Mail Archives: djgpp/1999/06/17/03:23:31
On Wed, 16 Jun 1999, Michael Stewart wrote:
> It creates a selector, given the location and size. So if you wanted a
> descriptor which spanned the VGA memory (0xa0000, 64000 bytes) you would
> call it like so.
>
> if (make_ds (video_ds, 0xa0000, 64000))
> return -1; /* Error making descriptor */
I think it might be useful.
> int make_ds (int &ds, int base, int limit) {
> static char ds_data[8];
Why did you make the ds_data[] array static?
Also, I think it's better to declare it "unsigned char".
> limit--;
> ds_data[0] = limit & 0xff;
> ds_data[1] = limit >> 8 & 0xff;
It might be better to make sure that if the size of the segment is
more than 1MB, the lower 12 bits in the limit you pass to
__dpmi_set_descriptor are set. This is because the DPMI spec requires
such large segments to be page-aligned, and some servers will fail the
call otherwise. Too many people posted here code that didn't work
because they failed to comply to that requirement, and it would be a
good idea to make this function take care of that for the user.
> ds_data[6] = limit >> 16 & 0x0f + 0x40;
The 0x40 part is only correct for limits less than 64KB (and even then
I think it needs to be 0x41). For larger segments, you need to make
it page-granular, so 0xc1 is the value in those cases.
> ds_data[7] = 0x00;
This is only right for addresses in the first 16MB range. Why the
limitation?
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