A word is retrieved based on a portion of its contents

  A word is retrieved based on a portion of its contents 

Location

 

·         Refers to whether memory is internal and external to the computer.
- internal memory, processor - registers - main memory - cache
- external memory consist of peripheral storage devices like disk and tape.

 

Capacity

 

·         Memory is typically expressed in terms of bytes

 

Unit of transfer

 

·         For internal memory the unit of transfer is equal to the number of electrical lines into and out of the memory module

 

How do the terms word, addressable unit, and unit of transfer relate to internal memory?

 

•       Word: The "natural" unit of organization of memory. The size of a word is typically equal to the number of bits used to represent an integer and to the instruction length.
• Addressable units: In some systems, the addressable unit is the word. However, many systems allow addressing at the byte level. In any case, 2A = N.
• Unit of transfer: For main memory, this is the number of bits read out of or written into memory at a time. The unit of transfer need not equal a word or an addressable unit.

 

Methods of accessing units of data

 

-       Sequential access
- Direct access
- Random access
- Associative access

 

Sequential access

 

-       Memory is organized into units of data, called records
- access must be made in a linear sequence
- access time is variable

 

direct access

 

-       involves a shared read-write mechanism
- individual blocks or records have a unique address based on physical location
- access time is variable

 

random access

 

-       each addressable location in memory has a unique, physically wired-in addressing mechanism
- The time to access a given location is independent of the sequence of prior accesses and is constant
- Any location can be selected at random and directly addressed and accessed
- main memory

 

associative

 

-       A word is retrieved based on a portion of its contents rather than its address
- Each location has its own addressing mechanism and retrieval time is constant independent of location or prior access patterns

 

performance parameters

 

-       access time (latency)
- memory cycle time
- transfer rate

 

access time (latency)

 

-       For random-access memory it is the time it takes to perform a read or write operation
- For non-random-access memory it is the time it takes to position the read-write mechanism at the desired location

 

memory cycle time

 

-       Access time plus any additional time required before second access can commence

 

transfer rate

 

-       The rate at which data can be transferred into or out of a memory unit
- For random-access memory it is equal to 1/(cycle time)

 

 

physical types of memory

 

-       Semiconductor memory
- Magnetic surface memory
- Optical
- Magneto-optical

 

physical characteristics

 

-       volatile / erasable
- nonvolatile / nonerasable

 

Volatile memory

 

·         Information decays naturally or is lost when electrical power is switched off

 

Nonvolatile memory

 

-       Once recorded, information remains without deterioration until deliberately changed
- No electrical power is needed to retain information

 

 

Magnetic-surface memories

 

·         Are nonvolatile

 

Semiconductor memory

 

·         May be either volatile or nonvolatile

 

Nonerasable memory

 

-       Cannot be altered, except by destroying the storage unit
- Semiconductor memory of this type is known as read-only memory (ROM)

 

Organization

 

·         refers to the physical arrangement of bits to form words

 

There is a trade-off among capacity, access time, and cost

 

-       Faster access time, greater cost per bit
- Greater capacity, smaller cost per bit
- Greater capacity, slower access time

 

Disk cache

 

·         A portion of main memory can be used as a buffer to hold data temporarily that is to be read out to disk

 

Virtual memory

 

·         Facility that allows programs to address memory from a logical point of view, without regard to the amount of main memory physically available

 

 

Mapping Function

 

·         Because there are fewer cache lines than main memory blocks, an algorithm is needed for mapping main memory blocks into cache lines

 

Direct mapping

 

·         Maps each block of main memory into only one possible cache line

 

 

Associative mapping

 

·         Permits each main memory block to be loaded into any line of the cache

 

set-associative mapping

 

·         A compromise that exhibits the strengths of both the direct and associative approaches while reducing their disadvantages

 

Victim Cache

 

-       Originally proposed as an approach to reduce the conflict misses of direct mapped caches without affecting its fast access time
- Fully associative cache

 

The most common replacement algorithms are:

 

-       Least recently used (LRU)
- First-in-first-out (FIFO)
- Least frequently used (LFU)

 

Least recently used (LRU)

 

-       Replace that block in the set that has been in the cache longest with no reference to it

 

First-in-first-out (FIFO)

 

-       Replace that block in the set that has been in the cache longest

 

Least frequently used (LFU)

 

-       Replace that block in the set that has experienced the fewest references

 

 

Write through

 

·         All write operations are made to main memory as well as to the cache

 

Write back

 

-       Minimizes memory writes
- Updates are made only in the cache

 

 

multilevel cache

 

·         As logic density has increased it has become possible to have a cache on the same chip as the processor

 

split cache

 

·         split the cache into two :
- One dedicated to instructions
- One dedicated to data,
Both exist at the same level