RAM Mapping Script

[Michael ODonnell]

> You won't be able to do this from userland.  That spot of memory
> is off-limits because the kernel needs to preserve it in the event
> that another process wants to enter a vm86 mode.

Maybe I haven't been following this thread closely enough, but I
don't see how either User mode accesses or vm86 mode would matter.

I figure that once the kernel has pointed the IDT register elsewhere
that zero'th page of RAM loses any special status and goes into a pool
where it's eligible for allocation like any other.

Well, OK - maybe not like any other.  Using the "crash" utility
I queried the system's mem_map[] and it appears (at least on this
steam-powered RHEL3 machine, assuming I'm looking at the correct
bit definitions in include/linux/mm.h) that the first dozen pages
are reserved:

	.
	.
	.
crash> kmem -p
  PAGE     PHYSICAL   MAPPING    INDEX CNT FLAGS
c100002c          0         0         0  0 8000
c1000068       1000         0         0  0 8000
c10000a4       2000         0         0  0 8000
c10000e0       3000         0         0  0 8000
c100011c       4000         0         0  0 8000
c1000158       5000         0         0  0 8000
c1000194       6000         0         0  0 8000
c10001d0       7000         0         0  0 8000
c100020c       8000         0         0  0 8000
c1000248       9000         0         0  0 8000
c1000284       a000         0         0  0 8000
c10002c0       b000         0         0  0 8000
c10002fc       c000         0         0  0 0
c1000338       d000         0         0  0 0
c1000374       e000         0         0  0 0
c10003b0       f000         0         0  0 0
c10003ec      10000         0         0  0 0
c1000428      11000         0         0  0 0
c1000464      12000         0         0  0 0
c10004a0      13000         0         0  0 0
c10004dc      14000         0         0  0 0
	.
	.
	.

...so just for fun I wrote NULLs to that first page (with
before and after displays for verification) thus:

   dd bs=1k count=1              if=/dev/mem | hexdump -C
   dd bs=1k count=1 if=/dev/zero of=/dev/mem
   dd bs=1k count=1              if=/dev/mem | hexdump -C

...and the system kept on running, which indicates only that the
IDT is elsewhere, though I'm still not %100 certain the page isn't
in use for some other purpose.