Upper memory area

As the 640 KB memory restriction became ever more of an obstacle, techniques were found to fill the empty areas with RAM.

[2] DR DOS' built-in memory manager, EMM386.EXE, could perform most of the basic functionality of QEMM and comparable programs.

Starting from version 6.0 of MS-DOS, Microsoft even included a program called MEMMAKER which was used to automatically optimize conventional memory by moving terminate-and-stay-resident (TSR) programs to the upper memory.

For a period in the early 1990s, manual optimization of the DOS memory map became a highly prized skill, allowing for the largest applications to run on even the most complex PC configurations.

The technique was to first create as many UMBs as possible, including remapping allocated but unused blocks of memory, such as the monochrome display area on colour machines.

Then, DOS' many subcomponents had to be loaded into these UMBs in the correct sequence to use the blocks of memory as efficiently as possible.

Exceptions were that to successfully cache CD-ROMs, most disk caches had to be loaded after any CD-ROM drivers, and that the modules of most network stacks had to be loaded in a certain sequence, essentially working progressively up through the layers of the OSI model.

The AllCard, an add-on memory management unit for XT-class computers, allowed normal memory to be mapped into the 0xA0000-EFFFF address range, giving up to 952 KB for DOS programs.