Mail Archives: opendos/2000/11/30/11:13:13.1
X-Comment-To: Patrick Moran
Hi!
29-ξΟΡ-2000 19:19 pmoran22 AT yahoo DOT com (Patrick Moran) wrote to
<opendos AT delorie DOT com>:
>> W.R.T. EMS, I recall reading somewhere that, rather than being
>> "Extremely inefficient", EMS is actually more efficient than XMS
>> (when based on either real hardware or the 386's magic tricks).
>> There were also "EMS simulator" software (286 stuff?), which
>> had to copy data to/from the page frame, hence these were
>> indeed inefficient - perhaps you were thinking of these?
PM> That could be or maybe I am thinking of the way MS crap for DOS 5 did it.
PM> You first had to have Extended memory then that is converted to EMS memory.
Not necessary. For example, QEMM (unlike EMM386) gives access to all
extended memory through both EMS and XMS API simultaneously.
PM> However, no matter how it is done, you have a tiny little 64K window of
PM> memory to shove stuff through from expanded memory to conventional memory
PM> and conventional memory to expanded memory. When you use something like
Not necessary. You can work with data directly in EMS pages (four 16K
pages in 64K EMS frame).
PM> Desqview and you have a large program over 500k in size and have to swap
PM> portions of it out 64k at a time in 16k chunks.
Even in this case using EMS can be faster than XMS: for EMS you use
plain "MOV CX,8000h/REP MOVSW" eight times+eight times EMS pages remapping,
with XMS you use block copying with (possible) much higher overhead for
copy.
PM> XMS. When a program is run in protected mode, there is no swapping, it
PM> operates in exntended memory as originally designed to do. In virtual 86
Hm. Don't mix XMS and protected mode operation.
PM> mode you can actually have a full 8086 computers installed many times and
PM> each can share entended memory. This is what the DRDOS Task manger does. It
PM> does not use EMS like Desqview and other so-called multitaskers do via EMS
PM> memory.
Again: don't mix memory manager itself and XMS/EMS specifications.
PM> I am not positive what XMS memory is, it is not talked about in "DOS Beyond
PM> 640K" book. I do remember something about QEMM converting Extended memory to
PM> EMS then converting EMS memory back for use as Extended memory, which is
PM> really, really stupid. Why not just use Extended memory directly like DPMS
PM> does?????
? You something muddle. QEMM doesn't "convert" anything. They simply
gives access to extended memory through EMS and/or through XMS API.
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