Another system extending program that comes from Microsoft is the HIMEM.SYS program. Back when DOS was put together, it was designed for the Intel 808x chips.
These blazing CPUs sported a whopping 100,000 instructions per second of computational ability. Now the engineers had to draw a line in the silicon somewhere for how much memory this chip family could address. They went for the incredible number of 1,000,000 bytes of RAM. Some of this went to read the BIOS, and some to the various types of video cards, i.e. black and white or color (with graphics!).
Since the reigning operating system of the day was C/PM and it used 64k in total, granting a limit of 640k seemed like a great deal of room for growth. Even Bill Gates was impressed, proclaiming, Nobody could use more than 640k of RAM.
Alas, nature hates a void. The limit was hit, and users screamed for more. Without getting into the HI=MEM wars of the day, a solution was found by swapping memory up above the real mode region, into the High Memory Area (HMA) for management and use. This is controlled by HIMEM.SYS.
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