CS 147
Virtual Memory
Prof. Sin Min Lee
Anthony Palladino
VIRTUAL MEMORY
Why Virtual Memory?
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When computers were first being developed, memory was very costly
and hard to come by.
Computers were the size of warehouses, yet often only had memory
which amounted to 128 kilobytes.
The complication of running out of memory space during run-time
began to surface and a solution was needed in order to alleviate this
problem.
A way to provide nearly limitless cheap memory was sought by
programmers.
What was developed was virtual memory.
Initial Problems
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Many early programmers thought it too strange a concept to be
pursued, and argued against it.
Addressing initially was a problem before a stable process was
developed.
Many thought that virtual memory would simply be to expensive to
implement correctly, as well as use too large a portion of the
processors power.
It was unclear at first how to mass produce the process for home use.
Some Terminology
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Virtual Memory- VRAM- Virtual Random Access
Memory
Physical Memory – Actually hardware memory
MMU – Memory management Unit
Real Address – Physical address in memory
Virtual address – address of virtual memory
Memory address – address of physical memory
Virtual Memory Features
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Virtual memory is much larger than real memory.
Today’s Primary job for virtual memory is to avoid
software conflicts.
Conversions are done on the Page tables.
MMU Controls the process.
The TLB allows for paging to remain fast.
Programs are no longer limited by amount of real memory.
(much larger problem when computers were first being
developed)
Virtual Memory Drawbacks
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Addressing is more difficult
 Conflicts can terminate programs
 Disk drive is much slower than memory
(thousands to millions of times)
How can programs be
separated?
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At any given time, only a fraction of a program is
running (90/10 Rule)
 Only parts needed for immediate execution, or that
are used repeatedly are loaded.
 Less accessed parts are left on disk and out of
memory in order to save space. (programs can be
several gigs these days, much too big for the
amount of memory most people have in their
computers)
Virtual Addressing
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Virtual memory must use a smart memory controller that
can convert between the byte and word addressing,
between the two memory systems.
This was later designed to be done without arithmetic
computation, so that the CPU would not lose much
processing power.
The easiest way to implement this was by keeping
everything in powers of two.
Virtual Memory is not part of the actual underlying
physical memory.
Each Program is able to operate in its own virtual address
space
Virtual Memory Interface
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Virtual memory is able to use complex memory organization
structures, primarily through the use of the MMU previously
mentioned.
This can be complicated as different types of memory (such as SRAM
and DRAM) use different word sizes. Additionally, RISC processors
discussed previously in class have access to more than one memory
structure.
The MMU is at the root of all virtual memory control.
MMU
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The MMU is a processor run controller that handles the
memory.
It serves as a hardware translator, doing conversions
The MMU is able to connect memories with different word
sizes, as well as virtual and real.
As far as the processor knows, there is one large memory,
with one set of addresses.
Mapping and Arithmetic
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Memory addressing is started from zero, and this is expect, so the
MMU does proper adjustments to keep this illusion.
This helps in simulating one large contiguous memory.
In order to keep the MMU from having to do intense mathematical
calculations in order maintain memory structure (such as subtraction),
it finds boundaries in powers of two, allowing the lower-order bits to
easily be used in the addressing scheme.
When a computer is created, the virtual memory is determined. When
a user adds or removes physical memory, it may create a hole, or a
place where virtual memory cannot be mapped to physical.
This can create an error due to discontinuity.
Dynamic creation of Virtual
Memory
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The processor can determine the mappings before running,
determining what is needed and what is not.
The processor then switches to real mode, using only the physical
memory.
Finally, after mapping is done, the MMU is enabled.
Paging
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Paging was developed as a more efficient replacement for
segmentation.
Memory is broken into blocks with equal sizes, known as pages.
Paging’s efficiency relies on page replacement, which moves the least
likely need paged out of physical memory.
The page system often relies on a power of two system, again to avoid
costly arithmetic calculations.
The MMU controls the page table.
Blocks and Paging
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Paging is controlled by the Page Map
 The Virtual address space is separated into
portions known as pages.
 Physical memory is separated into pieces
known as page frames.
 Pieces are all of equal size.
The Page Table
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The page table is a structure controlled by
the operating system.
 It contains the mapping of virtual addresses
to their physical location in memory.
 Every process and program is given its own
page table.
 The page table contains all possible page
mappings.
Multiprogramming and Virtual
Spaces
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Multiprogramming is designed so that multiple programs can be run at the
same time.
Early on, this was considered a bad idea, as memory conflicts could arise
between the different programs, and both would end in abnormal termination.
This could be solved by having programs designed to run intentionally using
different memory addresses, but this a great deal more intensive, and requires
much programming, while still being risky.
To alleviate this problem, memory partitioning was developed.
Virtual partitioned greatly increased the functionality of this, making it much
more feasible.
One limitation of the process is that it limits the total memory available to any
given process.
END