MPLAB C Compiler for PIC24 MCUs and dsPIC DSCs ... - Microchip

16-Bit C Compiler User’s Guide
extern int external_memory
__attribute__((space(external(size(1024)))));
Above an external memory of size 1024 bytes is defined. This memory can be uniquely
identified by its given name of external_memory.
6.4.2 Define Variables within an External Space
The external 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:
__external__ int external_array[256]
__attribute__((space(external(external_memory))));
external_array will be allocated in the previous declared memory
external_memory.
Note that, like managed PSV pointers, we have qualified the variable with a new type
qualifier __external__. When attached to a variable or pointer it instructs the compiler
to generate the correct sequence for accessing.
Now that a variable has been declared it may be accessed using normal C syntax. The
compiler will generate code to access the variable via special helper functions that the
programmer must define. These are covered in the next subsection.
6.4.3 Define How to Access Memory Spaces
References to variables placed in external memories are controlled via the use of several
helper functions. Up to five (5) functions may be defined for reading and five (5) for
writing. One each of these is a generic function and will be called if any of the other four
is not defined but is required. If none of the functions are defined, the compiler will generate
an error message. A brief example will be presented in the next subsection. Generally
defining the individual functions will result in more efficient code generation.
6.4.3.1 FUNCTIONS FOR READING
read_external
void __read_external(unsigned int address,
unsigned int memory_space,
void *buffer,
unsigned int len)
This function is a generic Read function and will be called if one of the next functions
are required but not defined. This function should perform the steps necessary to fill
len bytes of memory in the buffer from the external memory named memory_space
starting at address address.
read_external8
unsigned char __read_external8(unsigned int address,
unsigned int memory_space)
Read 8 bits from external memory space memory_space starting from address
address. The compiler would like to call this function if trying to access an 8-bit sized
object.
DS51284H-page 86 © 2008 Microchip Technology Inc.