Question

An 8085 microprocessor executes an instruction in 4 machine cycles containing 4, 3, 3, and 3 T-states respectively. If the clock frequency is 3 MHz, the execution time is approximately

• A. \(3.33\ \mu\text{s} \)
• B. \(4.33\ \mu\text{s} \)
• C. \(4.67\ \mu\text{s} \)
• D. \(13.33\ \mu\text{s} \)

Hint:

Whenever dealing with microprocessor execution times, remember: \[ \text{Total Time} = \frac{\sum \text{T-states}}{\text{Frequency}} \] Keep values fractional until the final step (\(\frac{13}{3}\)) to avoid rounding errors and quickly locate the repeating decimal answer choice.

Answer 1

The Correct Option is B

Concept: In an 8085 microprocessor system, instructions are split into smaller operations called machine cycles (like Opcode Fetch, Memory Read, Memory Write). Each machine cycle is composed of several fundamental internal clock periods called T-states. The overall time needed to complete an instruction execution depends on two parameters:
• The total count of T-states (\(N_{\text{total}}\)) required across all its machine cycles.
• The time duration of a single T-state (\(T_{\text{clk}}\)), which is the reciprocal of the operating system clock frequency (\(f_{\text{clk}}\)). The final formula is: \[ \text{Execution Time} = N_{\text{total}} \times T_{\text{clk}} = \frac{N_{\text{total}}}{f_{\text{clk}}} \]

Step 1: Calculating the total number of T-states.
The instruction consists of 4 distinct machine cycles. Let's add up their individual T-state values: \[ N_{\text{total}} = 4 + 3 + 3 + 3 = 13 \text{ T-states} \]

Step 2: Finding the duration of a single T-state.
The operational system clock frequency provided is $3\text{ MHz}$. \[ f_{\text{clk}} = 3\text{ MHz} = 3 \times 10^6\text{ Hz} \] The time duration for one single T-state is: \[ T_{\text{clk}} = \frac{1}{f_{\text{clk}}} = \frac{1}{3 \times 10^6\text{ s}} = \frac{1}{3}\ \mu\text{s} \approx 0.3333\ \mu\text{s} \]

Step 3: Calculating total execution time.
Multiply the total T-states by the duration of one T-state: \[ \text{Execution Time} = 13 \times \left(\frac{1}{3}\ \mu\text{s}\right) = \frac{13}{3}\ \mu\text{s} \] Performing the fractional division: \[ \text{Execution Time} \approx 4.3333\ \mu\text{s} \] This precisely matches option (2). Hence, the correct choice is option (2).