memory and it does not require any additional space. Certain disadvantages are associated with the internal type of add-on memory, however. Its use may impinge on the manufacturer's warranty and many of these memories must be removed whenever maintenance work is being done on the computer. Finally, these units draw on the computer's main-frame power and cooling, and as such, may not be completely identical. The other approach to add-on memories, the external plug-in method, allows the additional memory to be cable connected. This type of memory is completely self-sufficient with its own power supply and cooling.