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Types of RAM
The Hard Disk
Virtual computer

DOS

Back in the days when the PC was first being introduced to the public, there was a race to have an operating system to work for the PC as well. Microsoft showed that their operating system was superior beyond all others, there scheme loaded DOS (Disk Operating System) into RAM first with the other vital system resources in second.
This however proved to be not fatal but caused some headaches when larger programs began to appear. With the introduction of the hard disk, programs and operating systems began to get larger and larger. The problem was that there was only 640k of free memory shared between DOS and programs and often you would run out of memory when you were working on a large text file or something that took up more then its fair share.
However this problem was not caused by Microsoft DOS, but was the genius thinking of the PC hardware industry. The 8088 and the 80186 could only address up to 1MB of RAM, and we were running out of RAM, no matter how much we installed.
This was not truly fixed unil the introduction of the 286 which included a new memory model and could address up to 16MB of RAM.

Cries for help

Expanded memory was introduced in the late years of the 8088, which used an expansion board for the extra additional RAM that also required special memory management software. Designed by Lotus, Intel, and Microsoft, this new memory management was archaic, it used an expansion board that used the ISA bus but was very slow and very expensive.

Extended memory

Extended memory was then introduced with the 286 that in its self has 2 other memory models which are knows as Protected Mode and Virtual Mode.

Protected mode was one of the first attempts to allocate Extended memory and could address up to 16 MB of RAM. When running in protected mode, the CPU has to protect program memory segments from other applications that are also using memory segments.

Virtual Mode was introduced with the 386 and allowed DOS programs that required expanded or extended memory to run while each program thinking that they had their own computer. This was the introduction of multitasking, which allows you to run multiple programs at one time and is still used by your computer today.