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 ...
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