Ever run MEMMAKER or edit CONFIG.SYS? Here's a story... One of my very first projects at Microsoft was for MS-DOS 6.2, and it involved moving the Doublespace compression engine and parts of SmartDrv into the HMA, or High Memory Area. Then you could run MEMMAKER, and it would find up to 25K more than you had before. 25K was a big deal in that place and time! The only problem was that I was new to x86 and had no idea what the HMA even was. It turns out that on an 80286, when you get to the top of the 1M area, what happens if you point "past" it by combining your segment and offset registers to a value greater than 1M? In other words, does your pointer wrap at 1M or go up into high memory that might be installed? On an 8088, all it can do is wrap. But on an 80286, it COULD be resolved into the next higher pages of memory. But which behavior should the system do? To solve this, IBM used the A20 line on the PC, which is actually a keyboard line. If set, your pointers wrapped, and if not, they went into high memory. This gave you the ability to reach into "memory above 1M" even on a 1M system, which is kinda crazy, but that's how it worked. So I put all the code I could stuff into the 64K right above the 1M boundary, then stubbed everything with jump tables in low memory. I forget why the net was only 25K and not 64K - but there were probably other HMA consumers as well, I imagine. Anyway, that's how I spent the first month of the summer of 1993! And there was much rejoicing.