diff --git a/bin/spack b/bin/spack
index 13312a69d6..294334ac43 100755
--- a/bin/spack
+++ b/bin/spack
@@ -15,6 +15,7 @@ sys.path.insert(0, SPACK_LIB_PATH)
 # clean up the scope and start using spack package instead.
 del SPACK_FILE, SPACK_PREFIX, SPACK_LIB_PATH
 import spack
+import spack.tty as tty
 
 # Command parsing
 parser = argparse.ArgumentParser(
@@ -43,4 +44,7 @@ spack.debug = args.debug
 
 # Try to load the particular command asked for and run it
 command = spack.cmd.get_command(args.command)
-command(parser, args)
+try:
+    command(parser, args)
+except KeyboardInterrupt:
+    tty.die("Got a keyboard interrupt from the user.")
diff --git a/lib/spack/spack/__init__.py b/lib/spack/spack/__init__.py
index ef9e448413..02af7db4f3 100644
--- a/lib/spack/spack/__init__.py
+++ b/lib/spack/spack/__init__.py
@@ -1,5 +1,6 @@
 from globals import *
 from utils import *
-from exception import *
+from error import *
 
 from package import Package, depends_on
+from multi_function import platform
diff --git a/lib/spack/spack/arch.py b/lib/spack/spack/arch.py
index 88f04cb478..6610006804 100644
--- a/lib/spack/spack/arch.py
+++ b/lib/spack/spack/arch.py
@@ -1,34 +1,67 @@
 import os
-import platform
+import platform as py_platform
 
+import spack
+import error as serr
 from version import Version
 from utils import memoized
 
-instances = {}
-macos_versions = [
-    ('10.8', 'mountain_lion'),
-    ('10.7', 'lion'),
-    ('10.6', 'snow_leopard'),
-    ('10.5', 'leopard')]
+
+class InvalidSysTypeError(serr.SpackError):
+    def __init__(self, sys_type):
+        super(InvalidSysTypeError, self).__init__(
+            "Invalid sys_type value for Spack: " + sys_type)
 
 
-class SysType(object):
-    def __init__(self, arch_string):
-        self.arch_string = arch_string
+class NoSysTypeError(serr.SpackError):
+    def __init__(self):
+        super(NoSysTypeError, self).__init__(
+            "Could not determine sys_type for this machine.")
 
-    def __repr__(self):
-        return self.arch_string
 
-    def __str__(self):
-        return self.__repr__()
+def get_sys_type_from_spack_globals():
+    """Return the SYS_TYPE from spack globals, or None if it isn't set."""
+    if not hasattr(spack, "sys_type"):
+        return None
+    elif hasattr(spack.sys_type, "__call__"):
+        return spack.sys_type()
+    else:
+        return spack.sys_type
+
+
+def get_sys_type_from_environment():
+    """Return $SYS_TYPE or None if it's not defined."""
+    return os.environ.get('SYS_TYPE')
+
+
+def get_mac_sys_type():
+    """Return a Mac OS SYS_TYPE or None if this isn't a mac."""
+    mac_ver = py_platform.mac_ver()[0]
+    if not mac_ver:
+        return None
+
+    return "macosx_{}_{}".format(
+        Version(mac_ver).up_to(2), py_platform.machine())
+
 
 @memoized
 def sys_type():
-    stype = os.environ.get('SYS_TYPE')
-    if stype:
-        return SysType(stype)
-    elif platform.mac_ver()[0]:
-        version = Version(platform.mac_ver()[0])
-        for mac_ver, name in macos_versions:
-            if version >= Version(mac_ver):
-                return SysType(name)
+    """Returns a SysType for the current machine."""
+    methods = [get_sys_type_from_spack_globals,
+               get_sys_type_from_environment,
+               get_mac_sys_type]
+
+    # search for a method that doesn't return None
+    sys_type = None
+    for method in methods:
+        sys_type = method()
+        if sys_type: break
+
+    # Couldn't determine the sys_type for this machine.
+    if sys_type == None:
+        raise NoSysTypeError()
+
+    if not type(sys_type) == str:
+        raise InvalidSysTypeError(sys_type)
+
+    return sys_type
diff --git a/lib/spack/spack/cmd/sys-type.py b/lib/spack/spack/cmd/sys-type.py
new file mode 100644
index 0000000000..7f57fb16ff
--- /dev/null
+++ b/lib/spack/spack/cmd/sys-type.py
@@ -0,0 +1,11 @@
+import spack
+import spack.arch as arch
+
+description = "Print the spack sys_type for this machine"
+
+def sys_type(parser, args):
+    configured_sys_type = arch.get_sys_type_from_spack_globals()
+    if not configured_sys_type:
+        configured_sys_type = "autodetect"
+    print "Configured sys_type:             %s" % configured_sys_type
+    print "Autodetected default sys_type:   %s" % arch.sys_type()
diff --git a/lib/spack/spack/error.py b/lib/spack/spack/error.py
new file mode 100644
index 0000000000..3c3c3dafc9
--- /dev/null
+++ b/lib/spack/spack/error.py
@@ -0,0 +1,13 @@
+
+class SpackError(Exception):
+    """This is the superclass for all Spack errors.
+       Subclasses can be found in the modules they have to do with.
+    """
+    def __init__(self, message):
+        super(SpackError, self).__init__(message)
+
+
+class UnsupportedPlatformError(SpackError):
+    """Raised by packages when a platform is not supported"""
+    def __init__(self, message):
+        super(UnsupportedPlatformError, self).__init__(message)
diff --git a/lib/spack/spack/exception.py b/lib/spack/spack/exception.py
deleted file mode 100644
index 32167cf36a..0000000000
--- a/lib/spack/spack/exception.py
+++ /dev/null
@@ -1,39 +0,0 @@
-
-
-class SpackException(Exception):
-    def __init__(self, message):
-        self.message = message
-
-
-class FailedDownloadException(SpackException):
-    def __init__(self, url):
-        super(FailedDownloadException, self).__init__("Failed to fetch file from URL: " + url)
-        self.url = url
-
-
-class InvalidPackageNameException(SpackException):
-    def __init__(self, name):
-        super(InvalidPackageNameException, self).__init__("Invalid package name: " + name)
-        self.name = name
-
-
-class CommandFailedException(SpackException):
-    def __init__(self, command):
-        super(CommandFailedException, self).__init__("Failed to execute command: " + command)
-        self.command = command
-
-
-class VersionParseException(SpackException):
-    def __init__(self, msg, spec):
-        super(VersionParseException, self).__init__(msg)
-        self.spec = spec
-
-
-class UndetectableVersionException(VersionParseException):
-    def __init__(self, spec):
-        super(UndetectableVersionException, self).__init__("Couldn't detect version in: " + spec, spec)
-
-
-class UndetectableNameException(VersionParseException):
-    def __init__(self, spec):
-        super(UndetectableNameException, self).__init__("Couldn't parse package name in: " + spec)
diff --git a/lib/spack/spack/globals.py b/lib/spack/spack/globals.py
index ee67a461c3..b864c08533 100644
--- a/lib/spack/spack/globals.py
+++ b/lib/spack/spack/globals.py
@@ -1,6 +1,7 @@
 import os
 from version import Version
 from utils import *
+import arch
 
 # This lives in $prefix/lib/spac/spack/__file__
 prefix = ancestor(__file__, 4)
@@ -20,7 +21,7 @@
 install_path  = new_path(prefix, "opt")
 
 # Version information
-spack_version = Version("0.1")
+spack_version = Version("0.2")
 
 # User's editor from the environment
 editor = Executable(os.environ.get("EDITOR", ""))
@@ -39,6 +40,21 @@
 # location per the python implementation of tempfile.mkdtemp().
 tmp_dirs = ['/nfs/tmp2', '/var/tmp', '/tmp']
 
+#
+# SYS_TYPE to use for the spack installation.
+# Value of this determines what platform spack thinks it is by
+# default.  You can assign three types of values:
+# 1. None
+#    Spack will try to determine the sys_type automatically.
+#
+# 2. A string
+#    Spack will assume that the sys_type is hardcoded to the value.
+#
+# 3. A function that returns a string:
+#    Spack will use this function to determine the sys_type.
+#
+sys_type = None
+
 # Important environment variables
 SPACK_NO_PARALLEL_MAKE = 'SPACK_NO_PARALLEL_MAKE'
 SPACK_LIB = 'SPACK_LIB'
diff --git a/lib/spack/spack/multi_function.py b/lib/spack/spack/multi_function.py
new file mode 100644
index 0000000000..7d21b02f80
--- /dev/null
+++ b/lib/spack/spack/multi_function.py
@@ -0,0 +1,146 @@
+"""This module contains utilities for using multi-functions in spack.
+You can think of multi-functions like overloaded functions -- they're
+functions with the same name, and we need to select a version of the
+function based on some criteria.  e.g., for overloaded functions, you
+would select a version of the function to call based on the types of
+its arguments.
+
+For spack, we might want to select a version of the function based on
+the platform we want to build a package for, or based on the versions
+of the dependencies of the package.
+"""
+import sys
+import functools
+
+import arch
+import spack.error as serr
+
+class NoSuchVersionError(serr.SpackError):
+    """Raised when we can't find a version of a function for a platform."""
+    def __init__(self, fun_name, sys_type):
+        super(NoSuchVersionError, self).__init__(
+            "No version of %s found for %s!" % (fun_name, sys_type))
+
+
+class PlatformMultiFunction(object):
+    """This is a callable type for storing a collection of versions
+       of an instance method.  The platform decorator (see docs below)
+       creates PlatformMultiFunctions and registers function versions
+       with them.
+
+       To register a function, you can do something like this:
+           pmf = PlatformMultiFunction()
+           pmf.regsiter("chaos_5_x86_64_ib", some_function)
+
+       When the pmf is actually called, it selects a version of
+       the function to call based on the sys_type of the object
+       it is called on.
+
+       See the docs for the platform decorator for more details.
+    """
+    def __init__(self, default=None):
+        self.function_map = {}
+        self.default = default
+        if default:
+            self.__name__ = default.__name__
+
+    def register(self, platform, function):
+        """Register a version of a function for a particular sys_type."""
+        self.function_map[platform] = function
+        if not hasattr(self, '__name__'):
+            self.__name__ = function.__name__
+        else:
+            assert(self.__name__ == function.__name__)
+
+    def __get__(self, obj, objtype):
+        """This makes __call__ support instance methods."""
+        return functools.partial(self.__call__, obj)
+
+    def __call__(self, package_self, *args, **kwargs):
+        """Try to find a function that matches package_self.sys_type.
+           If none is found, call the default function that this was
+           initialized with.  If there is no default, raise an error.
+        """
+        sys_type = package_self.sys_type
+        function = self.function_map.get(sys_type, self.default)
+        if function:
+            function(package_self, *args, **kwargs)
+        else:
+            raise NoSuchVersionError(self.__name__, sys_type)
+
+    def __str__(self):
+        return "<%s, %s>" % (self.default, self.function_map)
+
+
+class platform(object):
+    """This annotation lets packages declare platform-specific versions
+       of functions like install().  For example:
+
+       class SomePackage(Package):
+           ...
+
+           def install(self, prefix):
+               # Do default install
+
+           @platform('chaos_5_x86_64_ib')
+           def install(self, prefix):
+               # This will be executed instead of the default install if
+               # the package's sys_type() is chaos_5_x86_64_ib.
+
+           @platform('bgqos_0")
+           def install(self, prefix):
+               # This will be executed if the package's sys_type is bgqos_0
+
+       This allows each package to have a default version of install() AND
+       specialized versions for particular platforms.  The version that is
+       called depends on the sys_type of SomePackage.
+
+       Note that this works for functions other than install, as well.  So,
+       if you only have part of the install that is platform specific, you
+       could do this:
+
+       class SomePackage(Package):
+           ...
+
+           def setup(self):
+               # do nothing in the default case
+               pass
+
+           @platform('chaos_5_x86_64_ib')
+           def setup(self):
+               # do something for x86_64
+
+           def install(self, prefix):
+               # Do common install stuff
+               self.setup()
+               # Do more common install stuff
+
+       If there is no specialized version for the package's sys_type, the
+       default (un-decorated) version will be called.  If there is no default
+       version and no specialized version, the call raises a
+       NoSuchVersionError.
+
+       Note that the default version of install() must *always* come first.
+       Otherwise it will override all of the platform-specific versions.
+       There's not much we can do to get around this because of the way
+       decorators work.
+    """
+class platform(object):
+    def __init__(self, sys_type):
+        self.sys_type = sys_type
+
+    def __call__(self, fun):
+        # Record the sys_type as an attribute on this function
+        fun.sys_type = self.sys_type
+
+        # Get the first definition of the function in the calling scope
+        calling_frame = sys._getframe(1).f_locals
+        original_fun = calling_frame.get(fun.__name__)
+
+        # Create a multifunction out of the original function if it
+        # isn't one already.
+        if not type(original_fun) == PlatformMultiFunction:
+            original_fun = PlatformMultiFunction(original_fun)
+
+        original_fun.register(self.sys_type, fun)
+        return original_fun
diff --git a/lib/spack/spack/package.py b/lib/spack/spack/package.py
index ae94690971..63bcf7a630 100644
--- a/lib/spack/spack/package.py
+++ b/lib/spack/spack/package.py
@@ -14,7 +14,7 @@
 import os
 import re
 import subprocess
-import platform
+import platform as py_platform
 import shutil
 
 from spack import *
@@ -24,6 +24,8 @@
 import validate
 import version
 import arch
+
+from multi_function import platform
 from stage import Stage
 
 
@@ -226,7 +228,7 @@ class SomePackage(Package):
     clean() (some of them do this), and others to provide custom behavior.
     """
 
-    def __init__(self, arch=arch.sys_type()):
+    def __init__(self, sys_type=arch.sys_type()):
         attr.required(self, 'homepage')
         attr.required(self, 'url')
         attr.required(self, 'md5')
@@ -235,7 +237,7 @@ def __init__(self, arch=arch.sys_type()):
         attr.setdefault(self, 'parallel', True)
 
         # Architecture for this package.
-        self.arch = arch
+        self.sys_type = sys_type
 
         # Name of package is the name of its module (the file that contains it)
         self.name = inspect.getmodulename(self.module.__file__)
@@ -266,7 +268,7 @@ def __init__(self, arch=arch.sys_type()):
         self.dirty = False
 
         # stage used to build this package.
-        Self.stage = Stage(self.stage_name, self.url)
+        self.stage = Stage(self.stage_name, self.url)
 
 
     def add_commands_to_module(self):
@@ -289,7 +291,7 @@ def add_commands_to_module(self):
         # standard CMake arguments
         m.std_cmake_args = ['-DCMAKE_INSTALL_PREFIX=%s' % self.prefix,
                             '-DCMAKE_BUILD_TYPE=None']
-        if platform.mac_ver()[0]:
+        if py_platform.mac_ver()[0]:
             m.std_cmake_args.append('-DCMAKE_FIND_FRAMEWORK=LAST')
 
         # Emulate some shell commands for convenience
@@ -361,11 +363,13 @@ def all_dependents(self):
     def stage_name(self):
         return "%s-%s" % (self.name, self.version)
 
-
+    #
+    # Below properties determine the path where this package is installed.
+    #
     @property
     def platform_path(self):
         """Directory for binaries for the current platform."""
-        return new_path(install_path, self.arch)
+        return new_path(install_path, self.sys_type)
 
 
     @property
@@ -388,6 +392,9 @@ def prefix(self):
 
     def remove_prefix(self):
         """Removes the prefix for a package along with any empty parent directories."""
+        if self.dirty:
+            return
+
         if os.path.exists(self.prefix):
             shutil.rmtree(self.prefix, True)
 
@@ -448,10 +455,15 @@ def do_install(self):
             self.install(self.prefix)
             if not os.path.isdir(self.prefix):
                 tty.die("Install failed for %s.  No install dir created." % self.name)
+
         except subprocess.CalledProcessError, e:
-            if not self.dirty:
-                self.remove_prefix()
+            self.remove_prefix()
             tty.die("Install failed for %s" % self.name, e.message)
+
+        except KeyboardInterrupt, e:
+            self.remove_prefix()
+            raise
+
         except Exception, e:
             if not self.dirty:
                 self.remove_prefix()
@@ -576,7 +588,7 @@ def __str__(self):
 
 
 def depends_on(*args, **kwargs):
-    """Adds a depends_on local variable in the locals of
+    """Adds a dependencies local variable in the locals of
        the calling class, based on args.
     """
     # This gets the calling frame so we can pop variables into it
diff --git a/lib/spack/spack/packages/__init__.py b/lib/spack/spack/packages/__init__.py
index e571134538..c32738bd6a 100644
--- a/lib/spack/spack/packages/__init__.py
+++ b/lib/spack/spack/packages/__init__.py
@@ -10,6 +10,7 @@
 import spack.arch as arch
 import spack.version as version
 import spack.attr as attr
+import spack.error as serr
 
 # Valid package names
 valid_package = r'^[a-zA-Z0-9_-]*$'
@@ -19,6 +20,13 @@
 
 instances = {}
 
+class InvalidPackageNameError(serr.SpackError):
+    """Raised when we encounter a bad package name."""
+    def __init__(self, name):
+        super(InvalidPackageNameError, self).__init__(
+            "Invalid package name: " + name)
+        self.name = name
+
 
 def valid_name(pkg):
     return re.match(valid_package, pkg) and not re.search(invalid_package, pkg)
@@ -26,7 +34,7 @@ def valid_name(pkg):
 
 def validate_name(pkg):
     if not valid_name(pkg):
-        raise spack.InvalidPackageNameException(pkg)
+        raise spack.InvalidPackageNameError(pkg)
 
 
 def filename_for(pkg):
@@ -36,7 +44,7 @@ def filename_for(pkg):
 
 
 def installed_packages(**kwargs):
-    """Returns a dict from SysType to lists of Package objects."""
+    """Returns a dict from systype strings to lists of Package objects."""
     list_installed = kwargs.get('installed', False)
 
     pkgs = {}
@@ -44,7 +52,7 @@ def installed_packages(**kwargs):
         return pkgs
 
     for sys_type in os.listdir(spack.install_path):
-        sys_type = arch.SysType(sys_type)
+        sys_type = sys_type
         sys_path = new_path(spack.install_path, sys_type)
         pkgs[sys_type] = [get(pkg) for pkg in os.listdir(sys_path)
                           if os.path.isdir(new_path(sys_path, pkg))]
diff --git a/lib/spack/spack/packages/libdwarf.py b/lib/spack/spack/packages/libdwarf.py
index 070fc8360f..b8a2f3e052 100644
--- a/lib/spack/spack/packages/libdwarf.py
+++ b/lib/spack/spack/packages/libdwarf.py
@@ -11,6 +11,7 @@ class Libdwarf(Package):
 
     depends_on("libelf")
 
+
     def clean(self):
         for dir in dwarf_dirs:
             with working_dir(dir):
@@ -19,6 +20,7 @@ def clean(self):
 
 
     def install(self, prefix):
+        # dwarf build does not set arguments for ar properly
         make.add_default_arg('ARFLAGS=rcs')
 
         # Dwarf doesn't provide an install, so we have to do it.
@@ -43,3 +45,9 @@ def install(self, prefix):
             install('dwarfdump',     bin)
             install('dwarfdump.conf', lib)
             install('dwarfdump.1',    man1)
+
+
+    @platform('macosx_10.8_x86_64')
+    def install(self, prefix):
+        raise UnsupportedPlatformError(
+            "libdwarf doesn't currently build on Mac OS X.")
diff --git a/lib/spack/spack/stage.py b/lib/spack/spack/stage.py
index ed48a48758..c8ffa915bc 100644
--- a/lib/spack/spack/stage.py
+++ b/lib/spack/spack/stage.py
@@ -5,8 +5,16 @@
 import getpass
 
 import spack
+import spack.error as serr
 import tty
 
+class FailedDownloadError(serr.SpackError):
+    """Raised wen a download fails."""
+    def __init__(self, url):
+        super(FailedDownloadError, self).__init__(
+            "Failed to fetch file from URL: " + url)
+        self.url = url
+
 
 class Stage(object):
     """A Stage object manaages a directory where an archive is downloaded,
@@ -161,7 +169,7 @@ def fetch(self):
                          "your internet gateway issue and install again.")
 
         if not self.archive_file:
-            raise FailedDownloadException(url)
+            raise FailedDownloadError(url)
 
         return self.archive_file
 
diff --git a/lib/spack/spack/utils.py b/lib/spack/spack/utils.py
index 0128d283cf..09e4985275 100644
--- a/lib/spack/spack/utils.py
+++ b/lib/spack/spack/utils.py
@@ -26,7 +26,7 @@ def memoized(obj):
     def memoizer(*args, **kwargs):
         if args not in cache:
             cache[args] = obj(*args, **kwargs)
-            return cache[args]
+        return cache[args]
     return memoizer
 
 
diff --git a/lib/spack/spack/version.py b/lib/spack/spack/version.py
index 75b09c2f5a..52b89883f7 100644
--- a/lib/spack/spack/version.py
+++ b/lib/spack/spack/version.py
@@ -2,7 +2,29 @@
 import re
 
 import utils
-from exception import *
+import spack.error as serr
+
+
+class VersionParseError(serr.SpackError):
+    """Raised when the version module can't parse something."""
+    def __init__(self, msg, spec):
+        super(VersionParseError, self).__init__(msg)
+        self.spec = spec
+
+
+class UndetectableVersionError(VersionParseError):
+    """Raised when we can't parse a version from a string."""
+    def __init__(self, spec):
+        super(UndetectableVersionError, self).__init__(
+            "Couldn't detect version in: " + spec, spec)
+
+
+class UndetectableNameError(VersionParseError):
+    """Raised when we can't parse a package name from a string."""
+    def __init__(self, spec):
+        super(UndetectableNameError, self).__init__(
+            "Couldn't parse package name in: " + spec)
+
 
 class Version(object):
     """Class to represent versions"""
@@ -32,6 +54,15 @@ def component(self, i):
         else:
             return None
 
+    def up_to(self, index):
+        """Return a version string up to the specified component, exclusive.
+           e.g., if this is 10.8.2, self.up_to(2) will return '10.8'.
+        """
+        return '.'.join(str(x) for x in self[:index])
+
+    def __getitem__(self, idx):
+        return tuple(self.version[idx])
+
     def __repr__(self):
         return self.version_string
 
@@ -123,7 +154,7 @@ def parse_version_string_with_indices(spec):
         if match and match.group(1) is not None:
             return match.group(1), match.start(1), match.end(1)
 
-    raise UndetectableVersionException(spec)
+    raise UndetectableVersionError(spec)
 
 
 def parse_version(spec):
@@ -162,7 +193,7 @@ def parse_name(spec, ver=None):
         match = re.search(nt, spec)
         if match:
             return match.group(1)
-    raise UndetectableNameException(spec)
+    raise UndetectableNameError(spec)
 
 def parse(spec):
     ver = parse_version(spec)