Virtual Vectors (eCos/ROM Monitor Calling Interface)

Virtually all eCos platforms provide full debugging capabilities via RedBoot. This environment contains not only debug stubs based on GDB, but also rich I/O support which can be exported to loaded programs. Such programs can take advantage of the I/O capabilities using a special ROM/RAM calling interface (also referred to as virtual vector table). eCos programs make use of the virtual vector mechanism implicitly. Non-eCos programs can access these functions using the support from the newlib library.

Virtual Vectors

What are virtual vectors, what do they do, and why are they needed?

"Virtual vectors" is the name of a table located at a static location in the target address space. This table contains 64 vectors that point to service functions or data.

The fact that the vectors are always placed at the same location in the address space means that both ROM and RAM startup configurations can access these and thus the services pointed to.

The primary goal is to allow services to be provided by ROM configurations (ROM monitors such as RedBoot in particular) with clients in RAM configurations being able to use these services.

Without the table of pointers this would be impossible since the ROM and RAM applications would be linked separately - in effect having separate name spaces - preventing direct references from one to the other.

This decoupling of service from client is needed by RedBoot, allowing among other things debugging of applications which do not contain debugging client code (stubs).

Initialization (or Mechanism vs. Policy)

Virtual vectors are a mechanism for decoupling services from clients in the address space.

The mechanism allows services to be implemented by a ROM monitor, a RAM application, to be switched out at run-time, to be disabled by installing pointers to dummy functions, etc.

The appropriate use of the mechanism is specified loosely by a policy. The general policy dictates that the vectors are initialized in whole by ROM monitors (built for ROM or RAM), or by stand-alone applications.

For configurations relying on a ROM monitor environment, the policy is to allow initialization on a service by service basis. The default is to initialize all services, except COMMS services since these are presumed to already be carrying a communication session to the debugger / console which was used for launching the application. This means that the bulk of the code gets tested in normal builds, and not just once in a blue moon when building new stubs or a ROM configuration.

The configuration options are written to comply with this policy by default, but can be overridden by the user if desired. Defaults are:

  • For application development: the ROM monitor provides debugging and diagnostic IO services, the RAM application relies on these by default.

  • For production systems: the application contains all the necessary services.

Pros and Cons of Virtual Vectors

There are pros and cons associated with the use of virtual vectors. We do believe that the pros generally outweigh the cons by a great margin, but there may be situations where the opposite is true.

The use of the services are implemented by way of macros, meaning that it is possible to circumvent the virtual vectors if desired. There is (as yet) no implementation for doing this, but it is possible.

Here is a list of pros and cons:

Pro: Allows debugging without including stubs

This is the primary reason for using virtual vectors. It allows the ROM monitor to provide most of the debugging infrastructure, requiring only the application to provide hooks for asynchronous debugger interrupts and for accessing kernel thread information.

Pro: Allows debugging to be initiated from arbitrary channel

While this is only true where the application does not actively override the debugging channel setup, it is a very nice feature during development. In particular it makes it possible to launch (and/or debug) applications via Ethernet even though the application configuration does not contain networking support.

Pro: Image smaller due to services being provided by ROM monitor

All service functions except HAL IO are included in the default configuration. But if these are all disabled the image for download will be a little smaller. Probably doesn't matter much for regular development, but it is a worthwhile saving for the 20000 daily tests run in the Red Hat eCos test farm.

Con: The vectors add a layer of indirection, increasing application size and reducing performance.

The size increase is a fraction of what is required to implement the services. So for RAM configurations there is a net saving, while for ROM configurations there is a small overhead.

The performance loss means little for most of the services (of which the most commonly used is diagnostic IO which happens via polled routines anyway).

Con: The layer of indirection is another point of failure.

The concern primarily being that of vectors being trashed by rogue writes from bad code, causing a complete loss of the service and possibly a crash. But this does not differ much from a rogue write to anywhere else in the address space which could cause the same amount of mayhem. But it is arguably an additional point of failure for the service in question.

Con: All the indirection stuff makes it harder to bring a HAL up

This is a valid concern. However, seeing as most of the code in question is shared between all HALs and should remain unchanged over time, the risk of it being broken when a new HAL is being worked on should be minimal.

When starting a new port, be sure to implement the HAL IO drivers according to the scheme used in other drivers, and there should be no problem.

However, it is still possible to circumvent the vectors if they are suspect of causing problems: simply change the HAL_DIAG_INIT and HAL_DIAG_WRITE_CHAR macros to use the raw IO functions.

Available services

The hal_if.h file in the common HAL defines the complete list of available services. A few worth mentioning in particular:

  • COMMS services. All HAL IO happens via the communication channels.

  • uS delay. Fine granularity (busy wait) delay function.

  • Reset. Allows a software initiated reset of the board.

The COMMS channels

As all HAL IO happens via the COMMS channels these deserve to be described in a little more detail. In particular the controls of where diagnostic output is routed and how it is treated to allow for display in debuggers.

Console and Debugging Channels

There are two COMMS channels - one for console IO and one for debugging IO. They can be individually configured to use any of the actual IO ports (serial or Ethernet) available on the platform.

The console channel is used for any IO initiated by calling the diag_*() functions. Note that these should only be used during development for debugging, assertion and possibly tracing messages. All proper IO should happen via proper devices. This means it should be possible to remove the HAL device drivers from production configurations where assertions are disabled.

The debugging channel is used for communication between the debugger and the stub which remotely controls the target for the debugger (the stub runs on the target). This usually happens via some protocol, encoding commands and replies in some suitable form.

Having two separate channels allows, e.g., for simple logging without conflicts with the debugger or interactive IO which some debuggers do not allow.


As debuggers usually have a protocol using specialized commands when communicating with the stub on the target, sending out text as raw ASCII from the target on the same channel will either result in protocol errors (with loss of control over the target) or the text may just be ignored as junk by the debugger.

To get around this, some debuggers have a special command for text output. Mangling is the process of encoding diagnostic ASCII text output in the form specified by the debugger protocol.

When it is necessary to use mangling, i.e. when writing console output to the same port used for debugging, a mangler function is installed on the console channel which mangles the text and passes it on to the debugger channel.

Controlling the Console Channel

Console output configuration is either inherited from the ROM monitor launching the application, or it is specified by the application. This is controlled by the new option CYGSEM_HAL_VIRTUAL_VECTOR_INHERIT_CONSOLE which defaults to enabled when the configuration is set to use a ROM monitor.

If the user wants to specify the console configuration in the application image, there are two new options that are used for this.

Defaults are to direct diagnostic output via a mangler to the debugging channel (CYGDBG_HAL_DIAG_TO_DEBUG_CHAN enabled). The mangler type is controlled by the option CYGSEM_HAL_DIAG_MANGLER. At present there are only two mangler types:


This causes a mangler appropriate for debugging with GDB to be installed on the console channel.


This causes a NULL mangler to be installed on the console channel. It will redirect the IO to/from the debug channel without mangling of the data. This option differs from setting the console channel to the same IO port as the debugging channel in that it will keep redirecting data to the debugging channel even if that is changed to some other port.

Finally, by disabling CYGDBG_HAL_DIAG_TO_DEBUG_CHAN, the diagnostic output is directed in raw form to the specified console IO port.

In summary this results in the following common configuration scenarios for RAM startup configurations:

  • For regular debugging with diagnostic output appearing in the debugger, mangling is enabled and stubs disabled.

    Diagnostic output appears via the debugging channel as initiated by the ROM monitor, allowing for correct behavior whether the application was launched via serial or Ethernet, from the RedBoot command line or from a debugger.

  • For debugging with raw diagnostic output, mangling is disabled.

    Debugging session continues as initiated by the ROM monitor, whether the application was launched via serial or Ethernet. Diagnostic output is directed at the IO port configured in the application configuration.

    Note:: There is one caveat to be aware of. If the application uses proper devices (be it serial or Ethernet) on the same ports as those used by the ROM monitor, the connections initiated by the ROM monitor will be terminated.

And for ROM startup configurations:

  • Production configuration with raw output and no debugging features (configured for RAM or ROM), mangling is disabled, no stubs are included.

    Diagnostic output appears (in unmangled form) on the specified IO port.

  • RedBoot configuration, includes debugging features and necessary mangling.

    Diagnostic and debugging output port is auto-selected by the first connection to any of the supported IO ports. Can change from interactive mode to debugging mode when a debugger is detected - when this happens a mangler will be installed as required.

  • GDB stubs configuration (obsoleted by RedBoot configuration), includes debugging features, mangling is hardwired to GDB protocol.

    Diagnostic and debugging output is hardwired to configured IO ports, mangling is hardwired.

Footnote: Design Reasoning for Control of Console Channel

The current code for controlling the console channel is a replacement for an older implementation which had some shortcomings which addressed by the new implementation.

This is what the old implementation did: on initialization it would check if the CDL configured console channel differed from the active debug channel - and if so, set the console channel, thereby disabling mangling.

The idea was that whatever channel was configured to be used for console (i.e., diagnostic output) in the application was what should be used. Also, it meant that if debug and console channels were normally the same, a changed console channel would imply a request for unmangled output.

But this prevented at least two things:

  • It was impossible to inherit the existing connection by which the application was launched (either by RedBoot commands via telnet, or by via a debugger).

    This was mostly a problem on targets supporting Ethernet access since the diagnostic output would not be returned via the Ethernet connection, but on the configured serial port.

    The problem also occurred on any targets with multiple serial ports where the ROM monitor was configured to use a different port than the CDL defaults.

  • Proper control of when to mangle or just write out raw ASCII text.

    Sometimes it's desirable to disable mangling, even if the channel specified is the same as that used for debugging. This usually happens if GDB is used to download the application, but direct interaction with the application on the same channel is desired (GDB protocol only allows output from the target, no input).

The calling Interface API

The calling interface API is defined by hal_if.h and hal_if.c in hal/common.

The API provides a set of services. Different platforms, or different versions of the ROM monitor for a single platform, may implement fewer or extra service. The table has room for growth, and any entries which are not supported map to a NOP-service (when called it returns 0 (false)).

A client of a service should either be selected by configuration, or have suitable fall back alternatives in case the feature is not implemented by the ROM monitor.

Note:: Checking for unimplemented service when this may be a data field/pointer instead of a function: suggest reserving the last entry in the table as the NOP-service pointer. Then clients can compare a service entry with this pointer to determine whether it's initialized or not.

The header file cyg/hal/hal_if.h defines the table layout and accessor macros (allowing primitive type checking and alternative implementations should it become necessary).

The source file hal_if.c defines the table initialization function. All HALs should call this during platform initialization - the table will get initialized according to configuration. Also defined here are wrapper functions which map between the calling interface API and the API of the used eCos functions.

Implemented Services

This is a brief description of the services, some of which are described in further detail below.


Version of table. Serves as a way to check for how many features are available in the table. This is the index of the last service in the table.


[Presently unused by the stub code, but initialized] This vector defines a function to execute when the system receives a kill signal from the debugger. It is initialized with the reset function (see below), but the application (or eCos) can override it if necessary.


The communication procedure table used for console IO (see the Section called IO channels.


The communication procedure table used for debugger IO (see the Section called IO channels).


Flushes the data cache for the specified region. Some implementations may flush the entire data cache.


Flushes (invalidates) the instruction cache for the specified region. Some implementations may flush the entire instruction cache.


Change debugging communication channel.


Change console communication channel.


Vector used to communication between debugger functions in ROM and in RAM. RAM eCos configurations may install a function pointer here which the ROM monitor uses to get thread information from the kernel running in RAM.


Resets the board on call. If it is not possible to reset the board from software, it will jump to the ROM entry point which will perform a "software" reset of the board.


Set if a debugger interrupt request was detected while processing console IO. Allows the actual breakpoint action to be handled after return to RAM, ensuring proper backtraces etc.


Will delay the specified number of microseconds. The precision is platform dependent to some extend - a small value (<100us) is likely to cause bigger delays than requested.


For accessing configuration settings kept in flash memory.


Installs a breakpoint at the specified address. This is used by the asynchronous breakpoint support (see ).


When a platform is changed to support the calling interface, applications will use it if so configured. That means that if an application is run on a platform with an older ROM monitor, the service is almost guaranteed to fail.

For this reason, applications should only use Console Comm for HAL diagnostics output if explicitly configured to do so (CYGSEM_HAL_VIRTUAL_VECTOR_DIAG).

As for asynchronous GDB interrupts, the service will always be used. This is likely to cause a crash under older ROM monitors, but this crash may be caught by the debugger. The old workaround still applies: if you need asynchronous breakpoints or thread debugging under older ROM monitors, you may have to include the debugging support when configuring eCos.

Implementation details

During the startup of a ROM monitor, the calling table will be initialized. This also happens if eCos is configured not to rely on a ROM monitor.

Note:: There is reserved space (256 bytes) for the vector table whether it gets used or not. This may be something that we want to change if we ever have to shave off every last byte for a given target.

If thread debugging features are enabled, the function for accessing the thread information gets registered in the table during startup of a RAM startup configuration.

Further implementation details are described where the service itself is described.

New Platform Ports

The hal_platform_init() function must call hal_if_init().

The HAL serial driver must, when called via cyg_hal_plf_comms_init() must initialize the communication channels.

The reset() function defined in hal_if.c will attempt to do a hardware reset, but if this fails it will fall back to simply jumping to the reset entry-point. On most platforms the startup initialization will go a long way to reset the target to a sane state (there will be exceptions, of course). For this reason, make sure to define HAL_STUB_PLATFORM_RESET_ENTRY in plf_stub.h.

All debugging features must be in place in order for the debugging services to be functional. See general platform porting notes.

New architecture ports

There are no specific requirements for a new architecture port in order to support the calling interface, but the basic debugging features must be in place. See general architecture porting notes.

IO channels

The calling interface provides procedure tables for all IO channels on the platform. These are used for console (diagnostic) and debugger IO, allowing a ROM monitor to provided all the needed IO routines. At the same time, this makes it easy to switch console/debugger channels at run-time (the old implementation had hardwired drivers for console and debugger IO, preventing these to change at run-time).

The hal_if provides wrappers which interface these services to the eCos infrastructure diagnostics routines. This is done in a way which ensures proper string mangling of the diagnostics output when required (e.g. O-packetization when using a GDB compatible ROM monitor).

Available Procedures

This is a brief description of the procedures


Pointer to the controller IO base (or a pointer to a per-device structure if more data than the IO base is required). All the procedures below are called with this data item as the first argument.


Writes the buffer to the device.


Fills a buffer from the device.


Write a character to the device.


Read a character from the device.


Device feature control. Second argument specifies function:


Changes baud rate.


Returns the current baud rate.






Disable debugging receive interrupts on the device.


Enable debugging receive interrupts on the device.


Returns the ISR vector used by the device for debugging receive interrupts.


Set GETC timeout in milliseconds.


Forces driver to flush data in its buffers. Note that this may not affect hardware buffers (e.g. FIFOs).


ISR used to handle receive interrupts from the device (see ).


Read a character from the device with timeout.


The standard eCos diagnostics IO functions use the channel procedure table when CYGSEM_HAL_VIRTUAL_VECTOR_DIAG is enabled. That means that when you use diag_printf (or the libc printf function) the stream goes through the selected console procedure table. If you use the virtual vector function SET_CONSOLE_COMM you can change the device which the diagnostics output goes to at run-time.

You can also use the table functions directly if desired (regardless of the CYGSEM_HAL_VIRTUAL_VECTOR_DIAG setting - assuming the ROM monitor provides the services). Here is a small example which changes the console to use channel 2, fetches the comm procs pointer and calls the write function from that table, then restores the console to the original channel:

#define T "Hello World!\n"

    hal_virtual_comm_table_t* comm;


    CYGACC_COMM_IF_WRITE(*comm, T, strlen(T));


Beware that if doing something like the above, you should only do it to a channel which does not have GDB at the other end: GDB ignores raw data, so you would not see the output.


The use of this service is controlled by the option CYGSEM_HAL_VIRTUAL_VECTOR_DIAG which is disabled per default on most older platforms (thus preserving backwards compatibility with older stubs). On newer ports, this option should always be set.

Implementation Details

There is an array of procedure tables (raw comm channels) for each IO device of the platform which get initialized by the ROM monitor, or optionally by a RAM startup configuration (allowing the RAM configuration to take full control of the target). In addition to this, there's a special table which is used to hold mangler procedures.

The vector table defines which of these channels are selected for console and debugging IO respectively: console entry can be empty, point to mangler channel, or point to a raw channel. The debugger entry should always point to a raw channel.

During normal console output (i.e., diagnostic output) the console table will be used to handle IO if defined. If not defined, the debug table will be used.

This means that debuggers (such as GDB) which require text streams to be mangled (O-packetized in the case of GDB), can rely on the ROM monitor install mangling IO routines in the special mangler table and select this for console output. The mangler will pass the mangled data on to the selected debugging channel.

If the eCos configuration specifies a different console channel from that used by the debugger, the console entry will point to the selected raw channel, thus overriding any mangler provided by the ROM monitor.

See hal_if_diag_* routines in hal_if.c for more details of the stream path of diagnostic output. See cyg_hal_gdb_diag_*() routines in hal_stub.c for the mangler used for GDB communication.

New Platform Ports


If CYGSEM_HAL_VIRTUAL_VECTOR_DIAG is set, make sure the infra diag code uses the hal_if diag functions:

 #define HAL_DIAG_INIT() hal_if_diag_init()
 #define HAL_DIAG_WRITE_CHAR(_c_) hal_if_diag_write_char(_c_)
 #define HAL_DIAG_READ_CHAR(_c_) hal_if_diag_read_char(&_c_)

In addition to the above functions, the platform HAL must also provide a function cyg_hal_plf_comms_init which initializes the drivers and the channel procedure tables.

Most of the other functionality in the table is more or less possible to copy unchanged from existing ports. Some care is necessary though to ensure the proper handling of interrupt vectors and timeouts for various devices handled by the same driver. See PowerPC/Cogent platform HAL for an example implementation.

Note:: When vector table console code is not used, the platform HAL must map the HAL_DIAG_INIT, HAL_DIAG_WRITE_CHAR and HAL_DIAG_READ_CHAR macros directly to the low-level IO functions, hardwired to use a compile-time configured channel.

Note:: On old ports the hardwired HAL_DIAG_INIT, HAL_DIAG_WRITE_CHAR and HAL_DIAG_READ_CHAR implementations will also contain code to O-packetize the output for GDB. This should not be adopted for new ports! On new ports the ROM monitor is guaranteed to provide the necessary mangling via the vector table. The hardwired configuration should be reserved for ROM startups where achieving minimal image size is crucial.

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