Virtual Memory

By

Steven LaBarbera

What I plan to cover

The history of virtual memory How virtual memory has evolved How virtual memory is used today Why is virtual memory important

History of virtual memory

Early computers used a two-level storage system consisting of main memory and secondary memory.

main memory (RAM) for the first computers were magnetic cores.

The secondary memory (hard disks) were magnetic drums.

History of virtual memory

Early in computers infancy memory was neither inexpensive nor easy to find.

the most sophisticated computers were lucky to have 128 kilobytes of memory, a fraction of what a floppy disk can hold.

This lack of space had serious implications on older systems processing power.

Why did lack of space have serious implications?

Programmers had to re-code their program every time they switched machines or added more memory.

How did they deal with this?

programmers divided programs into sections that were transferred into main memory for a period of processing time. As the program proceeded, new sections moved into main memory and replaced sections that were not needed. In this early era of computing, the programmer was responsible for devising this overlay system.

The creation of virtual memory

The technique was invented at the University of Manchester for the Atlas Computer, completed in 1962.

How virtual memory works

when a memory location is read or written to hardware within the computer translates the memory address generated by the software (the virtual memory address) into a, usually distinct, real memory address

Continued

This is accomplished by preserving the low order bits of the binary representation of the input address while treating the high order bits as a key to one or more address translation tables.

a range of consecutive addresses in the virtual address space whose size is a power of two will be translated in a corresponding range of consecutive physical addresses.

Simple Paging

The memory referenced by such a range is called a page

In a simple paging system you view the users program as consisting of fixed length blocks

The page size is typically in the range of 512 to 8192 bytes

Simple Paging

In a paging system consider the addresses as consisting of two components.

1. The page number

2. The displacement into the page

Page 0

0FFF

Page 1

1FFF

Page 2

2FFF

Page 3

3FFF

The displacement component

This must be big enough to hold any displacement within a page.

Example 2^12 – byte (4k) page size, the displacement of this has to hold 2^12 – 1.

Thus a 2^12 displacement requires a 12 bit (3 hex digit) displacement field 2^n page requires n-bit displacement field

Translation from virtual memory to physical memory

Translations are managed by the Memory Management Unit (MMU) .

Translation from virtual memory to physical memory

( 1 ) or "logical address" consisting of a logical page number plus the location within that page (x) must be interpreted or "mapped" onto an actual (physical) main memory address by the operating system using an address translation function or mapper

Translation from virtual memory to physical memory

( 2 ) If the page is present in the main memory, the mapper substitutes the physical page frame number for the logical number

Translation from virtual memory to physical memory

( 3 ) If the mapper detects that the page requested is not present in main memory, a fault occurs and the page must be read into a frame in main memory from secondary storage

( 4 , 5 ).

What does the mapper do?

translates the logical page number generated by the program into the physical page frame number where the main memory holds the page

What does the mapper do?

This translation is accomplished by using a directly indexed table called the page table which identifies the location of all the program's pages in the main store

Page Table

As the operating system loads pages of a user program into memory, it uses a page table to keep track of the location of each page.

A page table is a one-dimensional array in main memory that contains a slot for every page in the users program.

Page Table

the page table shows that pages 0,1,2, and 3 are in frames 4,6,2,and 5

Page Number

0

1

2

3

Frame Number

4

6

2

5

DAT Unit

In the book the DAT Unit is used to translate virtual addresses to physical addresses.

Dynamic Address Translation Unit

Paging pros and cons

Pros

• Eliminates memory fragmentation problems seen in multiprogramming systems.

• Paging systems do not have compaction overhead that systems with relocation have.

Cons• Has to do two operations

every time CPU fetches an item from memory.

1. Obtain page number from page table.

2. Obtain desired item from main memory at the computed physical address.

How they fix this inefficiency

Carefully designing the DAT Unit. Every time CPU fetches an item from

memory you keep a copy of the page table in a local memory area of the DAT Unit.

So every time a DAT Unit translates an address from the program, it can get the framer number from local memory.

Demand Paging

Each page table contains two parts

1. The frame number

2. A valid bit

Valid bit field

The valid bit indicates if the corresponding page is in memory.

0 = not in memory

1= in memory

Why is Virtual Memory Important?

The advantage of virtual memory is that it allows a computer to multiplex its CPU and memory between multiple programs without the need to perform expensive copying of the programs' memory images. If the combination of virtual memory system and operating system supports swapping, then the computer may be able to run simultaneously programs whose total size exceeds the available physical memory.

Review

when a memory location is read or written to hardware within the computer translates the memory address generated by the software (the virtual memory address) into a, usually distinct, real memory address

Review

MMU or Memory Management Unit DAT Unit or Dynamic Address Translation

Unit

These are the techniques used to translate a virtual address to a physical address.

Review

Simple Paging

Fixed length blocks

Holds page number

Displacement field

Demand Paging

Holds frame number

Boolean valid bit

1 = in memory

0= not in memory

References

Virtual Memory Tutorial http://cne.gmu.edu/itcore/virtualmemory/vmhistory.html

Virtual Memory Modulehttp://cne.gmu.edu/modules/vm/