هرمية الذاكرة

Memory hierarchy

a typical memory hierarchy starts with a small, expensive, and relatively fast unit, called the cache, followed by a larger, less expensive, and relatively slow main memory unit. It is customary to call the fast memory level the primary memory. The solid-state memory is followed by larger, less expensive, and far slower magnetic memories that consist typically of the (hard) disk and the tape. It is customary to call the disk the secondary memory, while the tape is conventionally called the tertiary memory.


The objective behind designing a memory hierarchy is to have a memory system that performs as if it consists entirely of the fastest unit and whose cost is dominated by the cost of the slowest unit.

The memory hierarchy can be characterized by a number of parameters:

access time :The term access refers to the action that physically takes place during a read or write operation.

The capacity of a memory level: which is usually measured in bytes.
The cycle time :it is defined as the time elapsed from the start of a read operation to the start of a subsequent read.

The latency :it is defined as the time interval between the request for information and the access to the first bit of that information.

The bandwidth provides a measure of the number of bits per second that can be accessed.

The cost of a memory level is usually specified as dollars per megabytes. the following figure depicts a typical memory hierarchy:


Typical memory hierarchy

The effectiveness of a memory hierarchy depends on the principle of moving information into the fast memory infrequently and accessing it many times before replacing it with new information. This principle is possible due to a phenomenon called locality of reference; that is, within a given period of time, programs tend to reference a relatively confined area of memory repeatedly


The sequence of events that takes place when the processor makes a request for an item is as follows:
First, the item is sought in the first memory level of the memory hierarchy. The probability of finding the requested item in the first level is called the hit ratio, h1. The probability of not finding (missing) the requested item in the first level of the memory hierarchy is called the miss ratio.

When the requested item causes a “miss,” it is sought in the next subsequent memory level. The probability of finding the requested item in the second memory level, the hit ratio of the second level, is h2. The miss ratio of the second memory level is (1 _ h2). The process is repeated until the item is found