Next we will take a closer look at the way the 8051 hardware is defined in C programs. 

Special Function Registers

The 8051 has many memory locations that are given special names and are called Special Function Registers (SFRs). Each is just a space in memory with an address like any other location in memory. To simply writing software, these spaces are given names and we use the names in our programs rather than the addresses. In C the SFR names are assigned to the correct memory locations using 

where P0 is the SFR name (Port 0 in this case) and 0x80 is the memory address in hex. (80 Hex is the address of Port 0.)

We can collect all of the SFR definitions and put them in a seperate file and just include that file at the beginning of each program we write. We have collected these as 8052.h.

Accessing Individual Port Pins

In the first two examples we moved data to the ports a whole byte at a time. We can also work with the individual pins of each port. The individual bits of each Port are defined in the 8052.h file and can be accessed using an underscore. For example P0_0 is bit 0 of Port 0.

Here is a simple example that reads the input from pin 39 (Port 0, Bit 0) and writes it to pin 1 (Port 1, Bit 0). transfer.c. The line P1_0 = P0_0; does the trick. The LED comes on when you connect pin 39 to ground and goes off when you connect it to 5 volts.