This is Nachos 5.0j with ELF32 binary support. I added ELF32 binary support after the conclusion of my Operating Systems course, CS377, at the University of Masssachusetts Amherst.

This permits you to use more modern compilers when compiling binaries to work with the nachos VM. Previously, you had to rely on a very old, aging build of gcc... one that no longer works on modern Mac OS X since the builds were PPC binaries.

New files (written by me):
Elf.java -> nachos.machine
ElfProgram.java -> nachos.machine
ElfSection.java -> nachos.machine

Changed files (minor amounts of code written by me):
Machine.java -> nachos.machine
    I made a change to this because the code I wrote relied on having the ability to use reflection. Reflection is used
    by ElfProgram and ElfSection since they use EnumSets to describe various ELF attributes and flags. It works by
    passing the privilege object for the Nachos security manager and using it to execute two calls to the method
    EnumSet.noneOf, which uses reflection to determine what items are in an enumeration. Reflection is banned by 
    the Nachos security manager by default.
UserProcess.java -> nachos.userprog
    COFF executable support is stripped from this and modified to fit the requirements for loading ELF binaries. It should
    look really similar to the COFF code... because a lot of it is.
test/ directory -> overwrite the existing test/ directory in the nachos distribution
    I made a very small change to sh.c to open .elf extension files instead of .coff extension files.
    va-mips.h was changed because sgidefs.h will erroneously be included, the package doesn't contain it,
        and it's not necessary.
    Makefile is changed to generate correct output with binutils-2.21, gcc-4.5.2 and beyond, and to output binaries
        with .elf extensions.
    script is changed to generate the correct format binary, and align all sections on nachos page size boundaries.

Other errata:
- The AutoGrader apparently has support for changing the COFF binary loader. I have no idea in what circumstances that is used,
and I think my autograder code is incomplete so I didn't change that aspect of it. It might be sufficient to just make it return an Elf type
instead of a Coff type.

Cross-compiler building
==================

We need to make binutils and gcc for this to work. Both should go pretty smoothly.

I have tested this on Ubuntu Linux 10.04 with a proper compiler (usually installed by default) and Mac OS X 10.6.7 with gcc installed via Xcode 3.2.5.

There are a few updated minor versions of the packages I list below, and a major gcc update (4.6), but I haven't tested using those newer versions. Feel free to try; it'll probably work.

I made a directory, crosscompile, and downloaded the following packages and placed the archives in the crosscompile directory:
1. binutils-2.21 -- http://ftp.gnu.org/pub/gnu/binutils/binutils-2.21.tar.gz
2. gcc 4.5.2 -- http://ftp.gnu.org/pub/gnu/gcc/gcc-4.5.2/gcc-core-4.5.2.tar.gz (we're only building C language support)
and the following REQUIRED gcc dependencies:
3. gmp-5.0.1 -- http://ftp.gnu.org/pub/gnu/gmp/gmp-5.0.1.tar.gz
4. mpc-0.9 -- http://www.multiprecision.org/mpc/download/mpc-0.9.tar.gz
5. mpfr-3.0.0 -- http://ftp.gnu.org/pub/gnu/mpfr/mpfr-3.0.0.tar.gz

Assuming we're in your crosscompile build directory, extract all of the packages and place stuff in the correct directories (replace with proper versions if you change them):
tar -zxf binutils-2.21.tar.gz
tar -zxf gcc-core-4.5.2.tar.gz
tar -zxf gmp-5.0.1.tar.gz
tar -zxf mpc-0.9.tar.gz
tar -zxf mpfr-3.0.0.tar.gz
mv gmp-5.0.1 gcc-4.5.2/gmp
mv mpc-0.9 gcc-4.5.2/mpc
mv mpfr-3.0.0 gcc-4.5.2/mpfr

Building binutils:
mkdir binutils-build
cd binutils-build
../binutils-2.21/configure --prefix=/usr/local/mips-gcc --target=mips-generic-linux
# NOTE: Change the --prefix if you want to install binutils somewhere else.
make
sudo make install
# NOTE: Use sudo if you're not installing it to your home directory and are installing to /usr/local/mips-gcc

Building gcc:
Go back to your crosscompile directory. If you're in binutils-build still, just type cd ..
mkdir gcc-build
cd gcc-build
../gcc-4.5.2/configure --prefix=/usr/local/mips-gcc --program-prefix=mips- --target=mips-generic-linux --disable-threads --disable-shared --disable-multilib --with-endian=little,big --disable-libmudflap --enable-languages=c --disable-libgomp --disable-libssp
# The build will fail at some point without that insane number of options. Change --prefix setting if you want to install somewhere else.
# NOTE: I think there's actually one more option that needs to be disabled, and I don't remember what it is... if you actually try building gcc with those settings, you should discover what is flawed and might need to be disabled.
# Also, install to the same directory as binutils, or you'll need to put your new binutils install somewhere in your PATH.
make
sudo make install
# Same deal, sudo only if you're not installing in your home directory and placing it in a place like /usr/local/mips-gcc

Now you should have a working cross-compiler in /usr/local/mips-gcc, or wherever you've specified. The Makefile edits I made in the nachos test/ directory assume that you're using /usr/local/mips-gcc as the installation directory.