Reported by Borja Sotomayor <borja@cs.uchicago.edu>.

Some versions of ld accept /dev/null as a valid input file.  Others will
not.  The latter versions will give false negatives for --buil-id support,
which breaks linking of Pintos userspace programs.  So use a different
method, that I hope will be more reliable, to detect --build-id support.

Reported by Joseph Huang <jdhuang@stanford.edu>.

Here, $buf might have some data in it that has not yet been printed, but
we were discarding it (and trying to read more) without printing it.

(In Perl, "do" ensures that the inner block runs at least once; without
"do" the condition is evaluated first.)

Reported by Nathaniel Hardison <hardison@stanford.edu>.

Thanks to David Mazieres for reporting the problem and the fix.

Thanks to Godmar Back for reporting the problem and for the fix.

Rounding to nearest raises a host of issues that we don't want students
to worry about.

Suggested by Godmar Back.

The compiler can't optimize out the load of "ticks" here because
it can't look into intr_disable() and intr_set_level() and know that
they don't modify "ticks".

Thanks to Martin <mfleener@stanford.edu> for pointing this out.

We cannot expect that we know in detail ever interrupt that can occur
on real hardware, so we must cope with exceptional cases a little
better.

The Pintos loader writes status to the serial port as well as the
VGA console, but this doesn't work out so well on at least some
real hardware when nothing is connected to the serial port: each
character can take a second or so to display because the BIOS
waits for a serial timeout each time.  So, on the first serial
timeout, replace the "int $0x14" call by a pair of no-ops.

The rest of the changes are part of the struggle not to exceed
the maximum loader size.

Pintos used to contain a loader and I/O functions derived from MIT
code, but this has now been entirely eliminated, so there is no
longer any reason to retain the MIT copyright notice or license.

The contents of these functions was derived from code used in the
Massachusetts Institute of Technology's 6.828 advanced operating
systems course.

I deleted the contents of these functions and then rewrote them from
scratch without reference to the earlier code, working only from the
function comments (which I wrote earlier).

pinning is necessary
added FAQ for why stack growth may cause page faults above stack pointer
minor edits to other sections

http://gcc.gnu.org/onlinedocs/gcc/Return-Address.html )

added loadusersymbols to load user symbols from file
updated documentation to reflect behavior of qemu's and bochs's gdbstubs

This is in preparation for introducing new block devices, in
particular USB storage-based block devices so that Pintos can boot
from USB memory sticks on notebook and desktop PCs.

This block layer was inspired by one from Anthony Romano
<chz@vt.edu> but it has been extensively (perhaps entirely) rewritten.
Thus, bugs must certainly be blamed on the committer.

We now ignore pointers that are not null but still clearly wild.

We could check that there's really a valid PDE and PTE (I think Linux
does this) but it would take more code and be ugly.

Based on code from Anthony Romano <chz@vt.edu> but with extensive changes.

This makes it conform to the Pintos convention that a global symbol name
begin with its module's name.

This global variable was not named according to the convention that a
global variable name start with its module name.  Furthermore, its usage
was somewhat scattered across the source tree.

This commit introduces new functions shutdown() and shutdown_configure()
to improve the situation.

Also, make shutdown_reboot() actually behave that way, by trying to reboot
forever instead of giving up after some number of tries.

The "user_shortcut" Bochs option allows specifying a key combination to
send to the VM when the user clicks on the "user" button in the Bochs
headerbar.

From Godmar, crossported from his usb-integration-aug08 branch.

From Godmar, crossported from his usb-integration-aug08 branch.

It is important that Pintos and its loader use the same serial speed, so
that a terminal program connected to a serial port can be set to a sane
speed.  The ideal choice would be 115200 bps, for maximum speed, but the
Pintos loader uses the BIOS to do serial output, and the BIOS supports a
maximum speed of 9600 bps.

From Godmar, crossported from his usb-integration-aug08 branch.

Thanks to Godmar for pointing out the problem.

Suggested by Godmar.

The name "user_page_limit" did not follow the naming convention, which
stated that its name should start with palloc_ since it is defined in
palloc.c.  But it was only used in one function, so it was better off
as a function parameter anyhow.

Found by Godmar with his process-linker-map.pl.

The stubs are only referenced within intr-stubs.S so there is no reason
for them to be exported.

Found with Godmar's process-linker-map.pl.

This makes its name fit the convention that the file name is used as
a prefix for global symbol names.

Found by Godmar with his process-linker-map.pl script.

This makes its name fit the convention that the file name is used as
a prefix for global symbol names.

Found by Godmar with his process-linker-map.pl script.

Found by Godmar with his process-linker-map.pl.