Overall View on Computer Architecture: Processor, Memory, IO, Buses

The computers which we use every day are all designed based on the same general idea of cooperation of three base elements processor, memory and peripheral devices. Their names represent their functions in the system. The memory stores the data and program code, the processor manipulates the data by executing programs, and peripherals are used to keep the contact with the user, environment and other systems. To exchange the information these elements are connected together by interconnections named buses. The generic block schematic diagram of the exemplary computer is shown in Fig. 1.

It is often called “the brain” of the computer. Although it doesn’t think, the processor is the element which controls all other units of the computer. The processor is the device that manages everything in the machine. Every hardware part of the computer is controlled more or less by the main processor. Even if the device has its own processor - for example keyboard - it works under the control of the main one. Processor handles events. We can say that synchronous events are those that which processor handles periodically. The processor can’t stop. Of course when it has the power. But even when you don’t see anything special happening on the screen. In PC computer in an operating system without a graphical user interface, for example, plain Linux, or a command box in Windows, if you see only „C:\>” the processor works. In this situation, it executes the main loop of the system. In such a loop, it’s waiting for the asynchronous events. Such an asynchronous event occurs when the user pushes the key or moves the mouse, when the sound card stops playing one sound, the hard disk ends up transmitting the data. For all of those actions the processor handles executing the programs - or if you prefer – procedures.

Processor is characterised by its main parameters including the frequency of operation, and class. Other features we will explain further in this chapter.

Frequency is very important information which tells the user how many instructions can be executed in the time unit. To have the detailed information of a real number of instructions per second it must be combined with the average number of clock pulses required to execute the instruction. Older processors needed a few or even a dozen pulses of clock for a single instruction. Modern machines, thanks to parallel execution can achieve impressive results of a few instructions per single cycle.

The class of the processor tells what is the number of bits in the data word. It means what is the size of the arguments of the data which the processor can calculate with a single arithmetic, logic or other operation. The still popular 8-bit machines have a data length of 8 bits, while most sophisticated can use 32 or 64-bit arguments. The class of the processor determines the size of its internal data registers, the size of arguments of instructions and the number of lines of the data bus.

Memory is the element of the computer that stores data and programs. It is visible to the processor as the sequence of data words, where every word has its own address. Addressing allows the processor to access simple and complex variables and to read the instructions for execution. Although it is intuitive that the size of a memory word should correspond to the class of the processor it is not always true. For example in PC computers, independently of the processor class, the memory is always organised as the sequence of bytes and the size of it is provided in megabytes or gigabytes.

The size of the memory installed on the computer does not have to correspond to the size of the address space – the maximal size of the memory which is addressable by the processor. In modern machines, it would be impossible or hardly achievable, for example for x64 architecture the theoretical address space is 2^64 (16 exabytes). Even address space supported in hardware by processors is as big as 2^48 which equals 256 terabytes. On the opposite side in constrained devices, the size of the physical memory can be bigger than supported by the processor. To enable access to bigger memory than the addressing space of the processor or to support flexible placement of programs in big address space the paging mechanism is used. It is a hardware support unit for mapping the address used by the processor into the physical memory installed in the computer.

Called also input-output (I/O) devices. There is a variety of units belonging to this group. It includes timers, communication ports, general-purpose inputs and outputs, displays, network controllers, video and audio modules, data storage controllers and many others. The main function of peripheral modules is to exchange information between the computer and the user, collect information from the environment, send and receive data to and from elements of the computer not connected directly to the processor, and exchange data with other computers and systems. Some of them are used to connect the processor to the functional unit of the same computer. For example, the hard disk drive is not connected directly to the processor, it uses the specialized controller which is the peripheral module operating as an interface between the centre of the computer and the hard drive. Another example can be the display. It uses a graphic card which plays the role of peripheral interface between the computer and the monitor.

Processor, memory and peripherals exchange information using interconnections called buses. Although you can find in the literature and internet a variety of kinds of buses and their names, on the very low level there are three buses which connect the processor, memory and peripherals.

Address bus delivers the address generated by the processor to memory or peripherals. This address specifies the one, and only one memory cell or peripheral register that the processor wants to access. The address bus is used not only to address the data which the processor wants to transmit to or from memory or peripheral. It also addresses the instruction which processor fetches and later executes. Instructions are also stored in the memory. The address bus is one-directional, the address is generated by the processor and delivered to other units.

The number of lines in the address bus is fixed for the processor and determines the size of the addressing space the processor can access. For example, if the address bus of some processor has 16 lines, it can generate up to 16^2 = 65536 different addresses.

Data bus is used to exchange data between processor and memory or peripherals. The processor can read the data from memory or peripherals or write the data to these units previously sending their address with the address bus. As data can be read or written the data bus is bi-directional.

The number of bits of the data bus usually corresponds to the class of the processor. It means that an 8-bit class processor has 8 lines of the data bus.

Control bus is formed by lines mainly used for synchronisation between the elements of the computer. In the minimal implementation, it includes the read and write lines. Read line (#RD) is the information to other elements that the processor wants to read the data from the unit. In such a situation, the element, e.g. memory puts the data from the addressed cell on the data bus. Active write signal (#WR) informs the element that the data which is present on the data bus should be stored at the specified address. The control bus can also include other signals specific to the system, e.g. interrupt signals, DMA control lines, clock pulses, signals distinguishing the memory and peripheral access, signals activating chosen modules and others.