Mail Archives: djgpp/2001/06/29/23:00:04
"Charles Sandmann" <sandmann AT clio DOT rice DOT edu> wrote in message
news:3b3bca85 DOT sandmann AT clio DOT rice DOT edu...
> > I have a game which accesses VGA memory (A000:0000) directly. In 16-bit
> > mode, that's ok, but to get it to run under DJGPP I either have to
rewrite
> > the thing or map VGA memory to linear memory. There is a function to do
> > that, but I can only do it with a DPMI 1.0 server.
>
> If you only plan to run under DOS, and use CWSDPMI, this feature is
> available. But I don't recommend using it because of the portability
> reasons.
Portability is not an issue, since this is a DOS-only game. The Win32 port
for obvious reasons doesn't use cwsdpmi (or the section of code that I'm
playing with).
> > Is there a way to accomplish the same thing (i.e. direct linear access
to
> > A000:0000 from my ds selector) with a DPMI 0.9 server? Right now I'm
looking
> > at the following options, and neither of them look good:
> >
> > 1. Use __djgpp_nearptr_enable(). The documentation recommends against
this,
> > and considering that I just overhauled a number of asm files and am
aware of
> > a number of bugs already, I'm not so sure I like the thought of being
able
> > to kill my system.
>
> Just try it. You probably won't kill the system - that's the worse case.
> If worried you could do another method first, then fall back to near
pointers
> which are portable to everything except Windows NT type platforms.
I would prefer not to, but if I am left with no other alternative I will. It
seems more logical to me to map what I need from the physical address space
into my linear address space.
> > 2. Use function 02h to map the segment to a descriptor, use function 06h
to
> > locate the linear base of the descriptor, then use functions 07h and 08h
to
> > modify my ds segment so I can read that linear address.
>
> No, use sys/farptr.h instead. Best solution. Check the FAQ.
>
> > Is there any way to do it directly, or is there a DPMI 0.9 server which
> > supports function 0508h (map device -> linear)?
>
> CWSDPMI (if DOS only). dosmemget/dosmemput functions in the library.
> near pointers. far pointers. 4 different ways, which ever you are most
> comfortable. Have fun.
First of all, thank you for your response.
I've already looked at dosmemget/dosmemput, and I can't use them due to the
design of the game. I would prefer not to use near pointers due to the fact
that it allows me to overwrite all of conventional memory (especially when
you see comments like "why does this overwrite memory??"). Far pointers are
the closest solution except that nearly all the code makes direct access to
the VGA buffer and I would have to convert everything, including a number of
assembly files. The easiest way in my assessment is to map
As for cwsdpmi, prior to my original post I attempted mapping A000:0000
through the following code (My code actually checks return values, but I
figured it was best to keep the post shorter):
__dpmi_meminfo info;
info.address = 0;
info.size = (320 * 200);
__dpmi_allocate_linear_memory(&info);
__dpmi_map_conventional_memory_in_memory_block(&info, 0xA0000);
The first call failed with error code 1284 (504h). I looked in the DPMI spec
for function ax=504h and this is not listed as an error code. The DPMI
server doesn't set an error code, but it sets the carry flag indicating an
error. I made absolutely sure of this by attempting the same call using
int386.
I don't see what I'm doing wrong, if this is supposed to work?
-Matt
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