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<HTML
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>eCos Synthetic Target</TH
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><H1
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><A
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NAME="SYNTH">Overview</H1
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CLASS="REFNAMEDIV"
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NAME="AEN6"
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></A
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><H2
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>Name</H2
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>The eCos synthetic target -- Overview</DIV
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CLASS="REFSECT1"
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NAME="SYNTH-DESCRIPTION"
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><H2
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>Description</H2
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><P
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>Usually eCos runs on either a custom piece of hardware, specially
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designed to meet the needs of a specific application, or on a
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development board of some sort that is available before the final
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hardware. Such boards have a number of things in common:
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><P
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>Obviously there has to be at least one processor to do the work. Often
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this will be a 32-bit processor, but it can be smaller or larger.
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Processor speed will vary widely, depending on the expected needs of
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the application. However the exact processor being used tends not to
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matter very much for most of the development process: the use of
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languages such as C or C++ means that the compiler will handle those
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details.
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</P
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></LI
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><LI
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><P
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>There needs to be memory for code and for data. A typical system will
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have two different types of memory. There will be some non-volatile
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memory such as flash, EPROM or masked ROM. There will also be some
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volatile memory such as DRAM or SRAM. Often the code for the final
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application will reside in the non-volatile memory and all of the RAM
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will be available for data. However updating non-volatile memory
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requires a non-trivial amount of effort, so for much of the
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development process it is more convenient to burn suitable firmware,
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for example RedBoot, into the non-volatile memory and then use that to
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load the application being debugged into RAM, alongside the
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application data and a small area reserved for use by the firmware.
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</P
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></LI
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><LI
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><P
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>The platform must provide certain mimimal I/O facilities. Most eCos
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configurations require a clock signal of some sort. There must also be
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some way of outputting diagnostics to the user, often but not always
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via a serial port. Unless special debug hardware is being used, source
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level debugging will require bidirectional communication between a
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host machine and the target hardware, usually via a serial port or an
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ethernet device.
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</P
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></LI
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><LI
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><P
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>All the above is not actually very useful yet because there is no way
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for the embedded device to interact with the rest of the world, except
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by generating diagnostics. Therefore an embedded device will have
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additional I/O hardware. This may be fairly standard hardware such as
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an ethernet or USB interface, or special hardware designed
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specifically for the intended application, or quite often some
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combination. Standard hardware such as ethernet or USB may be
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supported by eCos device drivers and protocol stacks, whereas the
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special hardware will be driven directly by application code.
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</P
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></LI
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></OL
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><P
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>Much of the above can be emulated on a typical PC running Linux.
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Instead of running the embedded application being developed on a
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target board of some sort, it can be run as a Linux process. The
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processor will be the PC's own processor, for example an x86, and the
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memory will be the process' address space. Some I/O facilities can be
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emulated directly through system calls. For example clock hardware can
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be emulated by setting up a <TT
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CLASS="LITERAL"
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>SIGALRM</TT
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> signal, which
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will cause the process to be interrupted at regular intervals. This
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emulation of real hardware will not be particularly accurate, the
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number of cpu cycles available to the eCos application between clock
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ticks will vary widely depending on what else is running on the PC,
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but for much development work it will be good enough.
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</P
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><P
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>Other I/O facilities are provided through an I/O auxiliary process,
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ecosynth, that gets spawned by the eCos application during startup.
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When an eCos device driver wants to perform some I/O operation, for
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example send out an ethernet packet, it sends a request to the I/O
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auxiliary. That is an ordinary Linux application so it has ready
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access to all normal Linux I/O facilities. To emulate a device
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interrupt the I/O auxiliary can raise a <TT
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CLASS="LITERAL"
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>SIGIO</TT
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>
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signal within the eCos application. The HAL's interrupt subsystem
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installs a signal handler for this, which will then invoke the
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standard eCos ISR/DSR mechanisms. The I/O auxiliary is based around
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Tcl scripting, making it easy to extend and customize. It should be
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possible to configure the synthetic target so that its I/O
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functionality is similar to what will be available on the final target
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hardware for the application being developed.
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</P
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><DIV
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CLASS="INFORMALFIGURE"
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><P
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><IMG
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SRC="overview.gif"
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ALIGN="CENTER"></P
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></DIV
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><P
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></P
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></DIV
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><P
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>A key requirement for synthetic target code is that the embedded
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application must not be linked with any of the standard Linux
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libraries such as the GNU C library: that would lead to a confusing
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situation where both eCos and the Linux libraries attempted to provide
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functions such as <TT
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CLASS="FUNCTION"
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>printf</TT
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>. Instead the synthetic
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target support must be implemented directly on top of the Linux
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kernels' system call interface. For example, the kernel provides a
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system call for write operations. The actual function
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<TT
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CLASS="FUNCTION"
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>write</TT
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> is implemented in the system's C library,
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but all it does is move its arguments on to the stack or into certain
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registers and then execute a special trap instruction such as
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<TT
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CLASS="LITERAL"
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>int 0x80</TT
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>. When this instruction is executed
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control transfers into the kernel, which will validate the arguments
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and perform the appropriate operation. Now, a synthetic target
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application cannot be linked with the system's C library. Instead it
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contains a function <TT
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CLASS="FUNCTION"
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>cyg_hal_sys_write</TT
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> which, like
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the C library's <TT
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CLASS="FUNCTION"
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>write</TT
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> function, pushes its
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arguments on to the stack and executes the trap instruction. The Linux
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kernel cannot tell the difference, so it will perform the I/O
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operation requested by the synthetic target. With appropriate
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knowledge of what system calls are available, this makes it possible
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to emulate the required I/O facilities. For example, spawning the
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ecosynth I/O auxiliary involves system calls
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<TT
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CLASS="FUNCTION"
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>cyg_hal_sys_fork</TT
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> and
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<TT
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CLASS="FUNCTION"
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>cyg_hal_sys_execve</TT
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>, and sending a request to the
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auxiliary uses <TT
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CLASS="FUNCTION"
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>cyg_hal_sys_write</TT
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>.
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</P
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><P
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>In many ways developing for the synthetic target is no different from
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developing for real embedded targets. eCos must be configured
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appropriately: selecting a suitable target such as
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<TT
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CLASS="USERINPUT"
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><B
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>i386linux</B
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></TT
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> will cause the configuration system
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to load the appropriate packages for this hardware; this includes an
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architectural HAL package and a platform-specific package; the
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architectural package contains generic code applicable to all Linux
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platforms, whereas the platform package is for specific Linux
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implementations such as the x86 version and contains any
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processor-specific code. Selecting this target will also bring in some
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device driver packages. Other aspects of the configuration such as
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which API's are supported are determined by the template, by adding
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and removing packages, and by fine-grained configuration.
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</P
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><P
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>In other ways developing for the synthetic target can be much easier
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than developing for a real embedded target. For example there is no
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need to worry about building and installing suitable firmware on the
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target hardware, and then downloading and debugging the actual
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application over a serial line or a similar connection. Instead an
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eCos application built for the synthetic target is mostly
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indistinguishable from an ordinary Linux program. It can be run simply
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by typing the name of the executable file at a shell prompt.
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Alternatively you can debug the application using whichever version of
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gdb is provided by your Linux distribution. There is no need to build
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or install special toolchains. Essentially using the synthetic target
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means that the various problems associated with real embedded hardware
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can be bypassed for much of the development process.
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</P
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><P
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>The eCos synthetic target provides emulation, not simulation. It is
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possible to run eCos in suitable architectural simulators but that
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involves a rather different approach to software development. For
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example, when running eCos on the psim PowerPC simulator you need
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appropriate cross-compilation tools that allow you to build PowerPC
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executables. These are then loaded into the simulator which interprets
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every instruction and attempts to simulate what would happen if the
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application were running on real hardware. This involves a lot of
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processing overhead, but depending on the functionality provided by
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the simulator it can give very accurate results. When developing for
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the synthetic target the executable is compiled for the PC's own
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processor and will be executed at full speed, with no need for a
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simulator or special tools. This will be much faster and somewhat
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simpler than using an architectural simulator, but no attempt is made
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to accurately match the behaviour of a real embedded target.
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</P
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></TD
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></TR
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><TR
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><TD
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WIDTH="33%"
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ALIGN="left"
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| 355 |
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VALIGN="top"
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| 356 |
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>eCos Synthetic Target</TD
|
| 357 |
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><TD
|
| 358 |
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WIDTH="34%"
|
| 359 |
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ALIGN="center"
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| 360 |
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VALIGN="top"
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| 361 |
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> </TD
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| 362 |
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><TD
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| 363 |
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WIDTH="33%"
|
| 364 |
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ALIGN="right"
|
| 365 |
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VALIGN="top"
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| 366 |
|
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>Installation</TD
|
| 367 |
|
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></TR
|
| 368 |
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></TABLE
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></DIV
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