Hi
I am poor at English. I ask to be excused.
I am using the AT91SAM7S256.
I declared the RSTC mode register, with following

#define RSTC_MR *((volatile unsigned int *)(0xFFFFFD08))

and insert 0xA5000000. with following

RSTC_MR = 0xA5000000;

Then, I read the register to confirm whether the value is right. But the register is cleared at 0x0.

Otherwise declared the register, with following

#define RSTC_MR *((unsigned int *)(0xFFFFFD08))

and insert 0xA5000000.

Then I can read.

I wonder why this operation is done.

thanks, regards.

Without the volatile you see a compiler manifestation of the value you wrote, held over in an ARM register.

The "A5" key does not read back, the point is that you write *with* the key to confirm you are a valid writer, not some random read/write on memory. Otherwise what would be the point of protecting the peripheral register?