Using C, I was trying to assign a variable name to a register address so that my code would be readable. An example of how to do this is as follows:
#define DDRA (*(volatile unsigned char *)(0x22))
This means that if a register or memory location exists at address 0×22, I can use DDRA to read or write to it like so..
DDRA = 0x05
in my C code.
The #define looks really cryptic at first. The way to understand this is by breaking it down into pieces.
First of all,
unsigned char
means we are using a byte-sized memory location. Byte being 8-bits wide.
unsigned char *
means we are declaring a pointer that points to a byte-sized location.
(unsigned char *) (0x22)
means the byte-sized pointer points to address 0×22. The C compiler will refer to address 0×22 when the variable DDRA is used. The assembly code will end up using 0×22 in Load(LD) and Store (STR) insturctions.
(*(unsigned char *)(0x22))
The first asterisk from the left signifies that we want to manipulate the value in address 0×22. * means “the value pointed to by the pointer”.
volatile
volatile forces the compiler to issue a Load or Store anytime DDRA is accessed as the value may change without the compiler knowing it.
