Where is my const char array stored?

[haxan7]

Device: PIC 24EP.
Compiler: v5.025.

Where is a const character array defined as

[Ttelmah]

It'll still be in ROM. But allow pointers as if it is in RAM.

The pass_strings=in_ram, means that the compiler will 'virtualise' constants, using a temporary RAM buffer to transfer them as used. Only wanted if you are putting the constants in ROM. Otherwise does nothing.

CONST=ROM, is the default. Will still be selected, unless you use CONST=READ_ONLY....

Remember either way, ROM has to hold it when the chip boots. If you use 'READ_ONLY', it is all copied to RAM at boot, and is then a RAM variable. Supposedly doesn't allow writing, but since the chip has no protection to stop this, you can easily write to it using a pointer.

The default changes to CONST=READ_ONLY, if you have ANSI selected.

Why have a const variable declared larger than the data it holds?. Just wastes space.

[haxan7]

One interesting thing to note is that if I put #DEVICE CONST=ROM in the header file.
The compiler starts giving compilation errors like "Expression must evaluate to a constant: on lines such as:

[Ttelmah]

Not surprised at that.
Generally CCS wouldn't allow a variable to be used at compile time like this till recently. The code that allows this to be done, only works with values held in RAM (which it now simulates at compile time), but to use a const requires a ROM value to be simulated in RAM. Normally you'd use a #define instead.

[haxan7]

Sorry Ttelmah, but I did not understand...
My question was that if CCS by default stores the constants in ROM then why does including #DEVICE CONST=ROM cause compilation error.

[Ttelmah]

A variable does not exist at compile time.
It only exists when the code is running.
So defining an array with a variable as a 'size', is a problem.
Historically CCS did not allow this.
However they now 'simulate' variables stored _in RAM_ to contain a value they are initialised with, so you can use a defined and initialised variable as a size.
Problem is that this simulation does not extend to variables stored and thought of as in ROM....

It is this last point that creates the problem, the simulation thinks the value is a variable.