sentences:  SLS is now most often associated with IBM i (formerly known as "i5/OS" or "OS/400"), the operating system of the IBM System i, although 
  IBM first implemented SLS in 1978 in the System/38 and its Control Program Facility (CPF) operating system, the predecessor to IBM i.With a single-level storage the entire storage of a computer is thought of as a single two-dimensional plane of addresses, pointing to pages.Pages may be in primary storage (RAM) or in secondary storage (disk); however, the current location of an address is unimportant to a process.