محاضرة ساعة المناقشة - المقاييس

1.1 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.

2. Performance Measures
In this section, we consider the important issue of assessing (تقييم) the performance of a computer by studying three measures:
a) CPI (clock cycles per instruction)
b) MIPS (million instructions-per-second)
c) MFLOPS (Million floating-point instructions per second)

2.1 CPI (clock cycles per instruction)

Performance analysis should help answering questions such as how fast can a program be executed using a given computer? In order to answer such a question ,we need to determine the time taken by a computer to execute a given job. We define the clock cycle time as the time between two consecutive rising (صاعد) edges of a periodic clock signal :


The time required to execute a job by a computer is often expressed in terms of clock cycles. We denote the number of CPU clock cycles for executing a job to be the cycle count (CC), the cycle time by CT, and the clock frequency by f = 1/CT. The time taken by the CPU to execute a job can be expressed as :


It may be easier to count the number of instructions executed in a given program as compared to counting the number of CPU clock cycles needed for executing that program. Therefore, the average number of clock cycles per instruction (CPI) has been used as an alternate performance measure. The following equation shows how to compute the CPI:







It is known that the instruction set of a given machine consists of a number of instruction categories: ALU (simple assignment and arithmetic and logic instructions), load, store, branch, and so on. In the case that the CPI for each instruction category is known, the overall CPI can be computed as :