9 - THE DYNAMIC USE OF MEMORY

Abstract

This chapter discusses the use of memory by computing systems. The memory used by Fortran variables can be divided into three classes, namely, common, local, and parametric. These variables appear on the SUBROUTINE or FUNCTION card. All variables that are not common or parametric are local. The chapter also discusses the software techniques for using memory. It discusses techniques for using memory, which are dependent on hardware. These techniques cannot be implemented unless the hardware is available. The chapter presents an idealized example of virtual memory and then discusses some specific examples. It also presents some techniques that utilize a virtual memory. The concept of virtual memory applies to all of the systems routines except the page interrupt and the I/O routines. The page interrupt routine uses the normal system I/O routines to read and write the pages from the drum. The first practical application of the virtual-memory technique was on the I.C.T. Atlas computer. To be used efficiently, the virtual memory must be used in a time-sharing system. When a page interrupt occurs, the current program cannot proceed until the requested page is brought from the drum.