Echelon Neuron C - Manual (Page 246)

234 Neuron C Custom System Images
limit is cumulative, in other words, the two (or more) images cannot use
more than 64 bytes in total. Any unused RAM from this 64-byte area is
made available to the application program when it is linked. Custom
system images can only use far RAM variables. Include the far keyword
in any extern declarations of these variables (see point # 4, above).
6 You cannot access EEPROM variables from a custom system image.
Providing a Large RAM Space
The total amount of RAM space available to a custom system image at link time
is 64 bytes. However, if a custom system image needs a larger amount of RAM
for certain functions, a large RAM block can be declared by an application
program that uses the custom system image, and the application program can
make this known to the custom system image during reset. This functionality
can be placed inside a reset routine provided in an include file associated with
the custom image. An application using the custom image would then be
expected to include the include file, and call the reset routine from the
when(reset) task.
A pointer to this RAM block can be passed as a parameter to the appropriate
custom system image functions each time they are called. For a more efficient
implementation, the custom system image can use 2 bytes of its RAM space to
declare a global pointer to such a block of memory, and initialize the pointer to
NULL. When the application program resets, it has the responsibility of
correctly setting this global pointer variable to point to the block of memory, or at
least the responsibility of calling an initialization function (providing this
pointer).
Performing Neuron C Functions
The pure C code that is placed in a custom system image cannot contain
references to network variables, messages, I/O objects, timers, or other Neuron C
objects. However, the custom system image can be designed mainly for the
purpose of performing Neuron C related tasks, such as standard I/O device
management, message construction, or timer manipulation.
You can access Neuron C objects from a custom system image function by making
it the responsibility of the application program to actually perform the Neuron C
operation in a function. The custom system image can declare a RAM variable
that the application sets to a pointer to the application function. The custom
system image can then call the function in the Neuron C application program,
and effectively perform Neuron C operations.
For example, consider a custom system image that contains routines for
management of a standard LCD display device. This custom system image would
contain various routines for formatting information and for managing the display
in response to various commands from the application program. It is desirable to
have the custom system image code automatically perform the I/O operations to
update the device. It might also be necessary for such a custom system image to
have access to a large buffer in RAM. However, due to the pure C restriction and
the custom system image RAM memory restriction, neither of these requirements
can be implemented solely within the custom system image. The RAM buffer can
be provided by the application as discussed above.