Date: Mon, 7 Jul 1997 08:19:26 -0700 (PDT)
Message-Id: <199707071519.IAA23803@adit.ap.net>
Mime-Version: 1.0
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To: Johan Karlsson <k AT sm DOT luth DOT se>
From: Nate Eldredge <eldredge AT ap DOT net>
Subject: Re: Protected mode
Cc: djgpp AT delorie DOT com
Precedence: bulk

At 09:21  7/7/1997 +0200, you wrote:

>I tried to use the dpmi interrupt 2fH ax = 1686H
>see: http://www.delorie.com/djgpp/doc/dpmi/ch5.g.html
>to get the CPU mode, but it return that the program runs in Real Mode.
>Is there some problem with this code part:
>__dpmi_regs r;
>r.x.ax = 0x1686;
>__dpmi_int(0x2f, &r)
>
>Now if (r.x.ax == 0) then Protected mode else Real Mode.
>And r.x.ax is not 0 when __dpmi_int returns.
That's correct. __dpmi_int creates a *real mode* interrupt. I'm not sure
offhand how to get a protected mode interrupt, but I think it can be done
with int86.
 
>I have a memory board on the ISA Bus which I want to access.
>The memory is in the address range 0x00400000 to 0x0047fffe
Here's some code which ought to do that. Note that I don't have any
memory-mapped devices to test it with so I don't know if it will work. Also:
it calls abort in each place where something has gone wrong.
--cut--
#define MEM_BD_PHYSICAL_BASE 0x00400000
#define MEM_BD_SIZE 0x00080000 /* or so */
   __dpmi_meminfo meminfo_block;
   short mem_bd_selector;
   int mem_bd_linear;
   /* First, get a selector that we can use */
   if ((mem_bd_selector = __dpmi_allocate_ldt_descriptors(1)) == -1) abort();
   /* Now, the physical address has to be mapped to a linear address */
   meminfo_block.address = MEM_BD_PHYSICAL_BASE;
   meminfo_block.size = MEM_BD_SIZE;
   if ((mem_bd_linear = __dpmi_physical_address_mapping(&meminfo_block)) == -1)
      abort();
   /* Now we lock it so nobody gets any funny ideas about swapping it */
   meminfo_block.address = mem_bd_linear;
   meminfo_block.size = MEM_BD_SIZE;
   if (__dpmi_lock_linear_region(&meminfo_block) == -1) abort();
   /* Ok, now make the selector work on the mem board area */
   /* First, set the base address of the segment */
   if (__dpmi_set_segment_base_address(mem_bd_selector,mem_bd_linear) == -1)
abort();
   /* Now the limit, to catch any pointer overruns, etc */
   if (__dpmi_set_segment_limit(mem_bd_selector,MEM_BD_SIZE) == -1) abort();
   /* That's it! Now you can use mem_bd_selector to access your
      board with farptr functions or movedata */

HTH

Nate Eldredge
eldredge AT ap DOT net