![]() ![]() The fastest instructions require one machine cycle (12 crystal pulses), many others require two machine cycles (24 crystal pulses), and the two very slow math operations require four machine cycles (48 crystal pulses). Since we know the crystal is pulsing 11,059,000 times per second and that one machine cycle is 12 pulses, we can calculate how many instruction cycles the 8051 can execute per second. ![]() That is to say, if an instruction takes one machine cycle to execute, it will take 12 pulses of the crystal to execute. Thus, the program can be easily modified by itself since it is stored in read-write memory. Therefore, it is impossible for program contents to be modified by the program itself. ![]() Typically, code (or program) memory is read-only and data memory is read-write. It is possible to access program memory and data memory simultaneously. The most obvious characteristic of the Harvard Architecture is that it has physically separate signals and storage for code and data memory. ![]() It is important to note that although there are different flavors of the 8051 in terms of speed and amount of on-chip ROM, they are all compatible with the original 8051 as far as the instructions are concerned. This has led to many, versions of the 8051 with different speeds and amounts of on-chip ROM marketed by more than half a dozen manufacturers. ![]()
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