bit wise manipulation on a 32-bit integer

Question

I am trying to read a 32 register,modify its first 8 bits[BIT7:BIT0] and write back its value. Does the code below achieve that?

reg_val = register_read(register_object);
reg_val = ((reg_val & 0xffffff00) | new_value));
register_write(register_object,reg_val);

Also is it the most efficient way to achieve that.any suggestions or comments are appreciated.

Answer 1

Unless new_value is guaranteed only 8 bits wide you should ensure it:

reg_val = ((reg_val & 0xffffff00) | (new_value & 0xff));

Also is it the most efficient way to achieve that?

Any compiler worth its salt will translate that into The Right Thing.

Answer 2

假设保证new_value <256，并假定它们是无符号整数，则可以，这是正确的方法。