Internal ROM in 8051.

The 8051 is organized so that data memory and program code memory can be in two entirely different physical memory entities.

Each has the same address ranges. The structure of the internal RAM has been discussed previously. A corresponding block of internal program code contained in an internal ROM occupies code address space 0000h to FFFFh.

The program addresses higher than 0FFFh, which exceed the internal ROM capacity, will cause the 8051 to automatically fetch code bytes from external program memory. Code bytes can also be fetched exclusively from an external memory, addresses 0000h to FFFFh, by connecting the external access pin to ground.
 
The PC doesn't care where the code is the circuit designer decides whether the code is found totally in internal ROM, totally in external ROM, or in a combination of internal and external ROM.


NAME                         FUNCTION                 INTERNAL RAM ADDRESS (HEX)

A                     Accumulator                                         0E0
B                    Arithematic                                            0F0
DPH               Addressing External memory                83
DPL               Addressing External memory                 82
IE                   Interrupt enable control                         0A8
IP                   Interrupt enable control                         0B8
P0                  Input/output latch port                           80
P1                  Input/output latch port                           90
P2                  Input/output latch port                           A0
P3                  Input/output latch port                           0B0
PCON           Power Control                                        87
PSW            Program Status Word                              0D0
SCON           Serial Port Control                                 98
SBUF            Serial port data buffer                           99
SP                  Stack Pointer                                         81
TMOD           Timer/counter mode control                 89
TCON            Timer/counter control                           88
TL0               Timer 0 low byte                                    8A
TH0               Timer 0 HIGH byte                               8C
TL1               Timer 1 low byte                                    8B
TH1               Timer 0 high byte                                  8D


    These are the hexadecimal commands to the respective functions.

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