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hi...could somebody describe the architecture of an 8051 microcontroller cpu?
Pages 10 and 11 of the first link discuss the CPU clock. They have a diagram of how an oscillator is connected to the part. And it has a discussion of the notion of instruction cycles.
The 8051 core usually measures execution in terms of instruction cycles. Individual instructions take between 1..3 instruction cycles to execute depending on how complex they are. (See the manual.) The original 8051 required 12 clocks to progress through one instruction cycle, so one instruction might take 12..36 clock cycles. With a 12MHz clock input, this means one instruction cycle takes one microsecond, which some people find convenient for timing.
More modern parts generally need fewer clocks per instruction cycle. 4 clocks/cycle is common, but I've also seen 6, 2, and 1 clock/cycle parts.
The clock input to peripherals like a timer, baud rate generator, and so on, is generally the oscillator frequency sent through some sort of input stage that lets you divide or "prescale" the clock to a lower frequency so that you get a more useful or larger range for your timers. Details vary in this area, so check the data sheet for your particular part.