================ Start Lecture #9 ================

MVT also introduces the ``Replacement Question'', which victim to swap out.

We will study this question more when we discuss demand paging.

Considerations in choosing a victim

NOTEs:
  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.