(* (Volatile unsigned char *) 0x20)

For the (volatile unsigned char *) 0x20 Let us analyze, which is composed of two parts: 

1) (unsigned char *) 0x20, 0x20 is just a value, preceded (unsigned char *) indicates 0x20 is an address, but this address type is unsigned char, meaning that read and write to this address, unsigned char values ​​to be written into and read out is unsigned char. 

2) volatile, volatile keyword to ensure that this directive will not be optimized C compiler have been omitted, and require each direct reading. For example while ((unsigned char *) 0x20), sometimes the system might not actually read the value of 0x20, but with the first read-out value, if so, then the loop may be an endless loop. With a volatile read is required every time the actual value of 0x20. Then (volatile unsigned char *) 0x20 is a fixed pointer is immutable, not a variable. The char * u is a pointer variable. And then in the front plus "*": * (volatile unsigned char *) 0x20 becomes a variable (unsigned char variables common and not a pointer variable), if # define i (* (volatile unsigned char *) 0x20), then with unsigned char i is the same, but in front of the address of i is fixed. So your question can be answered, (* (volatile unsigned char *) 0x20) can be regarded as a normal variable, this variable has a fixed address, pointing to 0x20. The 0x20 is just a constant, not a pointer but not variable.