Lecture 09

INTRODUCTION TO COPMUTER

LECTURE

09

FACTORS EFFECTING PROCESSING SPEED Although all microcomputers have a CPU and memory, all microcomputers are by no means the same. Computing power usually refers to the speed with which the computer processes data. One reason the speed increase is because the distance between transistors is reduced. In the future, CPU construction will be so small that it will be at the atomic level. Factors that affect the processing speed are following. i)

ii)

iii)

REGISTER The registers in the first PCs could hold two bytes-16-bits each. Most CPUs sold today for both PCs and Macintosh computers, have 32-bit registers. Some powerful computers, such as minicomputers and some high-end workstations, have 64-bit register. The size of registers, which some time called the word size, indicates the amount of data with which the computer can work at any given time. The bigger the word size, the more quickly the computer can process a set of data. If all other factors kept equal, a CPU with 32-bit registers can process data twice as first as one with 16-bit registers. MEMORY The amount of RAM in a computer can have a profound effect on the computer’s power. For one thing, more RAM means the computer can use bigger, more powerful programs, and those programs can access bigger data files. More RAM also can make the computer run faster. The computer does not necessarily have to load an entire program into memory to run it, but the more of the program it can fit into memory, the faster the program will run. For example a PC with 12 MB of RAM is capable of running Microsoft Windows 98, even through the program actually occupies about 50 MB of disk storage space. When you run Windows, the program does not need to load all its files into memory to run properly. It loads only the most essential part in the memory. When the computer needs access to the other parts of the program on the disk, it can unload, or swap out, nonessential parts from RAM back to the hard disk. Then the computer can load, or swap in, the program code or date it needs. If your PC has 16 MB of RAM or more, you will notice a dramatic difference in how fast Microsoft Windows 98 runs because the CPU will need to sap program instructions between RAM and the hard disk much less often. THE COMPUTER’S INTERNAL CLOCK

1

INTRODUCTION TO COPMUTER

LECTURE

09

Every microcomputer has a system clock, but the clock’s primary purpose is not to keep the time of day. Like most modern wristwatches, the clock is driven by a quartz crystal. When electricity is applied, the molecules in the crystal vibrate millions of times par second, a rate that never changes. The speed of the vibration is determined by the thickness of the crystal. The computer uses the vibrations of the quartz in the system clock to time its processing operations. Over the years, clock speaks have increased steadily. For example, the first PC operated at 4.77 megahertz. Hertz is a measure of cycles per second. One clock cycle means the time it takes to turn a transistor off and back on again Megahertz. (MHz) means “millions of cycles per second d.” Clock speeds of 400 MHz and higher are common. Processor speeds are increasing rapidly. By the time you read this, this, clock speeds probably will have exceeded 500 MHz. Experts predict that clock speeds of 1 GHz (GHz) will be achieved shortly after the turn of the century. All other factors being equal (although they never are), a CPU operating at 300 MHz can process data more than twice as fast as the same one operating at 133 MHz. iv) THE BUS In microcomputers, the term Bus refers to the paths between the components of a computer. There are two main buses in a computer. The data bus an address bus. The one that you hear the most about is the data bus, so when people just say “the bus.” They usually mean the data bus. a) The Data Bus The data bus is an electrical path that connects the CPU, memory, and the other hardware devices on the motherboard. Actually, the bus is a group of parallel wires. The number of wires in the bus affects the speed at which data can travel between hardware components, just as the number of lanes on a highway affects how long it takes people to get to their destinations. Because each wire can transfer one bit at a time, an eight-wire bus can move eight at a time, which is a full byte. A 16-bit bus can transfer two bytes, and a 32-bit bus can transfer four bytes at a time. b) The Address Bus The second bus found in every microcomputer is the address bus. The address bus is a set of wires similar to the data bus that connects the CPU and RAM and carries the memory addresses. (Remember, each byte in RAM is associated with a number, which is the memory address.) The address bus is important because the umber of wires in it determines. The maximum number of memory addresses.

2

INTRODUCTION TO COPMUTER v)

LECTURE

09

CACHE MEMORY Moving data between RAM and the CPU’s registers is one of the most time consuming operations a CPU must perform, simply because RAM is much slower than the CPU. A partial solution to this problem is to include a cache memory in the CPU. Cache (pronounced “cash”) memory is similar to RAM, except that it is extremely fast compared to normal memory, and it is used in a different way. When a program is running and the CPU needs to read data of instructions from RAM, the CPU first checks to see whether the data is in cache memory. If the data that it also loads a copy of the data into cache memory. The next time the CPU needs that same data, it finds it in the cache memory and saves the time needed to load the data from RAM. Knowing the size of most programs and many data files, you might think that the odds of the CPU finding the data it needs in the cache memory are small. But it actually does find the data it needs there often enough to improve the performance of a PC.

PARALLEL PROCESSING Another school of thought on producing faster computers is to build them with more than one processor. This is not a new idea in the mainframe and supercomputer arena. In fact, the IBM 3090 has two to four processors, and Cray X MP 4 has four processors, Some companies are developing computers with 256, 512 and even thousands of microprocessors, known as massively parallel processors (MPP)

3

INTRODUCTION TO COPMUTER

LECTURE

09

4