Difference between __I and __IO and __O in CMSIS core

Question

in core_cm4.h header file defined some thing like this:

#ifdef __cplusplus
  #define   __I     volatile             /*!< Defines 'read only' permissions                 */
#else
  #define   __I     volatile const       /*!< Defines 'read only' permissions                 */
#endif
#define     __O     volatile             /*!< Defines 'write only' permissions                */
#define     __IO    volatile             /*!< Defines 'read / write' permissions              */

What's the difference between __I , __O and __IO while they are defined in same way.

• and how they work to specify "read/write only" while they are only defined as a volatile variable ?!

Answer 1

"I" means: Input

"O" means: Output

"IO" means: Input and Output

As Long Pham notes, it is a naming convention, but also it is normal to use meanings to a type. Like an integer can be a counter, a timestamp, a date etc.

There are some reasons to use this:

• It is good for readability
• Whenever in future the type of an I, O or IO would change, (user) source code using I, O and IO does not need to be changed, only the type definitions / defines itself.

Answer 2

Since this is for the definition of a hardware register map:

I think the reason is that C, unlike C++, allows the declaration of const qualified variables without providing an initializer:

volatile const uint32_t REG; // Ok in C, invalid in C++

Similarly, C++ doesn't allow const members of struct/classes to be uninitialized either. This is inconvenient when we have read-only hardware registers and wish to make a register map by using structs (classes).

This C++ language limitation is likely why this header uses a dirty hack #ifdef __cplusplus to remove const from a register that should have been const qualified.

Answer 3

This header file was created to support the c++ as well,

Hence in case of C const variable denoted constant and cannot be modified by the user,However there is no any further optimization for this const variable by the C compiler.

But in case of C++ ,this const variable can be replaced by the constant value assigned to that variable.

In case of c++ structure member variables are not optimized hence const can be used there.

Therefore, this header file provides __I ,__O , __IO for the scalar variables and
__IM ,__OM ,__IOM for the structure members.