Old MS-DOS & WIN Software

Chuck Guzis cclist at sydex.com
Wed Dec 7 12:25:48 CST 2005

On 12/7/2005 at 10:11 AM Gil Carrick wrote:

>On Pentium systems, at least the current ones, a larger "page" is used to
>map most of the kernel memory into every logical address space with a
>entry. IIRC the default mapping is 4 MB. 

True, but that doesn't address where the video ROM image in NTVDM comes
from.  I believe it's merely a mapped in RAM copy of the ROM.  To do
otherwise would likely involve taking a speed hit whenever a routine inside
a 16-bit NTVDM session accessed the video ROM space, which seems unlikely.
Still, it'd be easy enough to check--just put a probe on the CS pin of the
video rom and see what happens when a 16-bit app accesses the memory area.

BTW, this little topic is a bone of contention for some.  There's a group
who insist that enabling the BIOS option "Cache Video ROM" is necessary for
maximum performance under Win 2K and XP.  Perhaps during early startup, but
once the generic VGA driver's loaded that's all water under the bridge--and
caching is wasted physical memory.

What's really water under the bridge is 16-bit mode in Windows.  Vista
doesn't support it, period.   Maybe someone will write a 16-bit emulator
for Vista and we'll get to keep our old apps in some form.

I can understand MS wanting to deprecate a lot of the PC legacy features.
NT startup has to be one of the more complex computing tasks around and I
suspect that the folks in Redmond have envied Apple for a long time in not
having to accomodate all sorts of bizarre departures from the norm in
hardware.  All of which has me wondering how much longer things like
8237-type-DMA will be part of the PC platform. 


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