UNIT I Fundamentals of Computer Design

The concept of stored program computers appeared in 1945 when John von Neumann drafted the first version of EDVAC (Electronic Discrete Variable Computer). Those ideas have since been the milestones of computers: • an input device through which data and instructions can be entered • storage in which data can be read/written; instructions are like data, they reside in the same memory • an arithmetic unit to process data • a control unit which fetches instructions, decode and execute them • output devices for the user to access the results. Four lines of evolution have emerged from the first computers (definitions are very loose and in many case the borders between different classes are blurring): 1. Mainframes: large computers that can support very many users while delivering great computing power. It is mainly in mainframes where most of the innovations (both in architecture and in organization) have been made. 2. Minicomputers: have adopted many of the mainframe techniques, yet being designed to sell for less, satisfying the computing needs for smaller groups of users. It is the minicomputer group that improved at the fastest pace (since 1965 when DEC introduced the first minicomputer, PDP-8), mainly due to the evolution of integrated circuits technology (the first IC appeared in 1958). 3. Supercomputers: designed for scientific applications, they are the most expensive computers (over one million dollars), processing is usually done in batch mode, for reasons of performance. 4. Microcomputers: have appeared in the microprocessor era (the first microprocessor, Intel 4004, was introduced in 1971). The term micro refers only to physical dimensions, not to computing performance. A typical microcomputer (either a PC or a workstation) nicely fits on a desk. Microcomputers are a direct product of technological advances: faster CPUs,  semiconductor memories, etc. For many years the evolution of computers was concerned with the problem of object code compatibility. A new architecture had to be, at least partly, compatible with older ones. The assembly language is no longer the language in which new applications are written, although the most sensitive parts continue to be written in assembly language, and this is due to advances in languages and compiler technology.  Performance Definition As a user you are interested in reducing the response time (also called the execution time or latency). The computer manager is more interested in increasing the throughput (also called bandwidth), the number of jobs done in a certain amount of time. Response time, execution time and throughput are usually connected to tasks and whole computational events. Latency and bandwidth are mostly used when discussing about memory performance. CPU Performance What is the time the CPU of your machine is spending in running a program? Assuming that your CPU is driven by a constant rate clock generator (and this is sure the case), we have: CPUtime = Clock_cycles_for_the_program * clock cycle time The above formula computes the time CPU spends running a program, not the elapsed time: it does not make sense to compute the elapsed time as a function of Tck, mainly because the elapsed time also includes the I/O time, and the response time of I/O devices is not a function of Tck. If we know the number of instructions that are executed since the program starts until the very end, lets call this the Instruction Count (IC), then we can compute the average number of clock cycles per instruction (CPI) as follows: The CPUtime can then be expressed as: CPUtime IC * CPI * clock cycle time The scope of a designer is to lower the CPUtime, and here are the parameters that can be modified to achieve this: • IC: the instruction count depends on the instruction set architecture and the compiler technology • CPI: depends upon machine organization and instruction set architecture. RISC tries to reduce the CPI • Tck: hardware technology and machine organization.  CPI has to be measured and not simply calculated from the system's specification. This is because CPI strongly depends of the memory hierarchy organization: a program running on the system without cache will certainly have a larger CPI than the same program running on the same machine but with a cache. What Drives the Work of a Computer Designer Designing a computer is a challenging task. It involves software (at least at the level of designing the instruction set), and hardware as well at all levels: functional organization, logic design, implementation. Implementation itself deals with designing/specifying ICs, packaging, noise, power, cooling etc. It would be a terrible mistake to disregard one aspect or other of computer design, rather the computer designer has to design an optimal machine across all mentioned levels. You can not find a minimum unless you are familiar with a wide range of technologies, from compiler and operating system design to logic design and packaging. Architecture is the art and science of building. Vitruvius, in the 1st century AD, said that architecture was a building that incorporated utilitas, firmitas and venustas, in English terms commodity, firmness and delight. This definition recognizes that architecture embraces functional, technological and aesthetic aspects. Thus a computer architect has to specify the performance requirements of various parts of a computer system, to define the interconnections between them, and to keep it harmoniously balanced. The computer architect's job is more than designing the Instruction Set, as it has been understood for many years. The more an architect is exposed to all aspects of computer design, the more efficient she will be. • the instruction set architecture refers to what the programmer sees as the machine's instruction set. The instruction set is the boundary between the hardware and the software, and most of the decisions concerning the instruction set affect the hardware, and the converse is also true, many hardware decisions may beneficially/adversely affect the instruction set. • the implementation of a machine refers to the logical and physical design techniques used to implement an instance of the architecture. It is possible to have different implementations for some architecture, in the same way there are different possibilities to build a house using the same plans, but other materials and techniques. The implementation has two aspects: • the organization refers to logical aspects of an implementation. In other words it refers to the high level aspects of the design: CPU design, memory system, bus structure(s) etc. • the hardware refers to the specifics of an implementation.