Point to const struct array

uslimey · Joined: 27 Aug 2011 Posts: 22 Location: Baltimore, MD

I created a typdef struct for a calibration table.

Ttelmah · Joined: 11 Mar 2010 Posts: 19436

Use rom, rather than const. (rom, not #ROM).

It's all down to the memory structure of the PIC, and history.
The PIC, has separate memory spaces for the program and code. Not the single linear memory space of chips like the 808x, etc..
Historically older PICs, could not directly read their ROM.

So when CCS first implemented const data tables, they 'cheated', and simulated the effect, using the method that worked within the PIC limitations, and generated a program that returned the bytes required. As such though it meant that a pointer could not be constructed to these.
To help avoid this being too much of a limitation, a 'shortcut' was added to the language, where using the construct (say):

putc("fred");

resulted in the putc function being called four times, with each character in turn.

Latter when running in ANSI emulation mode, the meaning of 'const' was changed to be compliant with this, being a RAM based table, that is 'read only'. To these a pointer can be constructed, but it means that the table has to be copied into RAM at runtime. Costly on RAM space. This is what happens if you switch the compiler to ANSI mode.

However CCS also decided that on the latter chips that did allow code to access the ROM, they would implement a form that did support pointers, and this is given by using the rom keyword, rather than const keyword. It makes things slightly bulkier, since extra code has to be added to support this, but does generally work.

So you have:

CCS mode
const data tables - small but no pointer support.
rom data tables - code larger, but supports pointers.

ANSI mode
const data tables - copied to RAM, and support pointers, but uses a lot of RAM
rom data tables - stored in ROM.

You can also switch the mode of ROM storage so all const tables allow pointers, using:

#device const=ROM

This though prevents the old CCS 'shortcut' usage where calling a function that expects an integer, with a constant string, repeatedly calls the function for each character in the string.

This is documented in the readme.txt with the compiler.

Best Wishes

uslimey · Joined: 27 Aug 2011 Posts: 22 Location: Baltimore, MD

I tried the rom method described...

Ttelmah · Joined: 11 Mar 2010 Posts: 19436

Look at the readme.
The point is what you have is a _rom_ pointer. Remember what I said about the data space of the chip. Something has to tell the code that this is rom, not ram. So you have to construct pointers as:

rom *fred;

fred is a pointer to rom etc...

It is in the readme.

Best Wishes

uslimey · Joined: 27 Aug 2011 Posts: 22 Location: Baltimore, MD

Tried...

rom *pointer;

resulted in a compile error... "...expecting a ("

in the ref manual the example for a pointer to a const using the follwoing syntax...

const char *cptr;

This only works with character data types...

Still stuck!

uslimey · Joined: 27 Aug 2011 Posts: 22 Location: Baltimore, MD

Seems like the pointer to the rom struct array should be declared as follows:

andrewg · Posted: Tue Nov 01, 2011 12:48 am