معمارية : المحاضرة الاولى- هرمية الذاكرة

Introduction
Computer architecture is concerned with the structure and behavior of the computer as seen by the user. It includes those attributes of the system that are visible to a programmer and have a direct impact on the execution of a program:
• Instruction sets
• Data representation – number of bits used to represent data
• Input/Output mechanisms
• Memory addressing techniques
Computer architecture is concerned with all aspects of the design and organization of the central processing unit and the integration of the CPU into the computer system itself. Architecture extends upward into computer software because a processor’s architecture must cooperate with the operating system and system software. It is impossible to design an operating system well without a knowledge of the underlying architecture. Moreover, the computer designer has to have an intimate understanding of software in order to implement the optimum architecture.
The main components of computer
The computer system cosists of three main components: the central processing unit (CPU), the main memory, the I/O system:

Figure 1: CPU main components and interactions with the memory and I/O
Memory unit
The registers are located at the top of typical memory hierarchy then a small, expensive, and relatively fast unit, called the cache is found, followed by a larger, less expensive, and relatively slow main memory unit which 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.

Figure 2: Typical memory hierarchy
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.

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:

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