From:	Emil0 AT aol DOT com
Date:	Wed, 23 Oct 1996 18:09:05 -0400
Message-ID:	<961023180905_339586309@emout17.mail.aol.com>
To:	leathm AT gbrmpa DOT gov DOT au
cc:	djgpp AT delorie DOT com
Subject:	Re: Another way to access VGA memory

Leathal wrote:

>Hey dudes...
>
>I started using selectors (such as %fs) but gave up after about 2 days.
>Why?
>Every time I want to write to a section of the graphics screen, I call
>a routine - say blit. Everytime blit is called, I have to reload the
>selector. Every time I exit the blit routine, I have to restore the
>selector.
>
>Using near pointers, you never have to worry about that. And I have
>never
>written a program where the __djgpp_conventional_base changes... could
>someone explain to me a situation where it does? I have never
>encountered
>this when only running my application (I do just about everything under
>Win95 too... :)
>
>Just have a variable gfx_screen which points to the physical memory area
>of the graphics screen, add the __djgpp_conventional_base to it, and
>then
>always just use that variable. Easy, no?
>
>Leathal.

Leathal,
I started this thread, but I only read the weekly digest, so I've only just
seen your reply. Using near pointers in no quicker because you still have to
load the %edi register with your pointer and then no doubt add some stuff to
this to access the right pixel. With my method you load %es or whatever and
then load stuff into %edi to point to the right pixel  - so it is not slower
(apart from perhaps another register to save on the stack, although a push
and a pop will hardly slow a program down even if done hundreds of times a
second). With my method you retain full memory protection and it will work
under all DPMI providers. It is a safe method which is guaranteed to work in
the future and with DOS emulators on other machines. The near pointer method
is really a hack, so for professional stuff I'd recommend my method which is
far neater. For example, say you're testing a bit blitter, a gp fault is
caused when you write outside video memory.  Another thing,
__djgpp_conventional_base is not guaranteed to stay the same - currently it
does, but in future this may change.
     I think  that the protection offered by my method far out-weighs the
hassle of having to save and restore a single register. My method is for
people who want to write bullet-proof code which runs fast.

Richard. :)

ps.     Web page authors! Put this method on your sites please!

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