Computer architecture

In computer science, Computer Architecture is the design, organization, optimization and verification of a computer system, usually focusing on the CPU, memory and other peripheral devices and the interfaces that those components communicate through. In a more concrete sense, a computer architecture may refer to a specific platform such as SPARC, X86 or PowerPC and ARM or 680x0.

Computer Architectures can span multiple purposes such as general desktop computing as in the case of the x86, PowerPC and SPARC architectures, or more specific purposes such as embedded computing where the ARM and 680x0 processor lines are preferred. The lines tend to blur though on exactly what system architecture is best and ultimately is left up to the engineer designing the system. For instance, the Intel 386 processor (X86 family) appeared initially on desktop and server systems for its first run but now resigns itself to small embedded devices because the power requirements and cost per unit is much less than faster processors which typically are not necessary.

The Motorola 68000 series processors once powered Apple's line of computers before being replaced by PowerPC and now Intel X86 processors. The 68000 can now be found in embedded platforms and in antiquated legacy hardware that has not been replaced with newer, faster systems.

How computers work: the stored program architecture

An exploded view of a modern personal computer:

1. Display
2. Motherboard
3. CPU (Microprocessor)
4. Primary storage (RAM)
5. Expansion cards
6. Power supply
7. Optical disc drive
8. Secondary storage (HD)
9. Keyboard
10. Mouse

While the technologies used in computers have changed dramatically since the first electronic, general-purpose computers of the 1940s, most still use the stored program architecture (sometimes called the von Neumann architecture). The design made the universal computer a practical reality.

The architecture describes a computer with four main sections: the arithmetic and logic unit (ALU), the control circuitry, the memory, and the input and output devices (collectively termed I/O). These parts are interconnected by bundles of wires (called "buses" when the same bundle supports more than one data path) and are usually driven by a timer or clock (although other events could drive the control circuitry).

Conceptually, a computer's memory can be viewed as a list of cells. Each cell has a numbered "address" and can store a small, fixed amount of information. This information can either be an instruction, telling the computer what to do, or data, the information which the computer is to process using the instructions that have been placed in the memory. In principle, any cell can be used to store either instructions or data.

The ALU is in many senses the heart of the computer. It is capable of performing two classes of basic operations. The first is arithmetic operations; for instance, adding or subtracting two numbers together. The set of arithmetic operations may be very limited; indeed, some designs do not directly support multiplication and division operations (instead, users support multiplication and division through programs that perform multiple additions, subtractions, and other digit manipulations). The second class of ALU operations involves comparison operations: given two numbers, determining if they are equal, or if not equal which is larger.

The I/O systems are the means by which the computer receives information from the outside world, and reports its results back to that world. On a typical personal computer, input devices include objects like the keyboard and mouse, and output devices include computer monitors, printers and the like, but as will be discussed later a huge variety of devices can be connected to a computer and serve as I/O devices.

The control system ties this all together. Its job is to read instructions and data from memory or the I/O devices, decode the instructions, providing the ALU with the correct inputs according to the instructions, "tell" the ALU what operation to perform on those inputs, and send the results back to the memory or to the I/O devices. One key component of the control system is a counter that keeps track of what the address of the current instruction is; typically, this is incremented each time an instruction is executed, unless the instruction itself indicates that the next instruction should be at some other location (allowing the computer to repeatedly execute the same instructions).

Since the 1980s the ALU and control unit (collectively called a central processing unit or CPU) have typically been located on a single integrated circuit called a microprocessor.

The functioning of such a computer is in principle quite straightforward. Typically, on each clock cycle, the computer fetches instructions and data from its memory. The instructions are executed, the results are stored, and the next instruction is fetched. This procedure repeats until a halt instruction is encountered.

The set of instructions interpreted by the control unit, and executed by the ALU, are limited in number, precisely defined, and very simple operations. Broadly, they fit into one or more of four categories: 1) moving data from one location to another (an example might be an instruction that "tells" the CPU to "copy the contents of memory cell 5 and place the copy in cell 10"). 2) executing arithmetic and logical processes on data (for instance, "add the contents of cell 7 to the contents of cell 13 and place the result in cell 20"). 3) testing the condition of data ("if the contents of cell 999 are 0, the next instruction is at cell 30"). 4) altering the sequence of operations (the previous example alters the sequence of operations, but instructions such as "the next instruction is at cell 100" are also standard).

Instructions, like data, are represented within the computer as binary code — a base two system of counting. For example, the code for one kind of "copy" operation in the Intel x86 line of microprocessors is 10110000 . The particular instruction set that a specific computer supports is known as that computer's machine language. Using an already-popular machine language makes it much easier to run existing software on a new machine; consequently, in markets where commercial software availability is important suppliers have converged on one or a very small number of distinct machine languages.

More powerful computers such as minicomputers, mainframe computers and servers may differ from the model above by dividing their work between more than one main CPU. Multiprocessor and multicore personal and laptop computers are also beginning to become available.

Supercomputers often have highly unusual architectures significantly different from the basic stored-program architecture, sometimes featuring thousands of CPUs, but such designs tend to be useful only for specialized tasks. At the other end of the size scale, some microcontrollers use the Harvard architecture that ensures that program and data memory are logically separate.