1. Cosc 3P92 Week 9 & 10 Lecture slides
Violence is the last refuge of the incompetent.
Isaac Asimov, Salvor Hardin in "Foundation"

2. Virtual Memory
The main idea is to allow programs to address more memory locations than are physically available.
first: overlays programmer manually divide programs into sections, and read, write them out during execution.
virtual memory: automatic transparent overly mgmt
virtual (logical) address: generated by program physical address: actual memory address during execution
Addresses generated by programs are called virtual addresses, and which are mapped into physical addresses during execution time.

3. Virtual Memory Three techniques: paging system segmentation system
program
virtual
address VM
algorithm
physical
Main
memory
secondary
storage
Three techniques:
paging system
segmentation system
paged segmentation system

4. Virtual Memory: Paging
Paging system: The virtual address space is divided into equal-sized pages and the physical memory is divided into frames of the same size.
Physical memory
Frame 1 2 3 4 5 6 7
(4K per frame)
(32K total)
Page Table
Virtual Memory
page 0
page 1
page 2
page 3
page 4
page 5
page 6
page 7
page 8
page 9
page 10
page 11
page 12
page 13
page 14
page 15
page #
offset
Virtual address
(4K per page)
(64K total)
3-bit
4-bit
12-bit
1-bit
IF PageTable[page #]. residence-bit =1 THEN
RETURN Frame[ PageTable[page #].frame ]+offset
ELSE
page fault

5. Example

6. Paging • page mapping mechanism is often done via special hardware
• page fault: page not resident
- read from secondary storage into main memory
- update page table with physical memory address
- repeat instruction that caused fault
• demand paging: get page only when asked for
vs. algorithms which evaluate page usage and do predictive
page fetches
• working set: the finite set of pages which a program will use
during execution (ie. progrms don't use infinite memory)
• when working set size > # page frames, thrashing likely
total
pages
used
time --->

7. Paging Page replacement:
random (not recommended)
least recently used
first-in-first-out (least recently paged in)
can use dirty bits: write back pages only when modified
fragmentation: page sizes fixed, so can have unused page portions (and therefore unused memory)
program
memory
space
virtual mapping

8. Paging Large page sizes: Small page sizes:
maximal use of slow secondary storage
less frequent page reads
smaller tables
more apt to have fragmentation
Small page sizes:
less fragmentation
good for programs that use small spread out memory references
need more IO calls, larger tables

9. Virtual Memory: Segmentation
Segmentation system: A program is divided into a collection of logical segments (e.g., procedures, arrays, stacks) which may be of different sizes. Each segment has a separate virtual address space.
segment B
segment A
Physial Memory 1
10K 7K 4K 5K
Segment Table
seg #
offset
Virtual address
2-bit
N-bit
unused
Virtual Memory
segment A 1
segment B 2
segment C
IF SegmentTable[seg #].valid-bit = 1 THEN
IF offset < SegmentTable[seg #].length THEN
RETURN SegmentTable[seg #].base + offset
ELSE
memory violation
segment fault 3
segment D

10. Segmentation
Segments: can be allocated for different program parts, data structures, users
Different modes, access privileges can be assigned
(permits memory violation checking, shared memory,...)
Sizes are alterable during execution.
The basis for multitasking multi-user systems

11. Segmentation
Garbage collection: best fit, first-fit, hole compaction, ...

12. Paged segmentation system
Merges ideas of previous 2 approaches
Segments are divided into groups of pages
Virtual address is a triple <s,p,d>
e.g MULTICS

13. Paged segmentation Address mapping: Speeding up virtual memory access
1. segment numbers leads to index in segment table
gives base address of page table
2. page no. p indexed into page table, and frame p' is found
3. physical address computed by adding displacement d to p'
Speeding up virtual memory access
address translation look aside buffer
like a page table cache
associative memory contains most recently used page info
common in mainframes
[fig next page]

14. Physical Address

15. Paged vs Segmentation

16. Example: Pentium II VM support
A paged-segmentation system
MMU: memory mgmt unit on CPU
PII has segment registers: DS (data), CS (code), ...
[6.12, 6.13, 6.14]
Scheme:
selector: 16-bits, 1 per segment, loaded into approp. segment register
local: application; global: OS, system
corresp. segment descriptor loaded for that segment
offset is checked to see if it is beyong segment bounds (TRAP - software interrupt)
seg size: G = 0 (seg size up to 1 Mb); G=1 (pages)
base added to offset to get a linear address (base split for back-compatibility with 286)
if paging off, this address is physical address
if paging on: --> it’s a virtual address
must be mapped: page directory (1K entries) --> page table (1K page entries) --> phys address [6.15]

20. Example: ultraSparc VM support
Paged virtual memory supported
only 44 bits of 64-bit addr space used for VM
varying page sizes: [6.17]
TLB: translation lookaside buffer [6.18]
maps virtual page # to physical page frame #
64 most recently used virt. page #’s for each of instns, data
also a context -- process ID
if not in TLB, a trap called, and OS software must fix
not same as a page miss! page may be in memory.
OS must also maintain a TSB (trans. storage buffer) - like a cache of pages
otherwise, if a TSB miss, then OS does whatever it wants to implement it --> no H/W support!

23. Comparing PII and UltraSparc
32-bit segments, fixed 1K segment/page table sizes
max 1 million pages per segment
can do pure segmentation, pure paging, or paged segmentation
only 1 segment per process in Windows, Unix
UltraSparc:
huge address space
2 billion pages: conventional page tables unworkable

24. The end