🚀 go-pugleaf

In article <1991May30.213856.1@freke.claremont.edu> dhosek@freke.claremont.edu (Don Hosek) writes:

   What is it about the first 64K of extended mem that makes it so
   special to DOS 5.0, DESQview, et alia? It seems that there should
   be no more trouble accessing any other 64K block of extended mem
   than the UMB, but I'm acknowledgedly ignorant of such things...

It's a consequence of Intel's segment/offset address calculation.  You see,
if you add a segment of 0ffffh and an offset of 10h, you get a linear address
of 100000h.  That's more than one meg.  In fact, it's the first byte of the
UMB.  Now, the 8088 didn't keep that bit, and so some programs might make use
of that wrap-around.  To be safe, IBM's AT design has a provision for forcing
address bit A20 to a zero.

Basically, accessing the HMA is just a matter of setting up the right segment,
and changing the hardware so that it respects A20.  That's what HIMEM.SYS
does.

--
--russ <nelson@clutx.clarkson.edu> I'm proud to be a humble Quaker.
Clear cutting is criminal, spiking trees is criminal, and using hyperbole of
this magnitude in a serious discussion is criminal.  -- Irv Chidsey