🚀 go-pugleaf

robert@alkymi.unit.no (Robert Schmidt) writes:

>In article <37619@uflorida.cis.ufl.edu>, ggh@reef.cis.ufl.edu (Greg Huey - Aymer deGregory) writes:
>> 
>>   Im trying to write a 'bare-bones' multi-processing os for the 386.
>> I dont wont to create my own dos, obviously I want to use MS-DOS...
>> Question is, how do I do this in protected mode?
>>   Presumably, I create the GDT, LDTs, TSSs, IDTs, and the viarious gates...
>> But no matter HOW I map virtual memory with the GDT, LDTs, I cannot make
>> the virtual memory transparent, due to the priv bits and table indicator
>> bit. I need to put the 386 in protected mode, but it has to look the
>> same to MS-DOS, otherwise MS-DOS will fail, wont it? For instance, MS-DOS
>> knows one of its buffer is at seg:offs -- when I put the 386 into protected
>> mode, seg, rather than being a simple address, will now be treated as a
>> selection index into the GDT or LDT.

Well, you might set up your GDT and LDT so that they map segments to the
same place they would have been otherwise.  Then maybe you can reserve
a memory space from DOS and then, after you've reserved it, change it
to point to somewhere in extended memory.  You probably couldn't use
it for code, but maybe you could use it to store data in.
    You'd have to remap the segment back where DOS expects it to be
before terminating your program, of course.  And this would't much help
in multi-tasking DOS programs made by others, it might just make your
own programs smaller in DOS memory so you can fit more in.