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Practice 2.1
Computer science
Computer
A computer is a general purpose device that can be programmed to carry out a set of arithmetic or logical operations. Since a sequence of operations can be readily changed, the computer can solve more than one kind of problem.
http://en.wikipedia.org/wiki/Computer
Computer parts
If you use a desktop computer, you might already know that there isn't any single part called the
"computer." A computer is really a system of many parts working together. The physical parts, which you can see and touch, are collectively calledhardware. (Software, on the other hand, refers to the instructions, or programs, that tell the hardware what to do.)
The illustration below shows the most common hardware in a desktop computer system. Your system may look a little different, but it probably has most of these parts. A laptop computer has similar parts but combines them into a single notebook-sized package.
System unit
Almost every other part of your computer connects to the system unit using cables. The cables plug into specific ports(openings), typically on the back of the system unit. Hardware that is not part of the system unit is sometimes called aperipheral device or device.
Storage
Your computer has one or more disk drives—devices that store information on a metal or plastic disk. The disk preserves the information even when your computer is turned off.
Hard disk drive
Your computer's hard disk drive stores information on a hard disk, a rigid platter or stack of platters with a magnetic surface. Because hard disks can hold massive amounts of information, they usually serve as your computer's primary means of storage, holding almost all of your programs and files. The hard disk drive is normally located inside the system unit.
CD and DVD drives
Nearly all computers today come equipped with a CD or DVD drive, usually located on the front of the system unit. CD drives use lasers to read (retrieve) data from a CD, and many CD drives can also write (record) data onto CDs. If you have a recordable disk drive, you can store copies of your files on blank CDs. You can also use a CD drive to play music CDs on your computer.
DVD drives can do everything that CD drives can, plus read DVDs. If you have a DVD drive, you can watch movies on your computer. Many DVD drives can record data onto blank DVDs.
Tip
If you have a recordable CD or DVD drive, periodically back up (copy) your important files to CDs or DVDs. That way, if your hard disk ever fails, you won't lose your data.
Floppy disk drive
Why are floppy disks "floppy"? Even though the outside is made of hard plastic, that's just the sleeve. The disk inside is made of a thin, flexible vinyl material.
Mouse
A mouse is a small device used to point to and select items on your computer screen. Although mice come in many shapes, the typical mouse does look a bit like an actual mouse. It's small, oblong, and connected to the system unit by a long wire that resembles a tail. Some newer mice are wireless.
A mouse usually has two buttons: a primary button (usually the left button) and a secondary button. Many mice also have a wheel between the two buttons, which allows you to scroll smoothly through screens of information.
When you move the mouse with your hand, a pointer on your screen moves in the same direction. (The pointer's appearance might change depending on where it's positioned on your screen.) When you want to select an item, you point to the item and then click (press and release) the primary button. Pointing and clicking with your mouse is the main way to interact with your computer. For more information, see
Keyboard
A keyboard is used mainly for typing text into your computer. Like the keyboard on a typewriter, it has keys for letters and numbers, but it also has special keys:
The function keys, found on the top row, perform different functions depending on where they are used.
The numeric keypad, located on the right side of most keyboards, allows you to enter numbers quickly.
The navigation keys, such as the arrow keys, allow you to move your position within a document or webpage.
Keyboard
You can also use your keyboard to perform many of the same tasks you can perform with a mouse. For more information, see.
Monitor
the information is called the screen. Like a television screen, a computer screen can show still or moving pictures.
There are two basic types of monitors: CRT (cathode ray tube) monitors and LCD (liquid crystal display) monitors. Both types produce sharp images, but LCD monitors have the advantage of being much thinner and lighter. CRT monitors, however, are generally more affordable.
LCD monitor (left); CRT monitor (right)
Printer
A printer transfers data from a computer onto paper. You don't need a printer to use your computer, but having one allows you to print e-mail, cards, invitations,
announcements, and other materials. Many people also like being able to print their own photos at home.
The two main types of printers are inkjet
printers and laser printers. Inkjet printers are the most popular
printers for the home. They can print in black and white or in full color and can produce high-quality photographs when used with special paper. Laser printers are faster and generally better able to handle heavy use.
Inkjet printer (left); laser printer (right)
Speakers
Modem
To connect your computer to the Internet, you need a modem. A modem is a device that sends and receives computer information over a telephone line or high-speed cable. Modems are sometimes built into the system unit, but higher-speed modems are usually separate components.
Computer functions
There are certain operations that are basic to almost any computer A sound understanding of these basic operations is a necessary prerequisite to examining the specific operations of a
particular computer.
Timing:
The activities of the central processor are cyclical. The processor fetches an instruction, performs the operations
required, fetches the next instruction, and so on. This orderly sequence of events requires precise timing, and the CPU therefore requires a free running oscillator clock which furnishes the reference for all processor actions The combined fetch and execution of a single instruction is referred to as an Instruction Cycle. The portion of a cycle identified with a clearly defined activity IS called a State. And the inter vat between pulses of the timing oscillator is referred to as a Clock Period. As a general rule, one or more clock periods are necessary for the completion of a state,
and there are several states in a cycle.
Instruction Fetch:
The first state(s) of any instruction cycle will be dedicated to fetching the next instruction. The CPU issues a read signal and the contents of the program counter are sent to memory, which responds by returning the next instruc tion word. The first byte of the instruction is placed in the instruction register. If the instruction consists of more than one byte, additional states are required to fetch
each byte of the instruction. When the entire instruction is present in the CPU, the program counter is incremented (in preparation for the next instruction fetch) and the instruction is decoded. The operation specified in the instruction will be executed in the remaining states of the
Memory Read:
An instruction fetch is merely a special memory read operation that brings the instruction to the CPU's instruction register. The instruction fetched may then call for data to be read from memory
into the CPU. The CPU again issues a read signal and sends the proper memory address; memory responds by returning the requested word. The data received is placed in the accumulator or one
of the other general purpose registers (not the instruction register).
Memory Write:
A memory write operation is similar to a read except for the direction of data flow. The CPU issues a write signal, sends the proper memory address, then sends the data word to be written into the
addressed memory location.
Wait (memory synchronization):
As previously stated, the activities of the processor are timed by a master clock oscillator. The clock period determines the timing of all processing activity.
The speed of the processing cycle, however, is limited by the memory's Access Time. Once the processor has sent a read address to memory, it cannot proceed until the memory has had time to
respond. Most memories are capable of responding much faster than the processing cycle requires. A few, however, cannot supply the addressed byte within the minimum time established
by the processor's clock.
Therefore a processor should contain a synchronization provision, which permits the memory to request a Wait state. When the memory receives a read or write enable signal, it places a request
signal on the processor's READY line, causing the CPU to idle temporarily. After the memory has had time to respond, it frees the processor's READY line, and the instruction cycle proceeds
Input/Output:
Input and Output operations are similar to memory read and write operations with the exception that a peripheral 1/0 device is addressed instead of a memory location. The CPU issues the appropriate input or output control signal, sends the proper device address and either receives the
data being input or sends the data to be output.
Data can be input/output in either parallel or serial form. All data within a digital computer is represented in binary coded form. A binary data word consists of a group 5
of bits; each bit is either a one or a zero. Parallel 1/0 consists of transferring all bits in the word at the same time, one bit per line. Serial 1/0 consists of transferring one bit at a time on a single line. Naturally serial 1/0 is much slower, but it requires considerably less hardware than does parallel
1/0.
Interrupts:
processor's efficiency. Consider the case of a computer that is processing a large volume of data, portions of which are to be output to a printer. The CPU can output a byte of data within a single machine cycle but it may take the printer the equivalent of many machine cycles to actually print the character specified by the data byte. The CPU could then remain idle waiting until the printer can accept the next data byte. If an interrupt capability is implemented on the computer, the CPU can output a data byte then return to data processing. When the printer is ready to accept the next data byte, it can request an interrupt. When the CPU acknowledges the interrupt, it suspends
main program execution and automatically branches to a routine that will output the next data byte. After the byte is output, the CPU continues with main program execution. Note that this is, in
principle, quite similar to a subroutine call, except that the jump is initiated externally rather than by the program.
More complex interrupt structures are possible, in which several interrupting devices share the same processor but have different priority levels. Interruptive processing is an important feature
that enables maximum utilization of a processor's capacity for high system throughput.
Hold:
Another important feature that improves the throughput of a processor is the Hold. The hold provision enables Direct Memory Access (DMA) operations.
In ordinary input and output operations, the processor itself supervises the entire data transfer. Information to be placed in memory is transferred from the input device to the processor, and then from the processor to the designated memory location. In similar fashion, information that
goes from memory to output devices goes by way of the processor.
Some peripheral devices, however, are capable of transferring information to and from memory much faster than the processor itself can accomplish the transfer. If any appreciable quantity of data must be transferred to or from such a device, then system throughput will be increased by having the device accomplish the transfer directly. The processor must temporarily suspend its operation during such a transfer, to prevent conflicts that would arise if processor and peripheral
Hardware
Computer hardware is the collection of physical elements that constitute a computer system. Computer hardware refers to the physical parts or components of a computer such as monitor, keyboard, Computer data storage, hard drive disk, mouse, system unit (graphic cards, sound cards,
memory, motherboard and chips), etc. all of which are physical objects that you can actually touch. In contrast, software is untouchable. Software exists as ideas, application, concepts, and symbols, but it has no substance. A combination of hardware and software forms a usable computing system.
Software
Computer software, or just software, is any set of machine-readable instructions (most often in the form of a computer program) that directs a computer's processor to perform specific operations.
The term is used to contrast with computer hardware, the physical objects (processor and related devices) that carry out the instructions. Hardware and software require each other and neither has
