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<!-- Author(s):   bartv                                              -->
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<!-- Date:        2002/02/24                                         -->
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<part id="hal-synth-arch"><title>eCos Synthetic Target</title>

<!-- {{{ Overview                       -->

<refentry id="synth">
  <refmeta>
    <refentrytitle>Overview</refentrytitle>
  </refmeta>
  <refnamediv>
    <refname>Overview</refname>
    <refpurpose>The eCos synthetic target</refpurpose>
  </refnamediv>

  <refsect1 id="synth-description"><title>Description</title>
    <para>
Usually eCos runs on either a custom piece of hardware, specially
designed to meet the needs of a specific application, or on a
development board of some sort that is available before the final
hardware. Such boards have a number of things in common:
    </para>
    <orderedlist>
      <listitem><para>
Obviously there has to be at least one processor to do the work.
Usually this will be a 32-bit processor, but it can be smaller or
larger. Processor speed will vary widely, depending on the expected
needs of the application. However usually the exact processor being
used does not matter very much for most of the development process:
the use of languages such as C or C++ means that the compiler will
handle those details.
      </para></listitem>
      <listitem><para>
There needs to be memory for code and for data. A typical system will
have two different types of memory. There will be some non-volatile
memory such as flash, EPROM or masked ROM. There will also be some
volatile memory such as DRAM or SRAM. Often the code for the final
application will reside in the non-volatile memory and all of the RAM
will be available for data. However updating non-volatile memory
requires a non-trivial amount of effort, so for much of the
development process it is more convenient to burn suitable firmware,
for example RedBoot, into the non-volatile memory and then use that to
load the application being debugged into RAM, alongside the
application data and a small area reserved for use by the firmware.
      </para></listitem>
      <listitem><para>
The platform must provide certain mimimal I/O facilities. Most eCos
configurations require a clock signal of some sort. There must also be
some way of outputting diagnostics to the user, often but not always 
via a serial port. Unless special debug hardware is being used, source
level debugging will require bidirectional communication between a
host machine and the target hardware, usually via a serial port or an
ethernet device.
      </para></listitem>
      <listitem><para>
All the above is not actually very useful yet because there is no way
for the embedded device to interact with the rest of the world, except
by generating diagnostics. Therefore an embedded device will have
additional I/O hardware. This may be fairly standard hardware such as
an ethernet or USB interface, or special hardware designed
specifically for the intended application, or quite often some
combination. Standard hardware such as ethernet or USB may be
supported by eCos device drivers and protocol stacks, whereas the
special hardware will be driven directly by application code.
      </para></listitem>
    </orderedlist>
    <para>
Much of the above can be emulated on a typical PC running Linux.
Instead of running the embedded application being developed on a
target board of some sort, it can be run as a Linux process. The
processor will be the PC's own processor, for example an x86, and the
memory will be the process' address space. Some I/O facilities can be
emulated through system calls. For example clock hardware can be
emulated by setting up a <literal>SIGALRM</literal> signal, which will
cause the process to be interrupted at regular intervals. This
emulation of real hardware will not be particularly accurate, for
example the number of cpu cycles available to the eCos application
between clock ticks will vary widely depending on what else is running
on the PC, but for much development work it will be good enough.
    </para>
    <para>
A key requirement for synthetic target code is that the embedded
application must not be linked with any of the standard Linux
libraries such as the GNU C library: that would lead to a confusing
situation where both eCos and the Linux libraries attempted to provide
functions such as <function>printf</function>. Instead the synthetic
target support must be implemented directly on top of the Linux
kernels' system call interface. For example, the kernel provides a
system call for write operations. The actual function
<function>write</function> is implemented in the system's C library,
but all it does is move its arguments on to the stack or into certain
registers and then execute a special trap instruction such as
<literal>int&nbsp;0x80</literal>. When this instruction is executed
control transfers into the kernel, which will validate the arguments
and perform the appropriate operation. Now, a synthetic target
application cannot be linked with the system's C library. Instead it
contains a function <function>cyg_hal_sys_write</function> which, like
the C library's <function>write</function> function, pushes its
arguments on to the stack and executes the trap instruction. The Linux
kernel cannot tell the difference, so it will perform the I/O
operation requested by the synthetic target. With appropriate
knowledge of what system calls are available, this makes it possible
to emulate the required I/O facilities. For example diagnostic output
can be achieved by a <function>cyg_hal_sys_write</function> system
call using file descriptor 1, which corresponds to stdout.
    </para>
    <para>
In many ways developing for the synthetic target is no different from
developing for real embedded targets. eCos must be configured
appropriately: selecting a suitable target such as
<userinput>i386linux</userinput> will cause the configuration system
to load the appropriate packages for this hardware; this includes an
architectural HAL package and a platform-specific package; the
architectural package contains generic code applicable to all Linux
platforms, whereas the platform package is for specific Linux
implementations such as the x86 version and contains any
processor-specific code. Selecting this target will also bring in some
device driver packages. Other aspects of the configuration such as
which API's are supported are determined by the template, by adding
and removing packages, and by fine-grained configuration.
    </para>
    <para>
In other ways developing for the synthetic target can be much easier
than developing for a real embedded target. For example there is no
need to worry about building and installing suitable firmware on the
target hardware, and then downloading and debugging the actual
application over a serial line or a similar connection. Instead an
eCos application built for the synthetic target is mostly
indistinguishable from an ordinary Linux program. It can be run simply
by typing the name of the executable file at a shell prompt.
Alternatively you can debug the application using whichever version of
gdb is provided by your Linux distribution. There is no need to build
or install special toolchains. Essentially using the synthetic target
means that the various problems associated with real embedded hardware
can be bypassed for much of the development process.
    </para>
    <para>
The eCos synthetic target provides emulation, not simulation. It is
possible to run eCos in suitable architectural simulators but that
involves a rather different approach to software development. For
example, when running eCos on the psim PowerPC simulator you need
appropriate cross-compilation tools that allow you to build PowerPC
executables. These are then loaded into the simulator which interprets
every instruction and attempts to simulate what would happen if the
application were running on real hardware. This involves a lot of
processing overhead, but depending on the functionality provided by
the simulator it can give very accurate results. When developing for
the synthetic target the executable is compiled for the PC's own
processor and will be executed at full speed, with no need for a
simulator or special tools. This will be much faster and somewhat
simpler than using an architectural simulator, but no attempt is made
to accurately match the behaviour of a real embedded target.
    </para>
    <note><para>
Support for emulating standard hardware such as ethernet is not yet
available in the current release of the eCos synthetic target code,
but is planned for future versions. Emulating application-specific
custom hardware is rather more difficult since there is no way of
knowing just what strange hardware a particular application might
need, but the I/O mechanisms planned should allow for
application-specific extensions.
    </para></note>
  </refsect1>

  <refsect1 id="synth-tools"><title>Toolchain</title>
    <para>
When developing eCos applications for a normal embedded target it is
necessary to use a suitable cross-compiler and related tools such as
the linker. Developing for the synthetic target is easier because you
can just use the standard GNU tools (gcc, g++, ld, &hellip;) which
were provided with your Linux distribution, or which you used to build
your own Linux setup. Any reasonably recent version of the tools, for
example gcc 2.96(Red Hat) as shipped with Red Hat Linux 7, should be
sufficient.
    </para>
    <para>
There is one important limitation when using these tools: current gdb
will not support debugging of eCos threads on the synthetic target. As
far as gdb is concerned a synthetic target application is also
indistinguishable from a normal Linux application, so it assumes that
any threads will be created by calls to the Linux
<function>pthread_create</function> function provided by the C
library. Obviously this is not the case since the application is never
linked with that library. Therefore gdb never notices the eCos thread
mechanisms and assumes the application is single-threaded. Fixing this
is possible but would involve non-trivial changes to gdb.
    </para>
    <para>
Theoretically it is possible to develop synthetic target applications
on, for example, a PC running Windows and then run the resulting
executables on another machine that runs Linux. This is rarely useful:
if a Linux machine is available then usually that machine will also be
used for building ecos and the application. However, if for some
reason it is necessary or desirable to build on another machine then
this requires a suitable cross-compiler and related tools. If the
application will be running on a typical PC with an x86 processor then
a suitable configure triplet would be
<userinput>i686-pc-linux-gnu</userinput>. The installation
instructions for the various GNU tools should be consulted for further
information. 
    </para>
  </refsect1>

  <refsect1 id="synth-hardware"><title>Hardware Preparation</title>
    <para>
Preparing a real embedded target for eCos development can be tricky.
Often the first step is to install suitable firmware, usually RedBoot.
This means creating and building a special configuration for eCos with
the RedBoot template, then somehow updating the target's flash chips
with the resulting RedBoot image. Typically it will also be necessary
to get a working serial connection, and possibly set up ethernet as
well. Although usually none of the individual steps are particularly
complicated, there are plenty of ways in which things can go wrong and
it can be hard to figure out what is actually happening. Of course
some board manufacturers make life easier for their developers by
shipping hardware with RedBoot preinstalled, but even then it is still
necessary to set up communication between host and target.
    </para>
    <para>
None of this is applicable to the synthetic target. Instead you can
just build a normal eCos configuration, link your application with the
resulting libraries, and you end up with an executable that you can
run directly on your Linux machine or via gdb. A useful side effect of
this is that application development can start before any real
embedded hardware is actually available.
    </para>
    <para>
Typically the memory map for a synthetic target application will be
set up such that there is a read-only ROM region containing all the
code and constant data, and a read-write RAM region for the data. The
default locations and sizes of these regions depend on the specific
platform being used for development. Note that the application always
executes out of ROM: on a real embedded target much of the development
would involve running RedBoot firmware there, with application code
and data loaded into RAM; usually this would change for the final
system; the firmware would be removed; and the application itself
would execute from ROM and perform the appropriate hardware
initialization. Therefore the synthetic target actually emulates the
behaviour of the final system, not the development environment.
    </para>
  </refsect1>

</refentry>

<!-- }}} -->
<!-- {{{ System calls                   -->

<refentry id="synth-syscalls">
  <refmeta>
    <refentrytitle>System Calls</refentrytitle>
  </refmeta>

  <refnamediv>
    <refname>cyg_hal_sys_xyz</refname>
    <refpurpose>Access Linux system facilities</refpurpose>
  </refnamediv>

  <refsynopsisdiv>
    <funcsynopsis>
      <funcsynopsisinfo>
#include &lt;cyg/hal/hal_io.h
      </funcsynopsisinfo>
      <funcprototype>
        <funcdef>int <function>cyg_hal_sys_xyz</function></funcdef>
        <varargs>
      </funcprototype>
    </funcsynopsis>
  </refsynopsisdiv>

  <refsect1 id="synth-syscalls-description"><title>Description</title>
    <para>
On a real embedded target eCos interacts with the hardware by peeking
and poking various registers, manipulating special regions of memory,
and so on. The synthetic target does not access hardware directly.
Instead I/O and other operations are emulated by making appropriate
Linux system calls. The HAL package exports a number of functions
which allow other packages, or even application code, to make these
same system calls. However this facility must be used with care: any
code which calls, for example, <function>cyg_hal_sys_write</function>
will only ever run on the synthetic target; that functionality is
obviously not provided on any real hardware because there is no
underlying Linux kernel to implement it.
    </para>
    <para>
The synthetic target only provides a subset of the available system
calls, specifically those calls which have proved useful to implement
I/O emulation. This subset can be extended fairly easily if necessary.
All of the available calls, plus associated data structures and
macros, are defined in the header file <filename
class="headerfile">cyg/hal/hal_io.h</filename>. There is a simple
convention: given a Linux system call such as
<function>open</function>, the synthetic target will prefix
<literal>cyg_hal_sys</literal> and provide a function with that name.
The second argument to the <function>open</function> system call is
a set of flags such as <constant>O_RDONLY</constant>, and the header
file will define a matching constant
<constant>CYG_HAL_SYS_O_RDONLY</constant>. There are also data
structures such as <structname>cyg_hal_sys_sigset_t</structname>,
matching the Linux data structure <structname>sigset_t</structname>.
    </para>
    <para>
In most cases the functions provided by the synthetic target behave as
per the documentation for the Linux system calls, and section 2 of the
Linux man pages can be consulted for more information. There is one
important difference: typically the documentation will say that a
function returns <literal>-1</literal> to indicate an error, with the
actual error code held in <varname>errno</varname>; the actual
underlying system call and hence the
<function>cyg_hal_sys_xyz</function> provided by eCos instead returns
a negative number to indicate an error, with the absolute value of
that number corresponding to the error code; usually it is the C
library which handles this and manipulates errno, but of course
synthetic target applications are not linked with that Linux library.
    </para>
    <para>
However, there are some exceptions. The Linux kernel has evolved over
the years, and some of the original system call interfaces are no
longer appropriate. For example the original
<function>select</function> system call has been superseded by
<function>_newselect</function>, and that is what the
<function>select</function> function in the C library actually uses.
The old call is still available to preserve binary compatibility but,
like the C library, eCos makes use of the new one because it provides
the appropriate functionality. In an attempt to reduce confusion the
eCos function is called <function>cyg_hal_sys__newselect</function>,
in other words it matches the official system call naming scheme. The
authoritive source of information on such matters is the Linux kernel
sources themselves, and especially its header files.
    </para>
    <para>
eCos packages and applications should never
<literal>#include</literal> Linux header files directly. For example,
doing a <literal>#include&nbsp;&lt;/usr/include/fcntl.h&gt;</literal>
to access additional macros or structure definitions, or alternatively
manipulating the header file search path, will lead to problems
because the Linux header files are likely to duplicate and clash with
definitions in the eCos headers. Instead the appropriate functionality
should be extracted from the Linux headers and moved into either
<filename class="headerfile">cyg/hal/hal_io.h</filename> or into
application code, with suitable renaming to avoid clashes with eCos
names. Users should be aware that large-scale copying may involve
licensing complications.
    </para>
    <para>
Adding more system calls is usually straightforward and involves
adding one or more lines to the platform-specific file in the
appropriate platform HAL, for example
<filename>syscall-i386-linux-1.0.S</filename>. However it is necessary
to do some research first about the exact interface implemented by the
system call, because of issues such as old system calls that have been
superseded. The required information can usually be found fairly
easily by searching through the Linux kernel sources and possibly the
GNU C library sources.
    </para>
  </refsect1>
</refentry>

<!-- }}} -->
<!-- {{{ Porting                        -->

<refentry id="synth-porting">
  <refmeta>
    <refentrytitle>Porting</refentrytitle>
  </refmeta>
  <refnamediv>
    <refname>Porting</refname>
    <refpurpose>The eCos synthetic target</refpurpose>
  </refnamediv>

  <refsect1 id="synth-porting-description"><title>Description</title>
    <para>
The initial development effort of the eCos synthetic target happened
on x86 Linux machines. Porting to other platforms involves addressing
a number of different issues. Some ports should be fairly
straightforward, for example a port to Linux on a processor other than
an x86. Porting to Unix or Unix-like operating systems other than
Linux may be possible, but would involve more effort. Porting to a
completely different operating system such as Windows would be very
difficult. The text below completes the eCos Porting Guide.
    </para>
  </refsect1>

  <refsect1 id="synth-porting-linux"><title>Other Linux Platforms</title>
    <para>
Porting the synthetic target to a Linux platform that uses a processor
other than x86 should be straightforward. The simplest approach is to
copy the existing <filename class="directory">i386linux</filename>
directory tree in the <filename class="directory">hal/synth</filename>
hierarchy, then rename and edit the ten or so files in this package.
Most of the changes should be pretty obvious, for example on a 64-bit
processor some new data types will be needed in the
<filename>basetype.h</filename> header file. It will also be necessary
to update the toplevel <filename>ecos.db</filename> database with an
entry for the new HAL package, and a new target entry will be needed
as well.
    </para>
    <para>
Obviously a different processor will have different register sets
and calling conventions, so the code for saving and restoring thread
contexts and for implementing <function>setjmp</function> and
<function>longjmp</function> will need to be updated. The exact way of
performing Linux system calls will also vary: on i386linux this
usually involves pushing some registers on the stack and then
executing an <literal>int&nbsp;0x080</literal> trap instruction, but
on a different processor the arguments might be passed in registers
instead and certainly a different trap instruction will be used. The
startup code is also written in assembler, but needs to do little more
than jump to the main <function>linux_entry</function> function
provided by the architectural synthetic target HAL package.
    </para>
    <para>
The header file <filename>hal_io.h</filename> provided by the
architectural HAL package provides various structure definitions,
function prototypes, and macros related to system calls. These are
correct for i386linux, but there may be problems on other processors.
For example a structure field that is currently defined as a 32-bit
number may in fact may be a 64-bit number instead.
    </para>
    <para>
The synthetic target's memory map is defined in two files in the
<filename class="directory">include/pkgconf</filename> subdirectory.
For x86 the default memory map involves eight megabytes of read-only
memory for the code at location 0x1000000 and another eight megabytes
for data at 0x2000000. These address ranges may be reserved for other
purposes on the new architecture, so may need changing. There may be
some additional areas of memory allocated by the system for other
purposes, for example the startup stack and any environment variables,
but usually eCos applications can and should ignore those.
    </para>
    <para>
Other HAL functionality such as interrupt handling, diagnostics, and
the system clock are provided by the architectural HAL package and
should work on different processors with few if any changes.
    </para>
    <para>
When porting to other processors, a number of sources of information
are likely to prove useful. Obviously the Linux kernel sources and
header files constitute the ultimate authority on how things work at
the system call level. The GNU C library sources may also prove very
useful: for a normal Linux application it is the C library that
provides the startup code and the system call interface.
    </para>
  </refsect1>

  <refsect1 id="synth-porting-unix"><title>Other Unix Platforms</title>
    <para>
Porting to a Unix or Unix-like operating system other than Linux would
be somewhat more involved. The first requirement involves toolchains:
the GNU compilers, gcc and g++, must definitely be used; use of other
GNU tools such as the linker may be needed as well, because eCos
depends on functionality such as prioritizing C++ static constructors,
and other linkers may not implement this or may implement it in a
different and incompatible way. A closely related requirement is the
use of ELF format for binary executables: if the operating system
still uses an older format such as COFF then there are likely to be
problems because they do not provide the flexibility required by eCos.
    </para>
    <para>
In the architectural HAL there should be very little code that is
specific to Linux. Instead the code should work on any operating
system that provides a reasonable implementation of the POSIX
standard. There may be some problems with program startup, but those
could be handled at the architectural level. Some changes may also be
required to the exception handling code. However one file which will
present a problem is <filename>hal_io.h</filename>, which contains
various structure definitions and macros used with the system call
interface. It is likely that many of these definitions will need
changing, and it may well be appropriate to implement variant HAL
packages for the different operating systems where this information
can be separated out. Another possible problem is that the generic
code assumes that system calls such as
<function>cyg_hal_sys_write</function> are available. On an operating
system other than Linux it is possible that some of these are not
simple system calls, and instead wrapper functions will need to be
implemented at the variant HAL level.
    </para>
    <para>
The remaining porting task is to implement one or more platform HAL
packages, one per processor type that is supported. This should
involve much the same work as a port to <link
linkend="synth-porting-linux">another processor running Linux</link>.
    </para>
    <para>
When using other Unix operating systems the kernel source code may not
be available, which would make any porting effort more challenging.
However there is still a good chance that the GNU C library will have
been ported already, so its source code may contain much useful
information. 
    </para>
    <caution><para>
Users should be aware that future enhancements to the synthetic target
code, especially in areas related to I/O, may be much more closely
tied to Linux. For example access to ethernet facilities might be
implemented using the <literal>PF_PACKET</literal> socket address
family, or the virtual tunnelling interface. Porting such code to
other Unix operating systems may prove rather more difficult.
    </para></caution>
  </refsect1>

  <refsect1 id="synth-porting-other"><title>Windows Platforms</title>
    <para>
Porting the current synthetic target code to some version of Windows
or to another non-Unix platform is likely to prove very difficult. The
first hurdle that needs to be crossed is the file format for binary
executables: current Windows implementations do not use ELF, instead
they use their own format PE which is a variant of the rather old and
limited COFF format. It may well prove easier to first write an ELF
loader for Windows executables, rather than try to get eCos to work
within the constraints of PE. Of course that introduces new problems,
for example existing source-level debuggers will still expect
executables to be in PE format.
    </para>
    <para>
Under Linux a synthetic target application is not linked with the
system's C library or any other standard system library. That would
cause confusion, for example both eCos and the system's C library
might try to define the <function>printf</function> function, and
introduce complications such as working with shared libraries. For
much the same reasons, a synthetic target application under Windows
should not be linked with any Windows DLL's. If an ELF loader has been
specially written then this may not be much of a problem.
    </para>
    <para>
The next big problem is the system call interface. Under Windows
system calls are generally made via DLL's, and it is not clear that
the underlying trap mechanism is well-documented or consistent between
different releases of Windows.
    </para>
    <para>
The current code depends on the operating system providing an
implementation of POSIX signal handling. This is used for I/O
purposes, for example <literal>SIGALRM</literal> is used for the
system clock, and for exceptions. It is not known what equivalent
functionality is available under Windows.
    </para>
    <para>
Given the above problems a port of the synthetic target to Windows may
or may not be technically feasible, but it would certainly require a
very large amount of effort.
    </para>
  </refsect1>

</refentry>

<!-- }}} -->

</part>
