Operating Systems

================ Start Lecture #18 ================

4.2.2: Memory Management with Linked Lists

Memory Management using Boundary Tags

Homework: 5.

MVT also introduces the ``Replacement Question''

That is, which victim should we swap out? Note that this is an example of the suspend arc mentioned in process scheduling.

We will study this question more when we discuss demand paging in which case we swap out part of a process.

Considerations in choosing a victim

  1. So far the schemes presented so far have had two properties:
    1. Each job is stored contiguously in memory. That is, the job is contiguous in physical addresses.
    2. Each job cannot use more memory than exists in the system. That is, the virtual addresses space cannot exceed the physical address space.

  2. Tanenbaum now attacks the second item. I wish to do both and start with the first.

  3. Tanenbaum (and most of the world) uses the term ``paging'' to mean what I call demand paging. This is unfortunate as it mixes together two concepts.
    1. Paging (dicing the address space) to solve the placement problem and essentially eliminate external fragmentation.
    2. Demand fetching, to permit the total memory requirements of all loaded jobs to exceed the size of physical memory.

  4. Tanenbaum (and most of the world) uses the term virtual memory as a synonym for demand paging. Again I consider this unfortunate.
    1. Demand paging is a fine term and is quite descriptive.
    2. Virtual memory ``should'' be used in contrast with physical memory to describe any virtual to physical address translation.

** (non-demand) Paging

Simplest scheme to remove the requirement of contiguous physical memory.

Example: Assume a decimal machine with page size = frame size = 1000.
Assume PTE 3 contains 459.
Then virtual address 3372 corresponds to physical address 459372.

Properties of (non-demand) paging.

Homework: 16.

Address translation

Choice of page size is discuss below.

Homework: 8.

4.3: Virtual Memory (meaning fetch on demand)

Idea is that a program can execute even if only the active portion of its address space is memory resident. That is, swap in and swap out portions of a program. In a crude sense this can be called ``automatic overlays''.


** 4.3.1: Paging (meaning demand paging)

Fetch pages from disk to memory when they are referenced, with a hope of getting the most actively used pages in memory.

Homework: 12.