Hitech C data buffers in program memory

Question

The C18 compiler allows variables in program memory with ROM qualifier, but the Hi-Tech C seems rather reluctant to utilize the Havard architecture to its best. So is there a way to create data buffers in program memory with the Hi-Tech C compiler (I am ready to compromise access speed).

I've seen indications of possibility with the psect but don't have any working implementation.

Answer 1

The HI-TECH PICC18 compiler places objects declared as const into program space by default. No special qualifiers like C18's RAM/ROM are needed:

3.5.3 Objects in Program Space

const objects are usually placed in program space. On the PIC18 devices, the program space is byte-wide, the compiler stores one character per byte location and values are read using the table read instructions. All const-qualified data objects and string literals are placed in the const psect. The const psect is placed at an address above the upper limit of RAM since RAM and const pointers use this address to determine if an access to ROM or RAM is required.

Note that placing frequently updated data in the microcontroller's flash memory may not be such a good idea, as flash has a limited number of program/erase cycles.

far pointers can be used to dereference program memory:

3.4.12.2 Const and Far Pointers

const and far pointers can either be 16 or 24 bits wide. Their size can be toggled with the --CP=24 or --CP=16 command line option. The code used to dereference them also changes with their size. The same pointer size must be used for all modules in a project. A pointer to far is identical to a pointer to const, except that pointers to far may be used to write to the address they hold. A pointer to const objects cannot be used to write as the const qualifier imposes that the object is read-only. const and far pointers which are 16 bits wide can access all RAM areas and most of the program space. At runtime when dereferenced, the contents of the pointer are examined. For addresses above the upper limit of RAM the program space is accessed using table read or table write instructions. Addresses below the upper limit of RAM access the data space. Even if the address held by a pointer to const is in RAM, the RAM location may not be changed. The default linker options always place const data at addresses above the upper limit of the data space so that the correct memory space is accessed when dereferencing with pointers. If the target device selected has more than 64k bytes of program space memory, then only the lower 64k bytes may be accessed with 16-bit wide pointers. Provided that all program space objects that need to be dereferenced are in the lower 64k bytes, 16-bit pointers to const and far objects may still be used. The smaller pointer size results in less RAM required and less code produced and so should be used whenever possible. const and far pointers which are 24 bits wide can access all RAM areas and all of the program space. At runtime when dereferenced, the contents of the pointer are examined. If bit number 21 in the address is set, the address is assumed to be a RAM address. Bit number 21 of the address is then ignored. If Bit number 21 is clear, then the address is assumed to be of an object in the program space and the access is performed using table read or table write instructions. Again, no writes to objects are permitted using a pointer to const. Note that when dereferencing a 24-bit pointer, the most significant implemented bit (bit number 21) of the TBLPTRU register may be overwritten. This bit may be used to enable access to the configuration area of the PIC18 device. If loading the table pointer registers from hand-written assembler code, make no assumptions about the state of bit number 21 prior to executing table read or write instructions.

The quotes are from HI-TECH PICC18 v9.51 manual.