The huge disparity in access times for various kinds of memory is what allows multiprogramming to succeed. In the table at the top of the next page, the approximate access times of registers, memory and secondary storage are listed. Note the huge jump between main memory and even the fastest hard disks -- about four orders of magnitude, or a factor of one thousand.

To read from a      it requires about
---------------------------------------------------
   register                      70 nanoseconds
   main memory cell             700 nanoseconds
   hard disk sector       1,000,000 nanoseconds