Discussion
Microcontroller 8051 operates with 12 MHz oscillator and one machine cycle is defined as 12 oscillator periods. The execution time for a 4 machine cycle instruction would therefore be :
Answer: A
The execution time is 4 µs.
\[ \begin{aligned} T_{osc} &= \frac{1}{12\ \text{MHz}} = \frac{1}{12}\ \mu s \\ T_{cycle} &= 12 \times \frac{1}{12}\ \mu s = 1\ \mu s \\ T_{instr} &= 4 \times 1\ \mu s = 4\ \mu s \end{aligned} \]
With the usual 12 MHz crystal, each machine cycle of the 8051 takes exactly 1 µs, so a 4-cycle instruction (such as MUL or DIV) takes 4 µs. 1 µs is a single machine cycle, and 8 µs or 2 µs would need a 6 MHz or 24 MHz clock.
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