ENVS 1800 Lecture Notes - Lecture 35: Linked List
14 May 2018
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ENVS 1800 Lecture 35 Notes – The Concept of Virtual Storage
Introduction
• There must be a single resource that identifies the entire pool of available memory
frames from which the memory manager may draw, when required.
• There are two common approaches in use.
• One is to provide an inverted page table, which lists every memory frame with its
associated process and page.
• This table shows the actual use of physical memory at every instant. Any frame without
an associated page entry is available for allocation.
• Well leave it as a siple eerise for ou to idetif the availale fraes.
• A second method maintains a list of available frames, usually as a simple linked list.
• When a process needs frames, it takes them from the top of the list.
• When a process exits, its frames are added to the end of the list.
• Since frame contiguity is unimportant, this is an effective way to manage the free frame
pool.
• The first two issues of memory management that we raised initially are thus solved.
• But, as the TV ifoerials sa, Wait—theres still ore!
• As we noted before, the third major challenge for memory management is the limited
total quantity of physical memory available.
• Even hundreds of megabytes of memory can only hold a few modern programs.
• Up to this point, we have assumed that there is a frame available for every page that
needs one.
• While page-to-frame translation has eliminated the question of how to fit programs into
existing memory space, the next step is more important and useful
• We will show you how the concept of virtual storage allows the system to extend the
address space far beyond the actual physical memory that exists.
• As you will see, the additional address space required to store a large number of
programs is actually provided in an auxiliary form of storage, usually disk
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