REPORTER: JOANNOU H. FEGARIDO

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.

Access Time Access Rate Access Mode Alter ability Permanence of Storage Cycle Time

The duration of time between the initiation of a read instruction signal and the availability of the required word in 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 of access mechanism. A basic measure of the operating speed of memory is its access time.

The 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 and high access rates are desirable memory characteristics.

It is defined as the order or sequence in which information can be accessed Memory classified into two: 1. Sequential access memories 2. Random access memories

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

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

DRAM - memories (Dynamic Random Access Module), which are inexpensive. They are used essentially for the computer's main memory SRAM - memories (Static Random Access Module), which are fast and costly. SRAM memories are used in particular for the processor’s cache memory

The method used to write information into a memory may be irreversible. Once information has been written it cannot be altered while the memory is in use

In some type of memories the stored information may be lost over a period of time unless appropriate action is taken There are three memory characteristics that can destroy information. Destructive Readout Dynamic Storage Volatile Memories

Some memories have the property that the method of reading the memory destroys the stored information is called destructive readout DRO. Memories in which reading does not affect the stored data are said to have Nondestructive readout (NRDO)

Memories which require periodic refreshing. Static memories are those which require no refreshing. Certain memories will discharge due to some physical decay process. Over a period of time a stored charge completely leaks causing a loss of information unless the charge is restored.

In some memories the contents will be present only as long as power supply is there. If the power supply is removed or power failure occurs then the contents are totally destroyed. This type of memories are known as volatile memories. Most semi conductor memories are volatile, while most magnetic memories are nonvolatile.

In 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 than access time. This time is called the cycle time of the memory

Hierarchical 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