DOS Memory Management - Expanded Memory

Expanded Memory

As memory prices declined, application programs such as spreadsheets and computer-aided drafting were changed to take advantage of more and more physical memory in the system. Virtual memory in the 8088 and 8086 was not supported by the processor hardware, and disk technology of the time would make it too slow and cumbersome to be practical. Expanded memory was a system that allowed application programs to access more RAM than directly visible to the processor's address space. The process was a form of bank switching. When extra RAM was needed, driver software would temporarily make a piece of expanded memory accessible to the processor; when the data in that piece of memory was updated, another part could be swapped into the processor's address space. For the IBM PC and IBM PC/XT, with only 20 address lines, special-purpose expanded memory cards were made containing perhaps a megabyte, or more, of expanded memory, with logic on the board to make that memory accessible to the processor in defined parts of the 8088 address space.

Allocation and use of expanded memory was not transparent to application programs. The application had to keep track of which bank of expanded memory contained a particular piece of data, and when access to that data was required, the application had to request (through a driver program) the expanded memory board to map that part of memory into the processor's address space. Although applications could use expanded memory with relative freedom, many other software components such as drivers and TSRs were still normally constrained to reside within the 640K "conventional memory" area, which soon became a critically scarce resource.