const placed in DATA segment

const char mysting[] ={"my string"};

code char mysting[] ={"my string"};

const char code myString[] =  "Hello";

Thank you Mark for your answer.

But if compiler knows that data read-only, why it's difficult to place it in ROM segment? I've seen a program compiled by IAR, and there were no statments "code" for "const" data.

Vladimir

It's not a difficulty. The idea is to give the user the flexibility to locate the strings in any memory space. The default is data space. While you could make an argument that as a const, it should default to code space, someone will then ask why it is not in data space.

'const' is similar read-only, but it is not a memory_storage_qualifier.

Look at the example:

struct CCC {
  const int x;
  int y;
} ccc = { 1, 2 };

void func(const char *str)
{ char * ptr;

  ptr = str;  // MUST produce WARNING

  *ptr = 'B'; // LEGAL
  *str = 'A'; // ILLEGAL

  ccc.y = 5;  // LEGAL
  ccc.x = 2;  // ILLEGAL
}

The compiler should not place the data in ROM because you may want to place a memory mapped read-only register in XDATA space. Thus if you were force into code space you could not read the register. By allowing const to do exactly what it does in ISO/ANSI C, Keil has allowed us to enforce compile-time read-only attributes while having the freedom of placing the read-only variable in any data space.

Another example, in the assembler startup routine you calculate a "first available memory" location for malloc and write it to an address for malloc to use. Malloc() declares that external variable as const but the variable must be in a writable memory space for the C startup module to be able to set it.

In the end, tell C51 exactly where you want everything to go. I put memory space qualifiers on every single variable I define. I am never surprised when I look in the map file.

- Mark

Hello Sergey,
many thanks for your example, I didn't consider this situation.

Vladimir

Thank you Tom,

may be you are right. At least it's supported by other compilers too. But I just wish to have an option at IDE for allocating cont in ROM area. Such options are exist for some compilers.

Regards,
Vladimir

Thank you Mark,

it really gives flexibility, and as it acceptable for many cross C-compilers I don't care any more.

Regards,
Vladimir