Monday, November 30, 2009

>>>tHe cOmPoNEnts Of SYsteM Unit<<<

the component system unit is where instructions are carried out on a computer. The system unit has several components, which can be considered as falling into three categories:
::the CPU - the central processor
::memory - RAM, which comes in several types
::other electronics - everything else in the system unit case

Before we "swim deeper"(ma'am,i'm using my own sweet language..huhu...) into the system unit components, this chapter discusses the fact that computers are digital devices. The real world is sometimes digital, but more often analog(i think this is the most brilliant intro from me...hehe...). To understand the difference, consider the difference between a light with an on/off switch and a light with a dimmer switch. Think of the standard on/off switch as a digital switch, because it has only two states (on and off) and has to be in one state or the other. Think of the light with a dimmer switch as analog, because it has many possible states, ranging from off through dim to very bright. Data in a computer system is held in binary digits, ones and zeros. You can think of these digits as switches that are either on (holding a one) or off (holding a zero). Since people tend to shorten phrases that they use a lot, computer people shorten the phrase "binary digit" to "bit".
Since a bit is not complex enough to represent more than two things (on and off), sequences of bits are used to represent characters. Two code systems using bit patterns (sequences) are discussed in the chapter: ASCII, which is used on most PCs, and EBCDIC, which is used on most IBM mainframes. The two systems are different, but they serve the same purpose: characters are assigned unique bit patterns, and those bit patterns are used by a computer to represent the characters in memory. In most cases, a character is represented by eight bits. We refer to a sequence of eight bits as a byte.
(for me,it's important,take a note please...huhu)

A drawback to using a byte to represent a character is that you can only represent 256 different characters by using 1s and 0s in a single byte. That sounds like plenty until you start thinking about more than one language, punctuation, money symbols, and lots of other characters the average user may not encounter every day. Unicode is a code system that uses two bytes for each character, which allows more than 65,000 possible characters. This is enough characters to describe all the symbols human languages currently use.
When transmitting signals between system, some bits can become lost or misunderstood. Various systems use some means of verifying the signals. One method is described on page 3.5: parity bits. A parity bit is an additional bit that is sent with each byte. A parity bit is either a 1 or a 0, like all bits.
An example will make this clearer. The sender counts the number of 1s in a byte. Let's assume the byte looks like this:
1110 0001
This byte has an even number of 1s. The sender now has to know which of two systems it is using: odd parity or even parity. If we are using odd parity, we would set the parity bit to 1, to make an odd total for the byte. If we are using even parity, we would set the parity bit to 0, to leave an even total of 1s. The way you set the parity bit is determined by your odd or even scheme and the actual number of 1s in each byte. (important!take note!)
Control Unit is the circuitry that locates, retrieves, interprets and executes each instruction in the central processing unit. The control unit directs electronic signals between primary storage and the ALU, and between the CPU and input/output devices.(the infos that i got are limited...sorry...huhu...)
As for example,when running a program, it is generally loaded into memory with relevant data. The data brought to the ALU from the main memory is manipulated by the ALU according to the commands issued BY THE CONTROL UNIT.
an arithmetic logic unit (ALU) is a digital cicuit that performs arithmetic and logical operations. (i wanna make it simple...take a note!) The ALU is a fundamental building block of the central processing unit (CPU) of a computer, and even the simplest microprocessors contain one for purposes such as maintaining timers. The processors found inside modern CPUs and graphics processing units (GPUs) accommodate very powerful and very complex ALUs(a single component may contain a number of ALUs).

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