Date:	Fri, 7 Jul 95 17:14 MDT
From:	mat AT ardi DOT com (Mat Hostetter)
To:	sledz AT cs DOT tu-berlin DOT de (Steffen Sledz)
Subject:	Re: Physical Memory Access?
Newsgroups:	comp.os.msdos.djgpp
References:	<3tja1f$rp2 AT news DOT cs DOT tu-berlin DOT de>
Cc:	djgpp AT sun DOT soe DOT clarkson DOT edu

>>>>> "Steffen" == Steffen Sledz <sledz AT cs DOT tu-berlin DOT de> writes:

    Steffen> How can I access physical under DJGPP?  I tried the DPMI
    Steffen> calls of V2beta with Quarterdecks QDPMI, but this results
    Steffen> in an SIGSEGV (Stack overflow) at runtime.

I just added linear frame buffer support to our Macintosh emulator,
for systems that have a VBE 2.0-compatible video driver.  It works,
and our graphics are now much faster.  Anyway, here's the routine we
use to get a selector for the physical memory corresponding to the
frame buffer.  You might find interesting.

Under DPMI 0.9 you have to use a special selector to reference such
memory; see the farptr.h functions, and the `movedata' routine.  You
can't access this memory directly.

-Mat


#include <dpmi.h>

int
selector_for_phys_mem (unsigned long base, unsigned long num_bytes)
{
  int sel;
  unsigned long seg_lim;
  __dpmi_meminfo minfo;

  /* Allocate a descriptor. */
  sel = __dpmi_allocate_ldt_descriptors (1);
  if (sel == -1)
    return -1;

  seg_lim = ((num_bytes + 4095) & ~4095) - 1;

  /* Map the physical memory into our address space. */
  minfo.handle  = 0;		/* unused */
  minfo.size    = seg_lim + 1;
  minfo.address = base;
  if (__dpmi_physical_address_mapping (&minfo) != 0)
    return -1;

  if (__dpmi_set_segment_base_address (sel, minfo.address) == -1)
    return -1;
  if (__dpmi_set_segment_limit (sel, seg_lim) == -1)
    return -1;

  return sel;
}

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