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From: "James" <countingtoten AT gmail DOT com>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: Accessing absolute address above 1MB
Date: 9 Nov 2006 07:06:24 -0800
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I was able to get it working. Thanks for your help.

James

Rod Pemberton wrote:
> "James" <countingtoten AT gmail DOT com> wrote in message
> news:1162922909 DOT 133028 DOT 98340 AT f16g2000cwb DOT googlegroups DOT com...
> > Charles Sandmann wrote:
> > > > I am trying to access the memory address 0x3FFF0000 in DOS using dpmi.
> > > ..
> > > >  buffer[0] = _farpeekb(selector, mi.address);
> > >
> > > mi.address should be 0 here, since the offset is built into the selector
> > > you just set up
> >
> > I tried that and still receive the same error message.
> >
>
> That's really strange, because that's the only error I could find. The
> offset, where you have mi.address, should be between 0 to 0xfff (segment
> limit-1).
>
> buffer[0] = _farpeekb(selector, 0);
>
> Perhaps, you're using a RM address currently and tripping the same line in
> the RM section?
>
> Your PM code should work for address below 1Mb as long as you don't call
> __dpmi_physical_address_mapping (shouldn't be used below 1Mb per DPMI
> specs.).  I commented out the RM section, the
> __dpmi_physical_address_mapping, and then used mi.address = 0xB8000
> (screen), and added a _farpokeb(selector, 0, 0x66) which output to the
> screen.  I also added the physical address mapping back w/0x3FFF0000 and no
> longer received the SIGSEGV.
>
> What OS are you running the code under?  RM DOS, FreeDOS, Win3.1/95/98/2k
> console, WinNT/2k/XP console, Linux/DOSEMU?  Are you running the code under
> an emulator (Bochs, QEMU, etc.)?
>
> Also, do you want 1Gb-1 (0x3FFF0000) or 256Kb-16 (3FFF:0000h, i.e.,
> 0x0003FFF0) ?  Most memory mapped devices are usually mapped below 1Mb.
>
> From this (first post):
> > fs: sel=01cf  base=3fff0000  limit=00000fff
>
> The fs selector is set to what you coded, but it doesn't show the access
> rights...
>
> If you're still having problems, I'd recommend adding a call to
> __dpmi_set_descriptor_access_rights and moving the
> __dpmi_physical_address_mapping call after the
> __dpmi_allocate_ldt_descriptors call.
>
> __dpmi_set_descriptor_access_rights(selector,0xC0F3) /* need to check value
> for your segment */
>
> I'd recommend reading the last two paragraphs of the first post of mine, and
> all of the second.
> http://groups.google.com/group/comp.os.msdos.djgpp/msg/fa3f19efbcba7754?hl=en
> http://groups.google.com/group/comp.os.msdos.djgpp/msg/3b2927a76908742a?hl=en
> 
> 
> Rod Pemberton