256 BYTES OF INTERNAL RAM

The standard 8051 microcontroller contains 128 bytes of Internal RAM that are available to the developer as working memory for variables and/or for the operating stack. Instructions that refer to addresses in the range of 00h through 7Fh refer to the 8051's Internal RAM, while addresses in the range of 80h through FFh refer to Special Function Registers (SFRs).

Although the 8052 has 256 bytes of Internal RAM, the above method of referencing them remains true. Any address between 00h and 7Fh refers to Internal RAM whereas address in the range of 80h through FFh refer to SFRs.

The 8052's additional Internal RAM may only be referred by Indirect Addressing. Indirect addressing always refers to Internal RAM, never to an SFR.

Thus, to read the value contained in Internal RAM address 90h, the developer would need to code something along the lines of the following:

    MOV R0,#90h  ;Set the indirect address to 90h

    MOV A,@R0   ;Read the contents of Internal RAM pointed to by R0

The above code first assigns the value 90h to the register R0. It subsequently reads, indirectly, the contents of the address contained in R0 (90h). Thus, after these two instructions have executed, the Accumulator will contain the value of Internal RAM address 90h.

It is very important to understand that the above code is not the same as the following:

    MOV A,90h   ;Reads the contents of SFR 90h (P1)

This instruction uses direct addressing; recall that direct addressing reads Internal RAM when the address is in the range of 00h through 7Fh, and reads an SFR when the address is in the range of 80h through FFh. Thus in the case of this second example, the move instruction reads the value of SFR 90h-which happens to be P1 (I/O Port 1).