A Deeper Look into Microprocessor Instruction Cycle

The Microprocessor Instruction Cycle Explained

A microprocessor provides an instruction capable of moving a string of bytes from one area of memory to another. The fetching and initial decoding of the instruction takes 15 clock cycles. Thereafter, it takes 20 clock cycles to transfer each byte. The microprocessor is clocked at a rate of 10 GHz.

1. Determine the Length of Instruction Cycle

For a string of 56 bytes:
The instruction cycle for a string of 56 bytes involves an initial fetching and decoding phase, which takes 15 clock cycles. The transfer of 56 bytes would require 56 * 20 = 1120 clock cycles. Therefore, the total instruction cycle for this case would be 15 (initial setup) + 1120 (byte transfer) = 1135 clock cycles.

2. Worst Case Delay for Acknowledging Interrupt (Non-Interruptible)

If the instruction is non-interruptible, any interrupt needs to wait until the entire instruction is processed. The worst-case delay for acknowledging an interrupt would be the whole duration of the instruction cycle, which is 1135 clock cycles. At a clock rate of 10 GHz, this equates to 113.5 nanoseconds.

3. Worst Case Delay for Acknowledging Interrupt (Instruction can be Interrupted)

If the instruction can be interrupted at the beginning of each byte transfer, the worst-case delay occurs when an interrupt happens just after a byte transfer has started. This leads to a wait of 20 clock cycles (or 2 nanoseconds at a 10 GHz clock rate) before the interrupt is acknowledged.