Levels of abstraction 1968: Dijkstra paper on his experiences with T.H. E Multiprograming system Designed using "levels of abstraction System design described in layers Higher levels could use services of lower levels Lower levels could not access higher levels Lowest level implemented first Provided a virtual machine"for implementation of next level Process continued until highest level completed a bottom up"technique Copyright Nancy Leveson, Sept 1999Levels of Abstraction 1968: Dijkstra paper on his experiences with T.H.E. Multiprograming system Designed using "levels of abstraction" System design described in layers Higher levels could use services of lower levels Lower levels could not access higher levels Lowest level implemented first Provided a "virtual machine" for implementation of next level Process continued until highest level completed. A "bottom up" technique Copyright Nancy Leveson, Sept. 1999 c �