Volatile

Almost every firmware interviewer will ask you about volatile variable. If you don't understand it, then you must read it carefully now!!!


A variable should be declared volatile whenever its value could change unexpectedly. In practice, only three types of variables could change:

1) A pointer points to a memory mapped hardware registers (for example, status registers)
2) Non-automatic variables which can be modified by an interrupt service routine
3) Shared variables within a multi-threaded application

Ex.

UINT1 * ptr = (UINT1 *) 0x1234;

// Wait for register to become non-zero.

while (*ptr == 0);

// Do something else.

should be changed to

UINT1 volatile * ptr = (UINT1 volatile *) 0x1234;

the meaning is: ptr point to a variable that may be unexpectedly modified, so every time when we deference it(*ptr), we need to actually retrieve data from its memory address.

Reference

http://book.opensourceproject.org.cn/embedded/oreillyembed/opensource/0596009836/id-i_0596009836_chp_7_sect_1.html