80386 and Subsequent Processors
Intel processors from the 386 onward allowed a virtual 8086 mode, which simplified the hardware required to implement expanded memory for MS DOS applications. Expanded memory managers such as Quarterdeck's QEMM product and Microsoft's EMM386 supported the expanded memory standard without requirement for special memory boards.
On 386 and subsequent processors, memory managers like QEMM might move the bulk of the code for a driver or TSR into extended memory and replace it with a small fingerhold that was capable of accessing the extended-memory-resident code. They might analyze memory usage to detect drivers that required more RAM during startup than they did subsequently, and recover and reuse the memory that was no longer needed after startup. They might even remap areas of memory normally used for memory-mapped I/O. Many of these tricks involved assumptions about the functioning of drivers and other components. In effect, memory managers might reverse-engineer and modify other vendors' code on the fly. As might be expected, such tricks did not always work. Therefore, memory managers also incorporated very elaborate systems of configurable options, and provisions for recovery should a selected option render the PC unbootable (a frequent occurrence).
Installing and configuring a memory manager might involve hours of experimentation with options, repeatedly rebooting the machine, and testing the results. But conventional memory was so valuable that PC owners felt that such time was well-spent if the result was to free up 30 KiB or 40 KiB of conventional memory space.
Read more about this topic: DOS Memory Management
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