The figure below illustrates a system in which operations occur as three consecutive clock cycles. Actions taking place in clock cycle 1 are scalable; that is, if the clock cycle time changes, the actions can be speeded up or slowed down correspondingly. In cycle 2, the action process 1 requires 25 ns and in clock cycle 3 the action process 2 requires 32 ns. If the clock cycle is less than the time required for process 1 or process 2, then one or more wait cycles have to be inserted for the process to complete.

Question

Cycle and process times

What is the time to complete an operation if the clock cycle time is

a. 50 ns

b. 40 ns

c. 30 ns

d. 20 ns

e. 10 ns

Answer 1

a. 50 ns If the cycle time is 50 ns the total time is 3 × 50 ns = 150 ns.
b. 40 ns If the cycle time is 40 ns the total time is 3 × 40 ns = 120 ns.
c. 30 ns If the cycle time is 30 ns, Process 2 will not fit into Cycle 3, and a wait state will be necessary. The total time will be 30 ns + 30 ns + 2 x 30 ns = 120 ns.
d. 20 ns If the cycle time is 20 ns both cycles 2 and 3 will require one wait state. The total time is 20 ns + 40 ns + 40 ns = 100 ns.
e. 10 ns If the cycle time is 10 ns, Cycle 2 will require two additional wait states (to accommodate the 25ns Process 1) and Cycle 3 will require three additional wait states to accommodate the 32 ns Process
2. The total time is 10 + 30 + 40 = 80 ns.
NOTE: I have added time in units of complete cycles. Real systems can increment the clock in units of one half cycle.