Memory Device Characteristic

Reporter: joannou h. fegaridoMemory Device Characteristic

Memory Device Characteristic

There are various type of memory systems which are used for storing data in computer.All these devices have advantages and some disadvantages over the other.

Characteristic of MemoryAccess TimeAccess RateAccess ModeAlter abilityPermanenceofStorageCycle Time

Access TimeThe duration of time between the initiation of a read instruction signal and the availability of the required wordin the memory buffer register

Similarly the duration between the write signal and storing of the information or word in the specified location is called write time.

Access time and Write time - Depends on the physical characteristics of the storage medium and the type ofaccess mechanism.

A basic measure of the operating speed of memory is its access time.

Access RateThe number of characters or words that can be accessed within a specified time interval.

It is reciprocal of the access time. It is measured in words/sec. Low cost andhigh access rates are desirable memory characteristics.

Access ModeIt is defined as the order or sequence in which information can be accessed

Memory classified into two:Sequential access memories Random access memories

Memory Classification

Sequential access memories Sequential access memory (SAM) is a class of data storage devices that read their data in sequence Are those where memory locations canbe accessed only in certain predetermined sequence.

Random Access MemoryMemory locations may be accessed in any order and access time is independent of the location being accessed

Types of random access memoryDRAM - memories (Dynamic Random Access Module), which are inexpensive. They are used essentially for the computer's main memorySRAM - memories (Static Random Access Module), which are fast and costly. SRAM memories are used in particular for the processorscache memory

Alter abilityThe method used to write information into a memory may be irreversible. Once information has been written it cannotbe altered while the memory is in use

PermanenceofStorageIn some type of memories the stored information may be lost over a period of time unless appropriate action is takenThere arethree memory characteristics that can destroy information. DestructiveReadoutDynamicStorageVolatileMemories

Destructive ReadoutSome memories have the property that themethod of reading the memory destroys the stored information is calleddestructive readout DRO.

Memories in which readingdoes not affect the stored data are said to have Nondestructive readout (NRDO)

Dynamic StorageMemories which require periodic refreshing. Static memories are those which require norefreshing.

Certain memories will discharge due to some physical decayprocess. Over a period of time a stored charge completely leaks causing aloss of information unless the charge is restored.

Volatile MemoriesIn some memories the contents will be present only as long aspower supply is there. If the power supply is removedor power failure occurs then the contents are totally destroyed. This type of memories are knownas volatile memories. Most semi conductor memories are volatile, while most magnetic memories are nonvolatile.

Cycle TimeIn DRO and dynamic memories memory access process cannot be continued one after the other unless the restore or refresh operation has been carried out.i.e., the minimum time that must elapse between the two different accesses by the memory can be greater thanaccess time. This time is called the cycle time of the memory

Hierarchical memory systemHierarchical memory system including separate cache memories for storing data and instructions

Additionally, each cache having a unique cache directory containing a plurality of control bits for assisting line replacement within the individual cache memories and for eliminating many accesses to main memory and to insure that unnecessary or incorrect data is never stored back into said main memory.

The cache architecture and controls permit normal instruction and data cache fetches and data cache stores.instructions are provided for setting the special control bits provided in both the instruction and data cache directories

Hierarchical Memory Systems