Date: Wed, 31 Jul 2002 02:19:31 +0200 From: Jens Rehsack <rehsack@liwing.de> To: hal@telefonica.net Cc: questions@freebsd.org Subject: Re: Accessing memory below 1 MB Message-ID: <3D472D13.BE63F5D9@liwing.de> References: <E17ZUer-0000LF-00@ash.drims.net>
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hal@telefonica.net wrote:
>
> Hello,
>
> I'm writing some graphics code (just for fun) and I need "legal" access to
> the memory addresses below 1 mb.
>
> The thing is, I know how to access those addresses (open /dev/mem, mmap, and
> that's it), but I'd like to be able to alloc some pages so that my accesses
> are correct and don't make the kernel panic...
>
> Does anyone know how I could do that?
Puh - I do not know if it's really make sense, because I think there should be
another way, f.e. framebuffer, ...
You may take a look into ghostscript, it's able to use X and a standard VGA card.
The way I know is writing a kernel module which gives you direct access to the
video memory (I assume you mean this by accessing memory below 1MB).
In any other case I strongly recommend find another way.
Matt Dillon writes in a mail before:
0xFFFFFFFF +-----------------------+-----
| |
| KERNEL | (1G)
| |
0xC0000000 +-----------------------+-----
| |
| USER STACK |
| | |
| V |
| |
+-----------------------+
| |
| |
| AVAILABLE FOR MMAP |
| | (stack, mmap, user data, program) = 3G
MAXDSIZ +-----------------------+
| |
| USER DATA (NON-MMAP)|
| |
+-----------------------+
| |
| PROGRAM BINARY |
| |
0x00000000 +-----------------------+-----
Any C program which uses shared libraries uses mmap(). Many library
functions and libraries also use mmap(), including portions of our
malloc() implementation (though the main area used by mmap is the
user data area). If you increase MAXDSIZ to the point where there
is not enough VM for the mmap()'s programs make then you will run
into the problems you are having.
I'm not sure why you are trying to have squid use all 4G of the
machine directly in its user data area. Squid caches a lot of
things in memory, sure, but it also caches things in files and
FreeBSD will use free physical memory to cache those files
regardless of how you configure the machine. So you should be
getting good utilization of your 4G of memory even if Squid cannot
use all 4G in its user data area directly. Setting MAXDSIZ to
2.9GB out of the 3G of user VM available puts a huge squeeze on
how much the program can mmap() before it runs out of VM.
-Matt
Matthew Dillon
<dillon@backplane.com>
> Thank you in advance.
>
> Alex
>
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