The terms input and output say if data flow into or out of the computer. The picture shows the major hardware components of a computer system. The arrows show the direction of data flow.
A bus is a group of wires on the main circuit board of the computer. It is a pathway for data flowing between components. Most devices are connected to the bus through a controller which coordinates the activities of the device with the bus.
Processor
The processor is an electronic device about a one inch square, covered in plastic. Inside the square is an even smaller square of silicon containing millions of tiny electrical parts. A modern processor may contain billions of transistors. It does the fundamental computing within the system, and directly or indirectly controls all the other components.
The processor is sometimes called the Central Processing Unit or CPU. A particular computer will have a particular type of processor, such as a Pentium processor or a SPARC processor.
Memory
The processor performs all the fundamental computation of the computer system. Other components contribute to the computation by doing such things as storing data or moving data into and out of the processor. But the processor is where the fundamental action takes place.
A processor chip has relatively little memory. It has only enough memory to hold a few instructions of a program and the data they process. Complete programs and data sets are held in memory external to the processor. This memory is of two fundamental types: main memory, and secondary memory.
Main memory is sometimes called volatile because it looses its information when power is removed. Secondary memory is usually nonvolatile because it retains its information when power is removed. (However, it needs power when information is stored into memory or retrieved from it.)
Main memory is sometimes called main storage and secondary memory is sometimes called secondary storage or mass storage.
Main memory:
closely connected to the processor.
stored data are quickly and easily changed.
holds the programs and data that the processor is actively working with.
interacts with the processor millions of times per second.
needs constant electric power to keep its information.
Secondary memory:
connected to main memory through the bus and a controller.
stored data are easily changed, but changes are slow compared to main memory.
used for long-term storage of programs and data.
before data and programs can be used, they must be copied from secondary memory into main memory.
does not need electric power to keep its information.
Main Memory
Main memory is where programs and data are kept when the processor is actively using them. When programs and data become active, they are copied from secondary memory into main memory where the processor can interact with them. A copy remains in secondary memory.
Main memory is intimately connected to the processor, so moving instructions and data into and out of the processor is very fast.
Main memory is sometimes called RAM. RAM stands for Random Access Memory. "Random" means that the memory cells can be accessed in any order. However, properly speaking, "RAM" means the type of silicon chip used to implement main memory.
When people say that a computer has "512 megabytes of RAM" they are talking about how big its main memory is. One megabyte of memory is enough to hold approximately one million (106) characters of a word processing document. (There will be more about bytes and megabytes later on in these notes.)
Nothing permanent is kept in main memory. Sometimes data are placed in main memory for just a few seconds, only as long as they are needed.
Secondary Memory
Secondary memory is where programs and data are kept on a long-term basis. Common secondary storage devices are the hard disk and optical disks.
The hard disk has enormous storage capacity compared to main memory.The hard disk is usually contained inside the case of a computer.The hard disk is used for long-term storage of programs and data.Data and programs on the hard disk are organized into files.A file is a collection of data on the disk that has a name.
A hard disk might have a storage capacity of 500 gigabytes (room for about 500 x 109 characters). This is about 100 times the capacity of main memory. A hard disk is slow compared to main memory. If the disk were the only type of memory the computer system would slow down to a crawl. The reason for having two types of storage is this difference in speed and capacity.
Large blocks of data are copied from disk into main memory. The operation is slow, but lots of data is copied. Then the processor can quickly read and write small sections of that data in main memory. When it is done, a large block of data is written to disk.
Often, while the processor is computing with one block of data in main memory, the next block of data from disk is read into another section of main memory and made ready for the processor. One of the jobs of an operating system is to manage main storage and disks this way.
Input and Output Devices
Input and output devices allow the computer system to interact with the outside world by moving data into and out of the system. An input device is used to bring data into the system.
Some input devices are:
Keyboard
Mouse
Microphone
Bar code reader
Graphics tablet
An output device is used to send data out of the system.
Some output devices are:
Monitor
Printer
Speaker
A network interface acts as both input and output. Data flows from the network into the computer, and out of the computer into the network.
I/O
nput/output devices are usually called I/O devices. They are directly connected to an electronic module attached to the motherboard called a device controller. For example, the speakers of a multimedia computer system are directly connected to a device controller called an audio card, which in turn is plugged into a bus on the motherboard.
With many recent computers, the functions of a device controller are integerated with the motherboard. Some motherboards have audio, graphics, and network controllers built in.
Sometimes secondary memory devices like the hard disk are called I/O devices (because they move data in and out of main memory). What counts as an I/O device depends on context. To a user, an I/O device is something outside of the computer case. To a programmer, anything outside of the processor and main memory is an I/O device. To an engineer working on the design of a processor everything outside of the processor is an I/O device.
Embedded Systems
A computer system that is part of a larger machine and which controls how that machine operates is an embedded system. Usually the processor constantly runs a single control program which is permanently kept in ROM (Read Only Memory).
ROM is used to make a section of main memory read-only. Main memory looks the same as before to the processor, except a section of it permanently contains the program the processor is running. This section of memory retains its data even when power is off.
A typical embedded system is a cell phone. This is obvious, but there are many less obvious embedded systems. Your car contains dozens of processors, and even more in its audio system. For instance, each airbag is controlled by its own computer chip. Digital cameras, DVD players, medical equipment, and even home appliances contain dedicated processors.
The overwhelming majority of processor chips are used in embedded systems. Less than 1% of processor chips are used in the familiar desktop computer!
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