NOTE: These notes are by Allan Gottlieb, and are reproduced here, with superficial modifications, with his permission. "I" in this text generally refers to Prof. Gottlieb, except in regards to administrative matters.


================ Start Lecture #8 (Feb. 24) ================

End of Fixed Length Partions

4.2.1: Memory Management with Bitmaps

Divide memory into blocks and associate a bit with each block, used to indicate if the corresponding block is free or allocated. To find a chunk of size N blocks need to find N consecutive bits indicating a free block.

The only design question is how much memory does one bit represent.

4.2.2: Memory Management with Linked Lists

Memory Management using Boundary Tags

PAGING

By far the most important and widely used memory management scheme in current machines. 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.

Address translation

Choice of page size is discuss below.

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''.

Advantages

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.

4.3.2: Page tables

A discussion of page tables is also appropriate for (non-demand) paging, but the issues are more acute with demand paging since the tables can be much larger. Why?

  1. The total size of the active processes is no longer limited to the size of physical memory. Since the total size of the processes is greater, the total size of the page tables is greater and hence concerns over the size of the page table are more acute.
  2. With demand paging an important question is the choice of a victim page to page out. Data in the page table can be useful in this choice.

We must be able access to the page table very quickly since it is needed for every memory access.

Unfortunate laws of hardware.

So we can't just say, put the page table in fast processor registers, and let it be huge, and sell the system for $1500.




For now, put the (one-level) page table in main memory.


Contents of a PTE

Each page has a corresponding page table entry (PTE). The information in a PTE is for use by the hardware. Information set by and used by the OS is normally kept in other OS tables. The page table format is determined by the hardware so access routines are not portable. The following fields are often present.

  1. The valid bit. This tells if the page is currently loaded (i.e., is in a frame). If set, the frame pointer is valid. It is also called the presence or presence/absence bit. If a page is accessed with the valid bit zero, a page fault is generated by the hardware.
  2. The frame number. This is the main reason for the table. It is needed for virtual to physical address translation.
  3. The Modified bit. Indicates that some part of the page has been written since it was loaded. This is needed when the page is evicted so the OS can know that the page must be written back to disk.
  4. The referenced bit. Indicates that some word in the page has been referenced. Used to select a victim: unreferenced pages make good victims by the locality property (discussed below).
  5. Protection bits. For example one can mark text pages as execute only. This requires that boundaries between regions with different protection are on page boundaries. Normally many consecutive (in logical address) pages have the same protection so many page protection bits are redundant. Protection is more naturally done with segmentation.