🚀 go-pugleaf

tmurphy%peruvian.utah.edu@cs.utah.edu (Thomas Murphy) writes:

>In article <1991Apr5.211351.4335@bnlux1.bnl.gov> kushmer@bnlux1.bnl.gov (christopher kushmerick) writes:
>>My IBM AT has a few Mb of extended ram...
>>Given this, does it necessarily follow that this ram will be recognized
>>by an ems driver, and therefore usefull for such things as ems overlays 
>>in turbo pascal etcetera?

>No it doesn't.  In most AT's you have 1 meg installed of which the
>part above 640 can be used as extended but not ems.  You may get
>around this with a NEAT set computer but I am not sure....memory
>though above the 1 meg barrier can be used as expanded or extended.

When you watch the memory count on an IBM AT with more than 640K installed, 
notice that the count APPEARS contiguous from 0 to maximum.  Logically,
however, it is more accurate to visualize the count stopping at 640K and
then JUMPING to 1024K to continue with the rest (the rest being extended 
memory).  There is no ram, in the conventional sense, in the A000-FFFF 
(640K-1024K) area.  IBM ATs and their conventional clones are very poor
at memory management.  Attempts to use the extra memory in any kind of 
real mode environment are awkward at best.  Using the extended memory
as a ram disk or as a cache, or using an XMS driver to gain limited access
to the ~64K HMA area above FFFF is about the only practical use you will
find.  The EMS emulators for 286 class machines are both slow (do to the
limitations of the 286 CPU when switching into protected mode to access
the extended memory) and memory intensive (the 64K EMS page frame is put
in conventional memory).  They are only useful if no other alternative 
exists.  That is, a particular program absolutely requires EMS to run and
you don't care how slow it is.  If EMS is required, the more practical
alternative is to have a dedicated hardware expanded memory board which
will but its page frame in upper memory above 640K.  This very problem
is why it is usually worth the extra cost to get at least a 386SX based 
machine.  Then, with the correct software (386 class XMS and EMS memory 
managers like QEMM and its cousins) you can pretty much do whatever you 
want with all the extended memory you might have.

--
Jimmy Liberato   liberato@dri.com
                 ...uunet!drivax!liberato