pgm_get_far_address(var)
This macro facilitates the obtention of a 32 bit "far" pointer (only 24 bits used) to data even passed the 64KB limit for the 16 bit ordinary pointer. It is similar to the '&' operator, with some limitations.
Comments:
The overhead is minimal and it's mainly due to the 32 bit size operation.
24 bit sizes guarantees the code compatibility for use in future devices.
hh8() is an undocumented feature but seems to give the third significant byte of a 32 bit data and accepts symbols, complementing the functionality of hi8() and lo8(). There is not an equivalent assembler function to get the high significant byte.
'var' has to be resolved at linking time as an existing symbol, i.e, a simple type variable name, an array name (not an indexed element of the array, if the index is a constant the compiler does not complain but fails to get the address if optimization is enabled), a struct name or a struct field name, a function identifier, a linker defined identifier,...
The returned value is the identifier's VMA (virtual memory address) determined by the linker and falls in the corresponding memory region. The AVR Harvard architecture requires non overlapping VMA areas for the multiple address spaces in the processor: Flash ROM, RAM, and EEPROM. Typical offset for this are 0x00000000, 0x00800xx0, and 0x00810000 respectively, derived from the linker script used and linker options. The value returned can be seen then as a universal pointer.