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CEMM, for Compaq Expanded Memory Manager was the first so-called PC "memory manager" for Intel 80386 CPUs, able to transform "XMS" extended memory into "EMS" expanded memory by using the virtual memory features and the virtual 8086 mode of the CPU.

It was present in Compaq DOS 3.31, in 1987.

In 1986, Compaq was the first vendor to ship a PC compatible computer with a 386 CPU, the Deskpro 386, and it was natural for them to develop solutions leveraging the specific features of their new hardware and in this case allowing existing EMS-compatible DOS programs to access all the memory.

Alas, interesting features often come with less interesting drawbacks. Although CEMM was called an "expanded memory manager", it was really a tiny, protected mode, single tasking operating system kernel, also called a virtual machine monitor (VMM). It had to catch all the interrupts and all the exceptions, and to reflect them to the virtual machine where DOS ran. This slowed the machine down a little, increased the latency of interrupts and changed the behavior of DOS programs trying to execute invalid machine instructions. Implementing a VMM was not CEMM's raison d'etre, but EMS emulation required paging, and paging required protected mode. The only way to run existing real-mode DOS and BIOS code was therefore through a VMM.

Because CEMM put the CPU into protected mode, other programs (DOS extenders) could not do this anymore by themselves. A special API, the......
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