Trying to understand the workings of a Memory Mapped I/O

Here is the sample area:

#define P0 ((*(__R volatile unsigned char *)(unsigned char)0x0))
#define P1 ((*(__R volatile unsigned char *)(unsigned char)0x1))
#define P2 ((*(__R volatile unsigned char *)(unsigned char)0x2))
#define P3 ((*(__R volatile unsigned char *)(unsigned char)0x3))


How would you describe what this is or does? If someone could break it down
so I could understand what is going on I would be in your debt.