An Operating Systems Vade Mecum

chapter 3SPACE MANAGEMENTThe management of both time and main store is motivated by the fact that processes generallyare designed to pretend that other processes don’t exist. It shouldn’t make anydifference to A whether or not B is also in the ready list. B’s actions should not havevisible repercussions on A’s store. Processes also ignore details about the machine, suchas how much main store there really is and where the kernel is in main store. Both ofthese features are consonant with the Beautification Principle, which makes the operatingsystem responsible for keeping ugly details out of view.However, multiprogramming works best when several processes are kept in mainstore at the same time, because process switching among them requires less work if dataneed not be transferred between main and backing store. The fact that several processesshare main store requires that the operating system allocate the main store resource effectively,in accord with the Resource Principle. In particular, the operating system mustprotect processes from each other, keep track of the current allocation state of main store,and arrange with the hardware for efficient execution.1 PRELIMINARIES1.1 SwappingThe operating system must arrange main store in such a way that processes can remainignorant of each other (following the Beautification Principle) and still share main store.57