Date: Wed, 15 Jan 1997 22:26:17 -0500 (EST) From: Thomas David Rivers <ponds!rivers@dg-rtp.dg.com> To: softweyr@xmission.com, ponds!lemis.de!grog Cc: ponds!freebsd.org!questions Subject: Re: the UNIX hole Message-ID: <199701160326.WAA05493@lakes.water.net>
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>
> Mike Branch writes:
> % could anybody give an explanation about the UNIX "hole"?
> % Is the hole filled in current versions of UNIX? Is it
> % still advisable to configure devices in contiguous memory
> % locations when building a kernel?
>
> Greg Lemis replied:
> > (You can tell how far behind I am with my mail :-()
> >
> > I didn't see a reply to this one, and since I've never heard of the
> > "UNIX hole", I thought I'd ask you what you mean.
>
> I skipped over this when I first read it because it sounded like the
> original author was from outer space. Now that I've seen it again, and
> my brain has had a few days to rebuild the associative links, I
> remember some bizarre problems running SVR3 apps on early i386 SVR4
> systems, something to do with a giant hole placed between the text and
> data segments when the program loaded.
>
> If this is what you're talking about, Mike, it has never existed in BSD
> UNIX, and still doesn't. ;^) And no, you can configure devices pretty
> much wherever you want in memory, though on the PC architecture* most
> devices use only I/O addresses and not memory addresses.
>
>
> * Sorry, the phrase "PC architecture" just sent my brain spinning, I'm
> trying to recover. The idea that someone actually *designed* this mess
> boggles...
>
Well - usually there is a "hole" - but it's done on purpose.
Generically (and you can invert the up/down in the following, particularly
on PCs - and I'm leaving out *lots* of details) - a program loaded on
a UNIX machine can be thought of as a few sections and a stack and a
heap (I'm also greatly simplifying here; typically there are actually
more sections and there's no reason they need to be contiguous, etc...
but I'm trying to illustrate) - When a program is running in it's
virtual address space - it looks like:
++++++++++++++++
| program |
| (.text) |
++++++++++++++++
| initialized |
| data |
| (.data) |
++++++++++++++++
| uninitialized|
| data (.bss) |
++++++++++++++++ <- end
| stack |
| | |
\|/
(hole)
/|\
| | | <- sbrk
| heap |
++++++++++++++++
The "heap" grows toward the program stack.
So, there is a virtual "hole" in-between them... but, so what?
Now, on PCs there is another "hole" between 640K and the first 1M.
Also, there is a "hole" in high memory used by the BIOS... but with
virtual memory these aren't "really" there; just physically there.
- Dave R. -
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