The 80C51 has separate address spaces for program and data memory. The Program memory can be up to 64k bytes long. The lower 4k can reside on-chip. Figure 1 shows a map of the 80C51 program memory. The 80C51 can address up to 64k bytes of data memory to the chip. The MOVX instruction is used to access the external data memory. The 80C51 has 128 bytes of on-chip RAM， plus a number of Special Function Registers （SFRs）。 The lower 128 bytes of RAM can be accessed either by direct addressing （MOV data addr） or by indirect addressing （MOV @Ri）。 Figure 2 shows the Data Memory organization.

　　The 128 bytes of RAM which can be accessed by both direct and indirect addressing can be divided into three segments as listed below and shown in Figure 3. 1. Register Banks 0-3： Locations 0 through 1FH （32 bytes）。 The device after reset defaults to register bank 0. To use the other register banks， the user must select them in software. Each register bank contains eight 1-byte registers 0 through 7. Reset initializes the stack pointer to location 07H， and it is incremented once to start from location 08H， which is the first register （R0） of the second register bank. Thus， in order to use more than one register bank， the SP should be initialized to a different location of the RAM where it is not used for data storage （i.e.， the higher part of the RAM）.