This post talks about the "C" keyword "const". It is very often thought that making a variable "const" ensures that it gets placed in the non-volatile memory (ROM or flash). But how true is this?
Well to start with w.r.t the ansi "C" compiler, the "const" keyword just tells the compiler that the user doesn't intend to modify the variable.
E.g.
const unsigned int Gctrl_NoOfGears= 10;
so if i try to do this
Gctrl_NoOfGears +=1;
I can either except a warning or an error. something like this
error: assignment of read-only variable `Gctrl_NoOfGears'
However, does this warrant me that the variable is now placed inside the memory in an non-volatile area.
Well, the answer is it is compiler (settings) dependent.
Why?
Most embedded compilers are very smart and automatically place the constants in the ROM/ flash area. However, some times the flash or ROM is used only as program memory and NOT as data memory. In which case there is no way (directly) that the compiler can place this in the program memory. Also In these processors the program memory is not directly accesible because the program & data are stored in differrent memory location & have different busses to access. In such processors writing
You will have to do this via some compiler pragma's like
Do I have to worry?
As a novice or some one programming for a simple system NO. However, when you are running low on resources and for various other reasons it makes sense to have a look at what the compilers is doing when you say "const". I repeat MOST compilers are clever enough to put the data into the ROM automatically.
The best practice is always to have a glance into the map file to ensure that you have everything at the right places.
Also, in one of the future posts i will be talking about another usages of const and how some constants are present even without your knowledge ( also little talk about const volatile...the common question in any interview on "C").
Please leave your comment. You can subscribe to this blog by using the links under "Subscribe" section.
Previous coding myths here {1} ,{2}
Well to start with w.r.t the ansi "C" compiler, the "const" keyword just tells the compiler that the user doesn't intend to modify the variable.
E.g.
const unsigned int Gctrl_NoOfGears= 10;
so if i try to do this
Gctrl_NoOfGears +=1;
I can either except a warning or an error. something like this
error: assignment of read-only variable `Gctrl_NoOfGears'
However, does this warrant me that the variable is now placed inside the memory in an non-volatile area.
Well, the answer is it is compiler (settings) dependent.
Why?
Most embedded compilers are very smart and automatically place the constants in the ROM/ flash area. However, some times the flash or ROM is used only as program memory and NOT as data memory. In which case there is no way (directly) that the compiler can place this in the program memory. Also In these processors the program memory is not directly accesible because the program & data are stored in differrent memory location & have different busses to access. In such processors writing
const char DisplayStr[] = "Welcome";
will not cause DisplayStr to go into the ROM area. You will have to do this via some compiler pragma's like
#pragma section start ROM
const char DisplayStr[] = "Welcome";
#pragma section end ROM
to make the compiler understand that it has to place the value in ROM & not in the RAM area.const char DisplayStr[] = "Welcome";
#pragma section end ROM
Do I have to worry?
As a novice or some one programming for a simple system NO. However, when you are running low on resources and for various other reasons it makes sense to have a look at what the compilers is doing when you say "const". I repeat MOST compilers are clever enough to put the data into the ROM automatically.
The best practice is always to have a glance into the map file to ensure that you have everything at the right places.
Also, in one of the future posts i will be talking about another usages of const and how some constants are present even without your knowledge ( also little talk about const volatile...the common question in any interview on "C").
Please leave your comment. You can subscribe to this blog by using the links under "Subscribe" section.
Previous coding myths here {1} ,{2}
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