Expanded memory

address  space of the 8088/86 processor. This may sound complicated, but in practice it is something with which every user is familiar in the form of the floppy disk drive and its diskettes. A single floppy drive may be limited to the storage capacity of a single diskette, 720KBytes say, but by inserting different diskettes it has an effectively infinite capacity. In the case of expanded memory it isn’t a diskette that is swapped in and out of a drive but a page of memory in and out a page frame.

In the first version of LIM, a 64KByte area of the address space is allocated as a page frame into which any of a number of 64KByte pages of expanded memory can be inserted. An application program that has been written to take advantage of LIM expanded memory can read and write data into a page and when it is full it can simply switch to another page. You can see that LIM provides a way of breaking the 640KByte barrier without actually removing it! A LIM application can make use of much more than 640KBytes of memory but at any one time it can only access the standard 640KBytes, plus of course the 64KByte page that is currently in the page frame.

LIM expanded memory has been redefined and improved a number of times-see the technical box LIM/EMS standards –but the version described above using a 64KByte page size, LIM 3.2, is ideal for applications that need to store more data than 640KBytes can cope with. There is a more sophisticated version, LIM 4.0, that allows the page size to be as large as the application chooses