mem: Delete the now unused Message*Port classes.

This port type is no longer used.

Change-Id: If4abbb774819644bea58fd82e00dfdec8f79b5a6
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/20822
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Gabe Black <gabeblack@google.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Gabe Black <gabeblack@google.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>

mem: Mark MemObject as deprecated.

It's constructor will now warn that it's deprecated and suggest using
ClockedObject directly. This change also gets rid of the params()
method and the Params typedef since they are functionally equivalent to
the ClockedObject versions.

It also removes the include of mem/port.hh which is not used in
mem_object.hh. This may break code which purposefully or (more likely)
accidentally depended on that transitive include from mem_object.hh.

Change-Id: I6dab3ba626e3f3ab6a6bd86edcf4f5cb4d6d2c45
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/20720
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>

mem: Put gem5 protocols in their own directory.

This reduces clutter in the src/mem directory, and makes it clear that
those protocols are for the classic gem5 memory system, not ruby, TLM,
etc.

Change-Id: I6cf6b21134d82f4f01991e4fe92dbea8c7e82081
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/20231
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Gabe Black <gabeblack@google.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>

mem: Split the various protocols out of the gem5 master/slave ports.

This makes the protocols easier to see in their entirity, and makes it
easier to add a new type of port which only supports the functional
protocol.

Change-Id: If5d639bef45062f0a23af2ac46f50933e6a8f144
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/20228
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>

arm, mem: Move the SecurePortProxy subclass into it's own file.

The idea of a "secure" memory area/access is specific to ARM and
shouldn't be in the common mem directory, although it's built in to the
generic memory protocol at this point.

Regardless, it should minimially be in its own file like the virtual
and physical port proxy classes are.

Change-Id: I140d4566ee2deded784adb04bcf6f11755a85c0c
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/18569
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>

mem: Add a QoS-aware Memory Controller type

This is the implementation of QoS algorithms support for gem5 memory
objects. This change-list provides a framework for specifying QoS
algorithm which can be used to prioritise service to specific masters in
the memory controller.
The QoS support implemented here is designed to be extendable so that
new QoS algorithms can be easily plugged into the memory controller as
"QoS Policies".

Change-Id: I0b611f13fce54dd1dd444eb806f8e98afd248bd5
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/11970
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>

mem: Add a memory delay simulator

Add a memory system component that delays traffic. The base
functionality to delay packets is implemented in the abstract MemDelay
class. This class exposes three methods that control packet delays:

* delayReq(pkt)
* delayResp(pkt)
* delaySnoopResp(pkt)

These methods should be specialized to implement delays for specific
packet types.

The class SimpleMemDelay uses the MemDelay base class to implement
constant delays for read/write requests and responses.

The intention is that these classes can be used for rapid prototyping
of components that add a small fixed delay and the same throughput as
the interconnect. I.e., any buffering done in the base class will be
small and proportional to the introduced delay.

Change-Id: I158cb85f20e32bfdbcbfed66a785b4b2dd47b628
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Nicholas Lindsey <nicholas.lindsay@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/11521
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>

x86, mem: Rewrite the multilevel page table class.

The new version extracts all the x86 specific aspects of the class,
and builds the interface around a variable collection of template
arguments which are classes that represent the different levels of the
page table. The multilevel page table class is now much more ISA
independent.

Change-Id: Id42e168a78d0e70f80ab2438480cb6e00a3aa636
Reviewed-on: https://gem5-review.googlesource.com/7347
Reviewed-by: Brandon Potter <Brandon.Potter@amd.com>
Maintainer: Gabe Black <gabeblack@google.com>

mem: hmc: serial link model

This changeset adds a serial link model for the Hybrid Memory Cube (HMC).
SerialLink is a simple variation of the Bridge class, with the ability to
account for the latency of packet serialization. Also trySendTiming has been
modified to correctly model bandwidth.

Committed by: Nilay Vaish <nilay@cs.wisc.edu>

mem: hmc: adds controller

This patch models a simple HMC Controller. It simply schedules the incoming
packets to HMC Serial Links using a round robin mechanism. This patch should
be applied in series with other patches modeling a complete HMC device.

Committed by: Nilay Vaish <nilay@cs.wisc.edu>

mem: Move trace functionality from the CommMonitor to a probe

This changeset moves the access trace functionality from the
CommMonitor into a separate probe. The probe can be hooked up to any
component that exports probe points of the type ProbePoints::Packet.

This patch moves the dependency on Google's Protocol Buffers library
from the CommMonitor to the MemTraceProbe, which means that the
CommMonitor (including stack distance profiling) no long depends on
it.

mem: Redesign the stack distance calculator as a probe

This changeset removes the stack distance calculator hooks from the
CommMonitor class and implements a stack distance calculator as a
memory system probe instead. The probe can be hooked up to any
component that exports probe points of the type ProbePoints::Packet.

mem: Add a stack distance calculator

This patch adds a stand-alone stack distance calculator. The stack
distance calculator is a passive SimObject that observes the addresses
passed to it. It calculates stack distances (LRU Distances) of
incoming addresses based on the partial sum hierarchy tree algorithm
described by Alamasi et al. http://doi.acm.org/10.1145/773039.773043.

For each transaction a hashtable look-up is performed. At every
non-unique transaction the tree is traversed from the leaf at the
returned index to the root, the old node is deleted from the tree, and
the sums (to the right) are collected and decremented. The collected
sum represets the stack distance of the found node. At every unique
transaction the stack distance is returned as
numeric_limits<uint64>::max().

In addition to the basic stack distance calculation, a feature to mark
an old node in the tree is added. This is useful if it is required to
see the reuse pattern. For example, Writebacks to the lower level
(e.g. membus from L2), can be marked instead of being removed from the
stack (isMarked flag of Node set to True). And then later if this same
address is accessed (by L1), the value of the isMarked flag would be
True. This gives some insight on how the Writeback policy of the
lower level affect the read/write accesses in an application.

Debugging is enabled by setting the verify flag to true. Debugging is
implemented using a dummy stack that behaves in a naive way, using STL
vectors. Note that this has a large impact on run time.

mem: Add MemChecker and MemCheckerMonitor

This patch adds the MemChecker and MemCheckerMonitor classes. While
MemChecker can be integrated anywhere in the system and is independent,
the most convenient usage is through the MemCheckerMonitor -- this
however, puts limitations on where the MemChecker is able to observe
read/write transactions.

mem: Add ExternalMaster and ExternalSlave ports

This patch adds two MemoryObject's: ExternalMaster and ExternalSlave.
Each object has a single port which can be bound to an externally-
provided bridge to a port of another simulation system at
initialisation.

mem: Add DRAMPower wrapping class

This patch adds a class to wrap DRAMPower Library in gem5.
This class initiates an object of class MemorySpecification
of the DRAMPower Library, passes the parameters from DRAMCtrl.py
to this object and creates an object of drampower library using
the memory specification.

mem: Rename Bus to XBar to better reflect its behaviour

This patch changes the name of the Bus classes to XBar to better
reflect the actual timing behaviour. The actual instances in the
config scripts are not renamed, and remain as e.g. iobus or membus.

As part of this renaming, the code has also been clean up slightly,
making use of range-based for loops and tidying up some comments. The
only changes outside the bus/crossbar code is due to the delay
variables in the packet.

mem: Simple Snoop Filter

This is a first cut at a simple snoop filter that tracks presence of lines in
the caches "above" it. The snoop filter can be applied at any given cache
hierarchy and will then handle the caches above it appropriately; there is no
need to use this only in the last-level bus.

This design currently has some limitations: missing stats, no notion of clean
evictions (these will not update the underlying snoop filter, because they are
not sent from the evicting cache down), no notion of capacity for the snoop
filter and thus no need for invalidations caused by capacity pressure in the
snoop filter. These are planned to be added on top with future change sets.

mem: adding architectural page table support for SE mode
This patch enables the use of page tables that are stored in system memory
and respect x86 specification, in SE mode. It defines an architectural
page table for x86 as a MultiLevelPageTable class and puts a placeholder
class for other ISAs page tables, giving the possibility for future
implementation.

mem: DRAMPower trace output

This patch adds a DRAMPower flag to enable off-line DRAM power
analysis using the DRAMPower tool. A new DRAMPower flag is added
and a follow-on patch adds a Python script to post-process the output
and order it based on time stamps.

The long-term goal is to link DRAMPower as a library and provide the
commands through function calls to the model rather than first
printing and then parsing the commands. At the moment it is also up to
the user to ensure that the same DRAM configuration is used by the
gem5 controller model and DRAMPower.

mem: Add DRAM power states to the controller

This patch adds power states to the controller. These states and the
transitions can be used together with the Micron power model. As a
more elaborate use-case, the transitions can be used to drive the
DRAMPower tool.

At the moment, the power-down modes are not used, and this patch
simply serves to capture the idle, auto refresh and active modes. The
patch adds a third state machine that interacts with the refresh state
machine.

mem: Rename SimpleDRAM to a more suitable DRAMCtrl

This patch renames the not-so-simple SimpleDRAM to a more suitable
DRAMCtrl. The name change is intended to ensure that we do not send
the wrong message (although the "simple" in SimpleDRAM was originally
intended as in cleverly simple, or elegant).

As the DRAM controller modelling work is being presented at ISPASS'14
our hope is that a broader audience will use the model in the future.

ruby: Move Ruby debug flags to ruby dir and remove stale options

This patch moves the Ruby-related debug flags to the ruby
sub-directory, and also removes the state SConsopts that add the
no-longer-used NO_VECTOR_BOUNDS_CHECK.

mem: Include the DRAMSim2 wrapper in NULL build

This patch makes sure DRAMSim2 is included in a build of the NULL ISA.

mem: Add a wrapped DRAMSim2 memory controller

This patch adds DRAMSim2 as a memory controller by wrapping the
external library and creating a sublass of AbstractMemory that bridges
between the semantics of gem5 and the DRAMSim2 interface.

The DRAMSim2 wrapper extracts the clock period from the config
file. There is no way of extracting this information from DRAMSim2
itself, so we simply read the same config file and get it from there.

To properly model the response queue, the wrapper keeps track of how
many transactions are in the actual controller, and how many are
stacking up waiting to be sent back as responses (in the wrapper). The
latter requires us to move away from the queued port and manage the
packets ourselves. This is due to DRAMSim2 not having any flow control
on the response path.

DRAMSim2 assumes that the transactions it is given are matching the
burst size of the choosen memory. The wrapper checks to ensure the
cache line size of the system matches the burst size of DRAMSim2 as
there are currently no provisions to split the system requests. In
theory we could allow a cache line size smaller than the burst size,
but that would lead to inefficient use of the DRAM, so for not we
fatal also in this case.

mem: Use the same timing calculation for DRAM read and write

This patch simplifies the DRAM model by re-using the function that
computes the busy and access time for both reads and writes.

arch: Resurrect the NOISA build target and rename it NULL

This patch makes it possible to once again build gem5 without any
ISA. The main purpose is to enable work around the interconnect and
memory system without having to build any CPU models or device models.

The regress script is updated to include the NULL ISA target. Currently
no regressions make use of it, but all the testers could (and perhaps
should) transition to it.

mem: Add tracing support in the communication monitor

This patch adds packet tracing to the communication monitor using a
protobuf as the mechanism for creating the trace.

If no file is specified, then the tracing is disabled. If a file is
specified, then for every packet that is successfully sent, a protobuf
message is serialized to the file.

ruby: add a prefetcher
This patch adds a prefetcher for the ruby memory system. The prefetcher
is based on a prefetcher implemented by others (well, I don't know
who wrote the original). The prefetcher does stride-based prefetching,
both unit and non-unit. It obseves the misses in the cache and trains on
these. After the training period is over, the prefetcher starts issuing
prefetch requests to the controller.

mem: Add a gasket that allows memory ranges to be re-mapped.

For example if DRAM is at two locations and mirrored this patch allows the
mirroring to occur.

DRAM: Introduce SimpleDRAM to capture a high-level controller

This patch introduces a high-level model of a DRAM controller, with a
basic read/write buffer structure, a selectable and customisable
arbiter, a few address mapping options, and the basic DRAM timing
constraints. The parameters make it possible to turn this model into
any desired DDRx/LPDDRx/WideIOx memory controller.

The intention is not to be cycle accurate or capture every aspect of a
DDR DRAM interface, but rather to enable exploring of the high-level
knobs with a good simulation speed. Thus, contrary to e.g. DRAMSim
this module emphasizes simulation speed with a good-enough accuracy.

This module is merely a starting point, and there are plenty additions
and improvements to come. A notable addition is the support for
address-striping in the bus to enable a multi-channel DRAM
controller. Also note that there are still a few "todo's" in the code
base that will be addressed as we go along.

A follow-up patch will add basic performance regressions that use the
traffic generator to exercise a few well-defined corner cases.

Bridge: Remove NACKs in the bridge and unify with packet queue

This patch removes the NACKing in the bridge, as the split
request/response busses now ensure that protocol deadlocks do not
occur, i.e. the message-dependency chain is broken by always allowing
responses to make progress without being stalled by requests. The
NACKs had limited support in the system with most components ignoring
their use (with a suitable call to panic), and as the NACKs are no
longer needed to avoid protocol deadlocks, the cleanest way is to
simply remove them.

The bridge is the starting point as this is the only place where the
NACKs are created. A follow-up patch will remove the code that deals
with NACKs in the endpoints, e.g. the X86 table walker and DMA
port. Ultimately the type of packet can be complete removed (until
someone sees a need for modelling more complex protocols, which can
now be done in parts of the system since the port and interface is
split).

As a consequence of the NACK removal, the bridge now has to send a
retry to a master if the request or response queue was full on the
first attempt. This change also makes the bridge ports very similar to
QueuedPorts, and a later patch will change the bridge to use these. A
first step in this direction is taken by aligning the name of the
member functions, as done by this patch.

A bit of tidying up has also been done as part of the simplifications.

Surprisingly, this patch has no impact on any of the
regressions. Hence, there was never any NACKs issued. In a follow-up
patch I would suggest changing the size of the bridge buffers set in
FSConfig.py to also test the situation where the bridge fills up.

ruby: banked cache array resource model

This patch models a cache as separate tag and data arrays. The patch exposes
the banked array as another resource that is checked by SLICC before a
transition is allowed to execute. This is similar to how TBE entries and slots
in output ports are modeled.

ruby: tag and data cache access support

Updates to Ruby to support statistics counting of cache accesses. This feature
serves multiple purposes beyond simple stats collection. It provides the
foundation for ruby to model the cache tag and data arrays as physical
resources, as well as provide the necessary input data for McPAT power
modeling.

Bus: Split the bus into a non-coherent and coherent bus

This patch introduces a class hierarchy of buses, a non-coherent one,
and a coherent one, splitting the existing bus functionality. By doing
so it also enables further specialisation of the two types of buses.

A non-coherent bus connects a number of non-snooping masters and
slaves, and routes the request and response packets based on the
address. The request packets issued by the master connected to a
non-coherent bus could still snoop in caches attached to a coherent
bus, as is the case with the I/O bus and memory bus in most system
configurations. No snoops will, however, reach any master on the
non-coherent bus itself. The non-coherent bus can be used as a
template for modelling PCI, PCIe, and non-coherent AMBA and OCP buses,
and is typically used for the I/O buses.

A coherent bus connects a number of (potentially) snooping masters and
slaves, and routes the request and response packets based on the
address, and also forwards all requests to the snoopers and deals with
the snoop responses. The coherent bus can be used as a template for
modelling QPI, HyperTransport, ACE and coherent OCP buses, and is
typically used for the L1-to-L2 buses and as the main system
interconnect.

The configuration scripts are updated to use a NoncoherentBus for all
peripheral and I/O buses.

A bit of minor tidying up has also been done.

MEM: Add the communication monitor

This patch adds a communication monitor MemObject that can be inserted
between a master and slave port to provide a range of statistics about
the communication passing through it. The communication monitor is
non-invasive and does not change any properties or timing of the
packets, with the exception of adding a sender state to be able to
track latency. The statistics are only collected in timing mode (not
atomic) to avoid slowing down any fast forwarding.

An example of the statistics captured by the monitor are: read/write
burst lengths, bandwidth, request-response latency, outstanding
transactions, inter transaction time, transaction count, and address
distribution. The monitor can be used in combination with periodic
resetting and dumping of stats (through schedStatEvent) to study the
behaviour over time.

In future patches, a selection of convenience scripts will be added to
aid in visualising the statistics collected by the monitor.

MEM: Enable multiple distributed generalized memories

This patch removes the assumption on having on single instance of
PhysicalMemory, and enables a distributed memory where the individual
memories in the system are each responsible for a single contiguous
address range.

All memories inherit from an AbstractMemory that encompasses the basic
behaviuor of a random access memory, and provides untimed access
methods. What was previously called PhysicalMemory is now
SimpleMemory, and a subclass of AbstractMemory. All future types of
memory controllers should inherit from AbstractMemory.

To enable e.g. the atomic CPU and RubyPort to access the now
distributed memory, the system has a wrapper class, called
PhysicalMemory that is aware of all the memories in the system and
their associated address ranges. This class thus acts as an
infinitely-fast bus and performs address decoding for these "shortcut"
accesses. Each memory can specify that it should not be part of the
global address map (used e.g. by the functional memories by some
testers). Moreover, each memory can be configured to be reported to
the OS configuration table, useful for populating ATAG structures, and
any potential ACPI tables.

Checkpointing support currently assumes that all memories have the
same size and organisation when creating and resuming from the
checkpoint. A future patch will enable a more flexible
re-organisation.

MEM: Remove legacy DRAM in preparation for memory updates

This patch removes the DRAM memory class in preparation for updates to
the memory system, with the first one introducing an abstract memory
class, and removing the assumption of a single physical memory.

MEM: Split SimpleTimingPort into PacketQueue and ports

This patch decouples the queueing and the port interactions to
simplify the introduction of the master and slave ports. By separating
the queueing functionality from the port itself, it becomes much
easier to distinguish between master and slave ports, and still retain
the queueing ability for both (without code duplication).

As part of the split into a PacketQueue and a port, there is now also
a hierarchy of two port classes, QueuedPort and SimpleTimingPort. The
QueuedPort is useful for ports that want to leave the packet
transmission of outgoing packets to the queue and is used by both
master and slave ports. The SimpleTimingPort inherits from the
QueuedPort and adds the implemention of recvTiming and recvFunctional
through recvAtomic.

The PioPort and MessagePort are cleaned up as part of the changes.

MEM: Move all read/write blob functions from Port to PortProxy

This patch moves the readBlob/writeBlob/memsetBlob from the Port class
to the PortProxy class, thus making a clear separation of the basic
port functionality (recv/send functional/atomic/timing), and the
higher-level functional accessors available on the port proxies.

There are only a few places in the code base where the blob functions
were used on ports, and they are all for peeking into the memory
system without making a normal memory access (in the memtest, and the
malta and tsunami pchip). The memtest also exemplifies how easy it is
to create a non-translating proxy if desired. The malta and tsunami
pchip used a slave port to perform a functional read, and this is now
changed to rely on the physProxy of the system (to which they already
have a pointer).

Merge with the main repo.

Merge yet again with the main repository.

Merge with main repository.

SE/FS: Turn on the page table class in FS.

SE/FS: Build in the tport in FS mode.

SE/FS: Build/expose vport in SE mode.

MEM: Add port proxies instead of non-structural ports

Port proxies are used to replace non-structural ports, and thus enable
all ports in the system to correspond to a structural entity. This has
the advantage of accessing memory through the normal memory subsystem
and thus allowing any constellation of distributed memories, address
maps, etc. Most accesses are done through the "system port" that is
used for loading binaries, debugging etc. For the entities that belong
to the CPU, e.g. threads and thread contexts, they wrap the CPU data
port in a port proxy.

The following replacements are made:
FunctionalPort > PortProxy
TranslatingPort > SETranslatingPortProxy
VirtualPort > FSTranslatingPortProxy

Ruby Debug Flags: Remove one, add another
The flag RubyStoreBuffer is being removed, instead RubySystem is being added

Ruby: Add infrastructure for recording cache contents
This patch changes CacheRecorder, CacheMemory, CacheControllers
so that the contents of a cache can be recorded for checkpointing
purposes.

Ruby: Process packet instead of RubyRequest in Sequencer
This patch changes the implementation of Ruby's recvTiming() function so
that it pushes a packet in to the Sequencer instead of a RubyRequest. This
requires changes in the Sequencer's makeRequest() and issueRequest()
functions, as they also need to operate on a Packet instead of RubyRequest.

scons: rename TraceFlags to DebugFlags

RubyPort: minor fixes to trace flag and dprintfs

ruby: get rid of ruby's Debug.hh

Get rid of the Debug class
Get rid of ASSERT and use assert
Use DPRINTFR for ProtocolTrace

ruby: Converted old ruby debug calls to M5 debug calls

This patch developed by Nilay Vaish converts all the old GEMS-style ruby
debug calls to the appropriate M5 debug calls.

SCons: Support building without an ISA

ruby: Convert most Ruby objects to M5 SimObjects.
The necessary companion conversion of Ruby objects generated by SLICC
are converted to M5 SimObjects in the following patch, so this patch
alone does not compile.
Conversion of Garnet network models is also handled in a separate
patch; that code is temporarily disabled from compiling to allow
testing of interim code.

ruby: Support for merging ALPHA_FS and ruby
Connects M5 cpu and dma ports directly to ruby sequencers and dma
sequencers. Rubymem also includes a pio port so that pio requests
and be forwarded to a special pio bus connecting to device pio
ports.

ruby: add RUBY sticky option that must be set to add ruby to the build
Default is false

ruby: Working M5 interface and updated Ruby interface.
This changeset also includes a lot of work from Derek Hower <drh5@cs.wisc.edu>

RubyMemory is now both a driver for Ruby and a port for M5. Changed
makeRequest/hitCallback interface. Brought packets (superficially)
into the sequencer. Modified tester infrastructure to be packet based.
and Ruby can be used together through the example ruby_se.py
script. SPARC parallel applications work, and the timing *seems* right
from combined M5/Ruby debug traces. To run,
% build/ALPHA_SE/m5.debug configs/example/ruby_se.py -c
tests/test-progs/hello/bin/alpha/linux/hello -n 4 -t

SCons: centralize the Dir() workaround for newer versions of scons.
Scons bug id: 2006 M5 Bug id: 308

Create a message port for sending messages as apposed to reading/writing a memory range.

SCons: add comments to SConscript documenting bug workaround

SCons: Manually specifying header only directories with Dir() works around the problem

Traceflags: Add SCons function to created a traceflag instead of having one file with them all.

Move SimObject python files alongside the C++ and fix
the SConscript files so that only the objects that are
actually available in a given build are compiled in.
Remove a bunch of files that aren't used anymore.

Rework the way SCons recurses into subdirectories, making it
automatic. The point is that now a subdirectory can be added
to the build process just by creating a SConscript file in it.
The process has two passes. On the first pass, all subdirs
of the root of the tree are searched for SConsopts files.
These files contain any command line options that ought to be
added for a particular subdirectory. On the second pass,
all subdirs of the src directory are searched for SConscript
files. These files describe how to build any given subdirectory.
I have added a Source() function. Any file (relative to the
directory in which the SConscript resides) passed to that
function is added to the build. Clean up everything to take
advantage of Source().
function is added to the list of files to be built.