In practice a computer spends a fraction of the time performing internal operations as well as memory accesses. Consequently, the effective speedup ratio is reduced, because cache memory has no effect on internal operations. The average time required to execute an instruction can be written

Finternal = fraction of time spent doing internal operations ( 0 to 1)
Fmemory = fraction of time spent doing memory accesses ( 0 to 1)
tcyc = clock cycle time
tc= cache access time expressed in clock cycles
td= delay penalty paid when not accessing cache memory expressed in clock cycles