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A typical vision of a computer architecture as a series of abstraction layers: hardware, firmware, assembler, kernel, operating system and applications (see also Tanenbaum 79).

In computer engineering, computer architecture is the conceptual design and fundamental operational structure of a computer system. It is a blueprint and functional description of requirements (especially speeds and interconnections) and design implementations for the various parts of a computer — focusing largely on the way by which the central processing unit (CPU) performs internally and accesses addresses in memory.

It may also be defined as the science and art of selecting and interconnecting hardware components to create computers that meet functional, performance and cost goals.

Computer architecture comprises at least three main subcategories ^[1]

• Instruction set architecture, or ISA, is the abstract image of a computing system that is seen by a machine language (or assembly language) programmer, including the instruction set, memory address modes, processor registers, and address and data formats.

• Microarchitecture, also known as Computer organization is a lower level, more concrete, description of the system that involves how the constituent parts of the system are interconnected and how they interoperate in order to implement the ISA^[2]. The size of a computer's cache for instance, is an organizational issue that generally has nothing to do with the ISA.

• System Design which includes all of the other hardware components within a computing system such as:

1. system interconnects such as computer busses and switches
2. memory controllers and hierarchies
3. CPU off-load mechanisms such as Direct memory access
4. issues like multi-processing.

Once both ISA and microarchitecture has been specified, the actual device needs to be designed into hardware. This design process is often called implementation. Implementation is usually not considered architectural definition, but rather hardware design engineering.

Implementation can be further broken down into three pieces:

• Logic Implementation/Design - where the blocks that were defined in the microarchitecture are implemented as logic equations.
• Circuit Implementation/Design - where speed critical blocks or logic equations or logic gates are implemented at the transistor level.
• Physical Implementation/Design - where the circuits are drawn out, the different circuit components are placed in a chip floor-plan or on a board and the wires connecting them are routed.

For CPUs, the entire implementation process is often called CPU design.

More specific usages of the term include more general wider-scale hardware architectures, such as cluster computing and Non-Uniform Memory Access (NUMA) architectures.

The most common goals in computer architecture revolve around the tradeoffs between cost and performance (i.e. speed), although other considerations, such as size, weight, reliability, feature set, expandability and power consumption, may be factors as well.

Generally cost is held constant, determined by either system or commercial requirements.

Computer performance is often described in terms of clock speed (usually in MHz or GHz). This refers to the cycles per second of the main clock of the CPU. However, this metric is somewhat misleading, as a machine with a higher clock rate may not necessarily have higher performance. As a result manufacturers have moved away from clock speed as a measure of performance.

Modern CPUs can execute multiple instructions per clock cycle, which dramatically speeds up a program. Other factors influence speed, such as the mix of functional units, bus speeds, available memory, and the type and order of instructions in the programs being run.

There are two main types of speed, latency and throughput. Latency is the time between the start of a process and its completion. Throughput is the amount of work done per unit time. Interrupt latency is the guaranteed maximum response time of the system to an electronic event (e.g. when the disk drive finishes moving some data). Performance is affected by a very wide range of design choices — for example, adding cache usually makes latency worse (slower) but makes throughput better. Computers that control machinery usually need low interrupt latencies. These computers operate in a real-time environment and fail if an operation is not completed in a specified amount of time. For example, computer-controlled anti-lock brakes must begin braking almost immediately after they have been instructed to brake.

The performance of a computer can be measured using other metrics, depending upon its application domain. A system may be CPU bound (as in numerical calculation), I/O bound (as in a webserving application) or memory bound (as in video editing). Power consumption has become important in servers and portable devices like laptops.

Benchmarking tries to take all these factors into account by measuring the time a computer takes to run through a series of test programs. Although benchmarking shows strengths, it may not help one to choose a computer. Often the measured machines split on different measures. For example, one system might handle scientific applications quickly, while another might play popular video games more smoothly. Furthermore, designers have been known to add special features to their products, whether in hardware or software, which permit a specific benchmark to execute quickly but which do not offer similar advantages to other, more general tasks.

The general scheme of optimization is to find the costs of the different parts of the computer. In a balanced computer system, the data rate will be constant for all parts of the system, and cost will be allocated proportionally to assure this. The exact form of the computer system will depend on the constraints and goals for which it was optimized.

Power consumption is another design criteria that factors in the design of modern computers. Power efficiency can often be traded for performance or cost benefits. With the increasing power density of modern circuits as the number of transistors per chip scales (Moore's Law), power efficiency has increased in importance. Recent processor designs such as the Intel Core 2 put more emphasis on increasing power efficiency. Also, in the world of embedded computing, power efficiency has long been and remains the primary design goal next to performance.

• Computer hardware
• CPU design
• Orthogonal instruction set
• Software architecture
• Computer organization

1. ^ John L. Hennessy and David A. Patterson (2003). Computer Architecture: A Quantitative Approach, Third Edition, Morgan Kaufmann Publishers, Inc. ISBN 1558605967. 
2. ^ Phillip A. Laplante (2001). Dictionary of Computer Science, Engineering, and Technology. CRC Press, 94–95. ISBN 0849326915. 

• ISCA: Proceedings of the International Symposium on Computer Architecture
• Micro: IEEE/ACM International Symposium on Microarchitecture
• HPCA: International Symposium on High Performance Computer Architecture
• ASPLOS: International Conference on Architectural Support for Programming Languages and Operating Systems
• ACM Transactions on Computer Systems
• IEEE Computer Society
• Microprocessor Report
• http://www.mkp.com/ Hennessy and Patterson: Computer Architecture: A Quantitative Approach, Fourth Edition, September 2006

• http://www.cs.wisc.edu/~arch/www
• http://www.cs.wisc.edu/arch/www/people.html
• ESCAPE - an easy-to-use, interactive portable PC-based simulation environment aimed at the support of computer architecture education
• http://www.codeproject.com/useritems/System_Design.asp - This approach allows beginners to easily break and design complex software systems.
• Technical University of Catalonia, Department of Computer Architecture
• The von Neumann Architecture of Computer Systems