There was to be a store, or memory, capable of holding 1,000 numbers of 40 decimal digits each (ca. kB ). An arithmetical unit , called the "mill", would be able to perform all four arithmetic operations , plus comparisons and optionally square roots . Initially it was conceived as a difference engine curved back upon itself, in a generally circular layout,  with the long store exiting off to one side. (Later drawings depict a regularized grid layout.)  Like the central processing unit (CPU) in a modern computer, the mill would rely upon its own internal procedures, roughly equivalent to microcode in modern CPUs, to be stored in the form of pegs inserted into rotating drums called "barrels", to carry out some of the more complex instructions the user's program might specify.