C pointers and the physical address

Question

I\'m just starting C. I have read about pointers in various books/tutorials and I understand the basics. But one thing I haven\'t seen explained is what are the numbers.

Answer 1

Lots of PC programmer replies as always. Here is a reply from a generic programming point-of-view.

You will be quite interested in the actual numerical value of the address when doing any form of hardware-related programming. For example, you can access hardware registers in a computer in the following way:

#define MY_REGISTER (*(volatile unsigned char*)0x1234)

This code assumes you know that there is a specific hardware register located at address 0x1234. All addresses in a computer are by tradition/for convenience expressed in hexadecimal format.

In this example, the address is 16 bits long, meaning that the address bus on the computer used is 16-bits wide. Every memory cell in your computer has an address. So on a 16-bit address bus you could have a maximum of 2^16 = 65536 addressable memory cells. On a PC for example, the address would typically be 32 bits long, giving you 4.29 billion addressable memory cells, ie 4.29 Gigabyte.

To explain that macro in detail:

• 0x1234 is the address of the register / memory location.
• We need to access this memory location through a pointer, so therefore we typecast the integer constant 0x1234 into an unsigned char pointer = a pointer to a byte.
• This assumes that the register we are interested in is 1 byte large. Had it been two bytes large, we would perhaps have used unsigned short instead.
• Hardware registers may update themselves at any time (their contents are "volatile"), so the program can't be allowed to make any assumptions/optimizations of what's stored inside them. The program has to read the value from the register at every single time the register is used in the code. To enforce this behavior, we use the volatile keyword.
• Finally, we want to access the register just as if it was a plain variable. Therefore the * is added, to take the contents of the pointer.

Now the specific memory location can be accessed by the program:

MY_REGISTER = 1;
unsigned char var = MY_REGISTER;

For example, code like this is used everywhere in embedded applications.

(But as already mentioned in other replies, you can't do things like this in modern PCs, since they are using something called virtual addressing, giving you a slap on the fingers should you attempt it.)