The following is extracted from:
http://www.pcguide.com/ref/ram/logicHMA-c.html
this is the best explanation for this question I found so far.
PC memory addresses are referred to using a segment:offset addressing scheme; the segment is multiplied by 16 (shifted one hexadecimal digit to the left) and then the offset is added to it to get the real address. The original IBM PC had only 20 address lines and so its highest memory address was FFFFFh. However, the weird segment:offset addressing makes it possible to generate a linear address that is higher than this number.
Take the address FFFF:FFFF. To convert this to a linear address you take the segment and multiply it by 16, to get FFFF0. Then you add the offset, FFFF. FFFF0+FFFF in hexadecimal results in 10FFEFh. There's a problem with this: that "1" at the front requires a 21st address line to represent it, and that doesn't exist on the 8088 or 8086 processors used in the first PCs. They deal with this problem by ignoring the "1"--they treat the address simply as 0FFEFh. Software must be able to handle this "wrap around" of the memory addresses.
When Intel created the 80286 processor, it supported both protected mode and real mode. When in real mode the 80286 was supposed to behave exactly the same as an 8088 or 8086, for compatibility. However, the 80286 does have a 21st address line (it has 24), and due to a bug in its design it didn't do the wrap around in the same way as the 8088 when in real mode. When it used address FFFF:FFFF and came up with 10FFEFh for a linear address, it kept it as 10FFEFh instead of wrapping it around to 0FFEFh like on the older CPUs. This allowed the first FFEFh of extended memory (100000-10FFEFh) to be accessed by the chip even while still in real mode. This block of memory is the high memory area (HMA).
There was still the problem of ensuring compatibility of the 80286 when in real mode. IBM solved this in the original AT by using spare lines in the keyboard controller chip to manage the 21st address line (which is called the A20 line because address bits are numbered starting with zero). The keyboard controller turns off the A20 line when the processor is running in real mode, to allow for full compatibility with the older PCs. It turns it back on when running in protected mode.
Many years later, when the 640 KB limit of conventional memory began to be quite cramping, the ability to access an additional 64 KB of memory in real mode was seen as a significant advantage. (People at this time were scratching and clawing to get even 8 KB more conventional memory to let them run large programs that insisted on certain minimums). Microsoft developed a special driver called HIMEM.SYS that allowed the A20 line to be manipulated under software control. This allows the high memory area to be put to good use.
In practical terms, the high memory area is normally used by DOS itself. Specifying "DOS=HIGH" in the DOS system file CONFIG.SYS tells DOS to load a portion of its own code into the high memory area instead of into conventional memory. This frees approximately 45 KB of conventional memory for use by programs.
The final step in institutionalizing this former bug as an official PC feature was removing manipulation of the A20 line from the keyboard controller. Since that was originally a hack anyway--the controller was used because there was no better way to do it, after all, it has nothing to do with the keyboard--in many newer PCs there is a BIOS option to allow the chipset to control the A20 line directly. This provides a small performance increase compared to letting the keyboard controller manage the line.
