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<HTML
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><HEAD
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><TITLE
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>Porting</TITLE
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"><LINK
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REL="HOME"
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TITLE="eCos Synthetic Target"
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HREF="hal-synth-arch.html"><LINK
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TITLE="Writing New Devices - host"
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><TH
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>eCos Synthetic Target</TH
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></TR
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><TR
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HREF="synth-new-host.html"
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>Prev</A
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WIDTH="80%"
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> </TD
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WIDTH="100%"></DIV
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><H1
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><A
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NAME="SYNTH-PORTING">Porting</H1
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><DIV
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CLASS="REFNAMEDIV"
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><A
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NAME="AEN1049"
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></A
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><H2
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>Name</H2
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>Porting -- Adding support for other hosts</DIV
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><DIV
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CLASS="REFSECT1"
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><A
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NAME="SYNTH-PORTING-DESCRIPTION"
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></A
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><H2
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>Description</H2
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><P
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>The initial development effort of the eCos synthetic target happened
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on x86 Linux machines. Porting to other platforms involves addressing
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a number of different issues. Some ports should be fairly
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straightforward, for example a port to Linux on a processor other than
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an x86. Porting to Unix or Unix-like operating systems other than
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Linux may be possible, but would involve more effort. Porting to a
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completely different operating system such as Windows would be very
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difficult. The text below complements the eCos Porting Guide.
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</P
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></DIV
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><DIV
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CLASS="REFSECT1"
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><A
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NAME="SYNTH-PORTING-LINUX"
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></A
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><H2
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>Other Linux Platforms</H2
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><P
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>Porting the synthetic target to a Linux platform that uses a processor
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other than x86 should be straightforward. The simplest approach is to
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copy the existing <TT
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CLASS="FILENAME"
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>i386linux</TT
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>
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directory tree in the <TT
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CLASS="FILENAME"
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>hal/synth</TT
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>
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hierarchy, then rename and edit the ten or so files in this package.
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Most of the changes should be pretty obvious, for example on a 64-bit
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processor some new data types will be needed in the
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<TT
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CLASS="FILENAME"
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>basetype.h</TT
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> header file. It will also be necessary
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to update the toplevel <TT
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CLASS="FILENAME"
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>ecos.db</TT
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> database with an
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entry for the new HAL package, and a new target entry will be needed.
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</P
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><P
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>Obviously a different processor will have different register sets and
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calling conventions, so the code for saving and restoring thread
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contexts and for implementing <TT
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CLASS="FUNCTION"
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>setjmp</TT
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> and
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<TT
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CLASS="FUNCTION"
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>longjmp</TT
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> will need to be updated. The exact way of
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performing Linux system calls will vary: on x86 linux this usually
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involves pushing some registers on the stack and then executing an
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<TT
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CLASS="LITERAL"
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>int 0x080</TT
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> trap instruction, but on a different
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processor the arguments might be passed in registers instead and
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certainly a different trap instruction will be used. The startup code
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is written in assembler, but needs to do little more than extract the
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process' argument and environment variables and then jump to the main
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<TT
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CLASS="FUNCTION"
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>linux_entry</TT
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> function provided by the
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architectural synthetic target HAL package.
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</P
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><P
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>The header file <TT
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CLASS="FILENAME"
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>hal_io.h</TT
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> provided by the
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architectural HAL package provides various structure definitions,
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function prototypes, and macros related to system calls. These are
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correct for x86 linux, but there may be problems on other processors.
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For example a structure field that is currently defined as a 32-bit
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number may in fact may be a 64-bit number instead.
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</P
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><P
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>The synthetic target's memory map is defined in two files in the
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<TT
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CLASS="FILENAME"
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>include/pkgconf</TT
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> subdirectory.
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For x86 the default memory map involves eight megabytes of read-only
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memory for the code at location 0x1000000 and another eight megabytes
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for data at 0x2000000. These address ranges may be reserved for other
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purposes on the new architecture, so may need changing. There may be
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some additional areas of memory allocated by the system for other
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purposes, for example the startup stack and any environment variables,
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but usually eCos applications can and should ignore those.
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</P
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><P
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>Other HAL functionality such as interrupt handling, diagnostics, and
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the system clock are provided by the architectural HAL package and
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should work on different processors with few if any changes. There may
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be some problems in the code that interacts with the I/O auxiliary
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because of lurking assumptions about endianness or the sizes of
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various data types.
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</P
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><P
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>When porting to other processors, a number of sources of information
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are likely to prove useful. Obviously the Linux kernel sources and
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header files constitute the ultimate authority on how things work at
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the system call level. The GNU C library sources may also prove very
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useful: for a normal Linux application it is the C library that
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provides the startup code and the system call interface.
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</P
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></DIV
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><DIV
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CLASS="REFSECT1"
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><A
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NAME="SYNTH-PORTING-UNIX"
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></A
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><H2
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>Other Unix Platforms</H2
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><P
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>Porting to a Unix or Unix-like operating system other than Linux would
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be somewhat more involved. The first requirement is toolchains: the
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GNU compilers, gcc and g++, must definitely be used; use of other GNU
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tools such as the linker may be needed as well, because eCos depends
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on functionality such as prioritizing C++ static constructors, and
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other linkers may not implement this or may implement it in a
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different and incompatible way. A closely related requirement is the
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use of ELF format for binary executables: if the operating system
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still uses an older format such as COFF then there are likely to be
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problems because they do not provide the flexibility required by eCos.
|
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</P
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><P
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>In the architectural HAL there should be very little code that is
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specific to Linux. Instead the code should work on any operating
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system that provides a reasonable implementation of the POSIX
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standard. There may be some problems with program startup, but those
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could be handled at the architectural level. Some changes may also be
|
| 224 |
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required to the exception handling code. However one file which will
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present a problem is <TT
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CLASS="FILENAME"
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>hal_io.h</TT
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>, which contains
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various structure definitions and macros used with the system call
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interface. It is likely that many of these definitions will need
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changing, and it may well be appropriate to implement variant HAL
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packages for the different operating systems where this information
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can be separated out. Another possible problem is that the generic
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code assumes that system calls such as
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<TT
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CLASS="FUNCTION"
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>cyg_hal_sys_write</TT
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> are available. On an operating
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system other than Linux it is possible that some of these are not
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simple system calls, and instead wrapper functions will need to be
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implemented at the variant HAL level.
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</P
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><P
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>The generic I/O auxiliary code should be fairly portable to other Unix
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platforms. However some of the device drivers may contain code that is
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specific to Linux, for example the <TT
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CLASS="LITERAL"
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>PF_PACKET</TT
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> socket
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address family and the ethertap virtual tunnelling interface. These
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may prove quite difficult to port.
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</P
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><P
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>The remaining porting task is to implement one or more platform HAL
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packages, one per processor type that is supported. This should
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involve much the same work as a port to <A
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HREF="synth-porting.html#SYNTH-PORTING-LINUX"
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>another processor running Linux</A
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>.
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</P
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><P
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>When using other Unix operating systems the kernel source code may not
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be available, which would make any porting effort more challenging.
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However there is still a good chance that the GNU C library will have
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been ported already, so its source code may contain much useful
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information.
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</P
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></DIV
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><DIV
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CLASS="REFSECT1"
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><A
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NAME="SYNTH-PORTING-OTHER"
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></A
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><H2
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>Windows Platforms</H2
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><P
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>Porting the current synthetic target code to some version of Windows
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or to another non-Unix platform is likely to prove very difficult. The
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first hurdle that needs to be crossed is the file format for binary
|
| 280 |
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executables: current Windows implementations do not use ELF, instead
|
| 281 |
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they use their own format PE which is a variant of the rather old and
|
| 282 |
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limited COFF format. It may well prove easier to first write an ELF
|
| 283 |
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loader for Windows executables, rather than try to get eCos to work
|
| 284 |
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within the constraints of PE. Of course that introduces new problems,
|
| 285 |
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for example existing source-level debuggers will still expect
|
| 286 |
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executables to be in PE format.
|
| 287 |
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</P
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| 288 |
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><P
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>Under Linux a synthetic target application is not linked with the
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system's C library or any other standard system library. That would
|
| 291 |
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cause confusion, for example both eCos and the system's C library
|
| 292 |
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might try to define the <TT
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CLASS="FUNCTION"
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>printf</TT
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> function, and
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| 296 |
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introduce complications such as working with shared libraries. For
|
| 297 |
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much the same reasons, a synthetic target application under Windows
|
| 298 |
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should not be linked with any Windows DLL's. If an ELF loader has been
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| 299 |
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specially written then this may not be much of a problem.
|
| 300 |
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</P
|
| 301 |
|
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><P
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| 302 |
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>The next big problem is the system call interface. Under Windows
|
| 303 |
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system calls are generally made via DLL's, and it is not clear that
|
| 304 |
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the underlying trap mechanism is well-documented or consistent between
|
| 305 |
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different releases of Windows.
|
| 306 |
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</P
|
| 307 |
|
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><P
|
| 308 |
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>The current code depends on the operating system providing an
|
| 309 |
|
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implementation of POSIX signal handling. This is used for I/O
|
| 310 |
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purposes, for example <TT
|
| 311 |
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CLASS="LITERAL"
|
| 312 |
|
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>SIGALRM</TT
|
| 313 |
|
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> is used for the
|
| 314 |
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system clock, and for exceptions. It is not known what equivalent
|
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functionality is available under Windows.
|
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</P
|
| 317 |
|
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><P
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>Given the above problems a port of the synthetic target to Windows may
|
| 319 |
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or may not be technically feasible, but it would certainly require a
|
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very large amount of effort.
|
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</P
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WIDTH="100%"><TABLE
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SUMMARY="Footer navigation table"
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><A
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HREF="synth-new-host.html"
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ACCESSKEY="P"
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>Prev</A
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><TD
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><A
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