Computer hardware structure explained

Many different computer hardware configurations exist.

Instructions and data are stored in the main storage, historically called "Core Memory". The terms Main Storage, Permanent Memory, Core Memory, and Memory are often used interchangeably. Connected to the memory are various processors. A processor is a hardware device capable of interpreting instructions and performing the indicated operation, for example "addition". Instructions as well as data are stored in the memory in coded form.

A computer has one or more Central Processing Units (CPU). A Central Processing Unit is a processor that manipulates and performs arithmetic operations upon data in the memory in the memory. It also executes instructions that control the other processors. For example, the CPU may execute a START I/O (SIO) instruction to initiate an I/O processor.

In many modern systems certain processors are specialized to perform specific tasks efficiently and/ or inexpensively. For example, the I/O (Input and Output) processors are designed to control I/O devices and handle the movement of data between the memory and the I/O devices. The processors range from being specialized for a certain I/O device to being quite sophisticated, simultaneously executing several I/O instructions and controlling several I/O devices.

Almost any conceivable device from a laser to a milking machine can be, and has been used as an I/O device. Many of the more common devices, for example say card readers, printers, disks, drums, tapes etc. require control circuitry that is applicable to other devices. For reasons of economy, this common hardware is sometimes separated out into a device called a Control Unit.

There may be many user programs competing for the system resources, such as memory, CPUs, I/O processors, control units, devices, etc.. What decides which users are to have access to the resources and how long a user may have a resource? What prevents, the system from collapsing if a device becomes inoperable? What assigns the "path" to a device through the I/O processors and control units? What manages all of these resources? The answer to all of these questions is the Operating System.