HP 21MX E-Series

The E-Series computers had the same basic architecture as the M-Series machines. The primary difference was in processing speed. The E-Series incorporated a technology "Variable Microcycle Timing" (VMT) which allowed the computer to allocate its fastest data paths (175 nanoseconds) to the most commonly used operations. This technology gave the E-Series about twice the processing power of the M-Series. The E-Series computers also offered optional high-speed memory cycle times of 350 nanoseconds, compared with 650 nanoseconds on M-Series memory.