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6.4 EXTERNAL POINTERS
6.3.3 Define Variables within PMP Space
Additional C Pointer Types
The pmp space attribute is also used to assign individual variables to the space. This
requires that the memory space declaration to be present. Given the declarations in the
previous subsection, the following variable declarations can be made:
__pmp__ int external_array[256]
__attribute__((space(pmp(external_PMP_memory))));
external_array will be allocated in the previous declared memory
external_PMP_memory. If there is only one PMP memory, and chip-selects are not
being used, it is possible to leave out the explicit reference to the memory. It is good
practice, however, to always make the memory explicit which would lead to code that
is more easily maintained.
Note that, like managed PSV pointers, we have qualified the variable with a new type
qualifier __pmp__. When attached to a variable or pointer it instructs the compiler to
generate the correct sequence for accessing via the PMP peripheral.
Now that a variable has been declared it may be accessed using normal C syntax. The
compiler will generate code to correctly communicate with the PMP peripheral.
Not all of Microchip’s 16-bit devices have a PMP peripheral, or not all memories are
suitable for attaching to a parallel port (serial memories sold by Microchip, for example).
The toolsuite provides a more general interface to any external memory, although, as
will be seen, the memory does not have to be external.
Like PMP memory space, the tool-chain needs to learn about external memories that
are being attached. Unlike PMP, however, the compiler does not know how to access
these memories. A mechanism is provided by which an application can specify how to
access such memories.
External pointers (and their addresses) consume 32 bits. The largest attachable memory
is 64K (16 bits); the other 16 bits is used to uniquely identify the memory. A total of
64K (16 bits) of these may be (theoretically) attached.
To use this feature:
• Declare a New Memory Space
• Define Variables within an External Space
• Define How to Access Memory Spaces
As an example:
• An External Example
6.4.1 Declare a New Memory Space
This is very similar to declaring a new memory space for PMP access.
The 16-bit toolsuite requires information about each external memory. In order for
16-bit device linker to be able to properly assign memory, information about the size of
memory available and, optionally the origin of the memory, needs to be provided.
In Chapter 2. “Differences Between 16-Bit Device C and ANSI C” the new external
memory space was introduced. This attribute serves two purposes: declaring
extended memory spaces and assigning C variable declarations to external memory
(this will be covered in the next subsection).
Declaring an extended memory requires providing the size of the memory. You may
optionally specify an origin for this memory; if none is specified 0x0000 will be
assumed.
© 2008 Microchip Technology Inc. DS51284H-page 85