INTRODUCTION TO COMPUTERS When we talk about computers, we really are talking about a Computer System. Computer System: It is a combination of Hardware and Software. This combination allows a computer to accept input, process data, and produce output. Hardware: Physical equipment Software: Instructions to run the hardware. Computer hardware: CPU Memory I/O Devices Peripheral Devices Computer software: Application Software System Software There are several categories of computers (Categorizing computers is a way to group them according to criteria such as usage, cost, speed, size, etc.): Personal computers (Desktop, Notebook, PDA, etc.) Workstations Mainframes Supercomputers We will come back to explain the differences between them, when we acquire more knowledge. In our present study we are going to restrict ourselves to Personal Computers, their hardware elements and their software components. But before we look into the computer hardware and software of PCs, we have to deal with the fact that computers don t represent data the way we do it. Computers represent data in a digital form, i.e. they work with two different states: current flowing through a wire, gate, circuit, etc. or current not flowing; location magnetized, or not magnetized; light reflected or scattered; etc. We can represent the first state in all cases with a 1, and the second state with a 0. Therefore the computers will need to keep information using this binary situation. As a matter of fact, the first unit of memory is a bit (from binary unit) that is a location that may hold one of the two values for a state (either a 1 or a 0). But this is a small unit, so we have a Byte, which is equivalent to 8 bits, and then we also have multiples of a Byte. We may summarize the units in the following way: 1 bit = 1 (binary unit) It will distinguish between two possible states. This is the minimal unit for transmission of information.
1 Byte = 8 bits It will distinguish between 256 possible states (2 8 = 256). This is the minimal unit for storage of information 1 KB (Kilobyte) = 1024 Bytes (2 10 Bytes) 1 MB (Megabyte) = 1024 KBs (2 20 Bytes) 1 GB (Gigabyte) = 1024 MBs (2 30 Bytes) 1 TB (Terabyte) = 1024 GBs (2 40 Bytes) On the other hand we represent data using numerical digits (0-9), alphabetic characters (A-Z, a-z), and other characters, like >,!, }, etc. So we need a way to convert our representation that has more variety, into the computer representation that only uses two states. Numeric representation: Numeric data consists of numbers that can be used in arithmetic operations. Our numerical representation using 10 digits is as arbitrary as the one used by the computer using only 2 digits. In our representation we have to learn the addition and the multiplication tables. The addition tables and multiplication tables for the digital representation used by the computer are very simple: Addition: Multiplication: 0 0 1 1 + 0 + 1 + 0 + 1 --- --- --- --- 0 1 1 10 0 0 1 1 * 0 * 1 * 0 * 1 --- --- --- --- 0 0 0 1 We can represent without difficulty all the numbers using the binary system, following the same rules that we use for representing numbers using the decimal system Binary Decimal 0 0 1 1 10 2
11 3 100 4 101 5 110 6 111 7 1000 8 1001 9 1010 10 1011 11 1100 12 1101 13 Example: Add 3 and 9 in both systems, to see that we obtain 12 using the binary addition rules. (Add any other two) String representation: A string is a set of characters, also called character data. Character data consists of letters, symbols and digits that are not used in arithmetic operations. Since the computer only understands 0 s and 1 s, we need to encode the characters. There are several codes accepted to encode characters, and among them we may point out the following: ASCII (American Standard Code for Information Interchange) Extended ASCII Unicode The ASCII code requires 7 bits for each character. The extra bit needed to have a Byte is either ignored or used for other purposes, as we will see it later. Although the code uses the binary system, very often we find a decimal number to supply the code. Since there is an equivalent decimal number corresponding any binary number and we are more familiar with the decimal number this one is used. See next figure. The Extended ASCII code uses the 8 bits of a Byte for each character. The first 128 characters have the same representation, but the first bit, with value 0 is taken into consideration. When the bit is changed to 1, we obtain another 128 characters, to make the 256 characters that can be represented with the Extended ASCII code. The Unicode uses more than one Byte to represent each script (a generalization of character). Using two Bytes we could represent up to 65536 different symbols. There is a backward compatibility with ASCII, so the first Byte will contain only 0 s in the Unicode representation of the 256 characters represented in the Extended ASCII code. This is a work in progress and there are at the moment several versions of Unicode. If we use 4 Bytes to represent a script, you need to include 3 Bytes with 0 s, together with the ASCII Byte, to represent the characters in the Extended ASCII code. For information on Unicode see: Unicode and Wikipedia
Music and pictures representation: We need to transform the analog data into digital data represented by 0s and 1s, so the computer can manipulate it. A picture is treated as a series of colored dots. Each dot is assigned a binary number according to its color, and therefore a picture is seen by a computer as a list of numbers, each representing a colored dot. Note that the more colored dots we have on a picture, the larger the list of numbers that we need to use to keep it. Sound is an analog signal that we must transform into 0s and 1s. An analog signal is converted to a digital signal at a given sampling rate and bit resolution. Sampling rate is a measure of how often you take a sample. Bit resolution is how big the integer number can
be for the greatest value of your amplitude. Generally speaking, the higher are the sampling rate and bit resolution the more fidelity we obtain. But this will increase the amount of digital data. See picture. The red represents the sound wave, while the black is its digital representation. (The integer number in the figure is 15, so the bit rate is 4, because you need 4 bits to represent 15) Hardware CPU (Central Processing Unit). This is what it makes a computer to be a computer. It performs 3 basic operations: moving data, additions and comparisons. It has an internal clock that sets the pace for executing instructions to perform basic operations. The speed of a microprocessor is specified in megahertz (MHz) or gigahertz (GHz), what means that the clock will click a million times on a second or a billion times on a second, respectively. The CPU contains three different parts: ALU (Arithmetic and Logic Unit) Control Unit Registers The ALU performs the additions and comparisons. The Control Unit moves the data, and the Registers contain the data and instructions to be executed.
Another important issue related to CPU is the word size, i.e. the number of bits that a microprocessor can manipulate at one time. The size of the registers must match this number of bits, and also the data bus, i.e. the circuits where the data travel from one component to another. Note that here the word bus means an electronic pathway. There is a data bus, an address bus, a command bus, a system bus, etc.., named according to the kind of information they carry. Memory By memory we mean storage locations. The units of memory are given by how many bytes you can store on that location. Therefore we know that we can talk about KBs, MBs, GBs, TBs, etc. We distinguish between two kinds of memory: Main Memory and Secondary (or Auxiliary) Memory. The Main Memory can transfer information directly to the CPU s registers. The Secondary Memory needs to transfer the information into the Main Memory, before it can be transferred to the registers. Main Memory: It is characterized by a fast access to the data. It is divided into: RAM: Random Access Memory ROM: Read Only Memory RAM Properties: ROM Properties: o RAM is a temporary holding area for data, application program instructions and the operating system. Its capacity is always specified on the description of a PC. o In the PC it is attached to the motherboard (computer main circuit board). o It is called Random Access Memory because the CPU can access any place in RAM as needed, without going through the same path. o For a program to run it must be stored in RAM. o RAM is volatile, i.e., when the power goes off, the contents on RAM are erased. o RAM has less capacity than secondary storage, and it is more expensive o ROM s contents can t be modified o ROM contains the information to startup a computer when the power goes on o ROM contains instructions to perform diagnostic tests.
o Because ROM s contents can t be modified, there is another kind of memory, CMOS (Complementary Metal Oxide Semiconductor) that contains the basic hardware settings of your computer. If you add more RAM, the settings need to be changed, and there are kept in CMOS memory. Even if RAM access is fast, sometimes there is an intermediate memory between the CPU and the RAM. It is called cache memory, it has smaller capacity than RAM, it is more expensive and it has a faster access. When the CPU needs some data to be placed in the registers, first check to see if it is in cache memory. If it is, it takes the data from there, otherwise it transfer the data from RAM to cache, and then to the registers. RAM and ROM are directly attached to the motherboard. Secondary Memory: It is a permanent memory, with a very slow access time (time it takes the data to be accessed in secondary memory, before it can be transferred to RAM). It is cheaper than RAM. There are several kinds of secondary memory, like: Hard disk: 360 GBs, average capacity, with access time of 9 msec. Floppy disk : 1.4 MBs. With access time 125 msec Flash drive: 2, 4, 6, 8 GBs CD-ROM : 640 MBs to 700 MBs of storage capacity. Transfer time X10 (24, 52).. Due to the fact that CDs and DVDs contain such amount of data, there is an interest not only in the access time, but also the speed at which data is transferred. The basic unit is 1200 Kb/s, referred as X1. So X10 is 1200*10 Kb/s = 12000 Kb/s = 1500 KB/s. Similarly you may compute that X40 is 6000 KB/s. Text books on a CD-ROM: 80 char/line *60 lines/page * 500 pages/book = 2400000 bytes/book 640 MB/ 3 MB = 213 books DVD-ROM: 4.7 GBs of storage capacity (single layer DVD-ROM) I/O Devices Keyboard: It transforms the character represented by the key that you press into the corresponding sequence of 0s and 1s given by the ASCII code. Monitor
Text mode: There is a LUT (Look Up Table) for ASCII values. Graphic mode: It uses PIXELS (Picture Elements) (See figure) Resolution: # of horizontal pixels * # of vertical ones Peripheral Devices They normally used some expansion slots on the motherboard to be connected to. Scanner: It works with an OCR (Optical Character Recognition program) for working with text. The picture of a character has to be changed to its binary representation. Printer: Modem: Impact - Dot matrix Inkjet Small droplets of ink are thrown to the paper Laser- Technology similar to the copier machines, using light to print the page To transform the binary information stored on the computer into the information that can be accepted by the transmission media.
The original modems were using the POTS (Plain Old Telephone System) to transmit the data (we call it now by dial-up ). They will change the binary information into two different audible frequencies (modulation) at one end, and then convert the two frequencies (demodulation) into the corresponding 0 or 1, at the other end.