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, class, … 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.