Fundamentals of Hardware and Software MEMORY STORAGE The computer is able to store information for future use. This proves to be very important especially when it comes down to the fact where you need to transfer information from one place to another or need to update or finish your work at a later date or time. The computer's memory stores information the computer is using at the time. Its capacity is limited and data and processing results are lost when the computer is switched off or power is disrupted from the computer. If data, programs and processing results are to be preserved for repeated and permanent use, additional storage capacity must be available. Secondary Storage Devices serves this purpose. Secondary Storage is still slower than Primary Storage in that it takes a longer time to access information stored but it is less expensive that Primary Storage. MAIN MEMORY This is the memory area in which all programs and data are stored while the computer is executing programs. Main Memory is very important to a computer because it can be written to and read from at high speed. Main Memory is also referred to as Primary Memory/Primary Storage/Immediate Access Storage. Primary Storage may also be defined as the component of the computer that temporarily stores program instructions and data being used. Functions of Main Memory 1. Stores program instructions prior to execution 2. Stores the data which can be immediately processed by the program instructions 3. Stores information temporarily after processing is complete TYPES OF MEMORY RANDOM ACCESS MEMORY (RAM) This is a type of memory that allows information to be written to and read from by the CPU. RAM stores information temporarily so that it is directly and speedily available to the Microprocessor. It is also a memory storage device (chip) used for short-term storage of data and program instructions. Random Access Memory is constantly being refreshed with electrical impulses. Random Access Memory is said therefore to be volatile (information is stored depending on electrical power). The contents of Random Access Memory are temporarily stored and are lost when electrical power is disrupted or when the computer is turned off. All programs and data must be transferred to RAM from an input device (such as a keyboard) or from a secondary storage device (such as Magnetic Disks) before programs can be executed and data can be processed. Why is it called Random Access Memory? The term Random Access doesn’t imply that the memory stores data randomly! A better term for RAM would be Direct Access Memory. In RAM, each memory location has an address, just like a post office box. Using this address, called a Memory Address, the CPU can store and retrieve data by going directly to a single location in memory. Fig. 1 In RAM, each memory location has an address like a P.O. Box. Using the Memory Address instructions can be stored and retrieved directly. 1001 1002 1003 1004 1005 1006 1007 1008 1009 DATA 1010 READ ONLY MEMORY (ROM) If everything in RAM is erased when the power is turned off from the computer, how does the computer maintains important instructions? The answer – ROM. The instructions to start the computer are stored in ROM Chips, which are non-volatile. This is a type of memory that allows information to be read from and cannot be written to (changed or altered) by the user. It is also a memory storage device (chip) used to store important data and program instructions permanently. Read Only Memory is said to be static or non-volatile (information storage does not depend on the availability of electricity). This means that what is stored in Read Only Memory is not lost when the electrical power to the computer is disrupted or when the computer is turned off. Read Only Memory can be used to store programs that will be required to perform specialized routine functions. Example, sometimes the Operating System which is a program which controls the operations of the computer and its resources, are stored into Read Only Memory so that every time the computer is switched on, the computer will check and load the Operating System Program into Random Access Memory from Read Only Memory. What is stored in this memory may not be destroyed if someone tries to write over/to erase/to modify (change) what is stored. The information stored in Read Only Memory is "burnt in" by the manufacturer and the user can only read and use the information and program instructions stored. The data and program instructions stored on Read only Memory Chips is stored in a way so that it cannot be changed by the user hence it is called Read Only Memory. So as to allow for upgrades to ROM, most computers use Flash Memory, or Flash BIOS, PROM and EPROM. In Personal Computers (PCs), ROM is used to store the computer’s Basic Input/Output System (BIOS). When you start your computer (also called Booting), the BIOS execute a Boot Sequence. In the Boot Sequence, the BIOS conducts various Power-On Self-Tests (POST), including memory tests, configuring the system’s hardware, and locating the Disk Storage containing the Boot Sector. The Boot Sector contains the computer’s basic functions for start up operations. Functions of Read Only Memory 1. To store start-up instructions for the computer permanently 2. To store program instructions/machine instructions the computer will need to use to control its operations THE ROLE OF READ ONLY MEMORY Vs. RANDOM ACCESS MEMORY It is important to note that every computer must have both Read Only Memory and Random Access Memory. One needs to understand that the computer requires a memory to store programs and instructions that are currently in use by the CPU. This type of memory is known as Random Access Memory. However, the computer also has to be comprised of a memory to store program instructions, programs and data that are essential to the running of the computer. This type of memory as known as Read Only Memory. NB A computer does not have to search its entire memory each time it needs to find data because the CPU stores and retrieves each piece of data (information) using MEMORY ADDRESSES. TERMS ASSOCIATED WITH MEMORY CACHE Memory RAM is fast but isn’t fast enough to support the processing speeds of today’s super fast microprocessors such as Pentium II. To enable these microprocessors to operate at such high speeds, today’s computer designers uses CACHE Memory. This memory is much faster than RAM, but it’s also more expensive. CACHE Memory helps to improve the computer system’s overall performance. The microprocessor can use this memory to store frequently and recently accessed instructions, programs and data. Bistable Device This is a device, which exists in two (2) discrete states. Example, the Binary System exists in only zero and 1. Programmable Read Only Memory (PROM) This is a type of Read Only Memory where the user can only read the program instructions stored and not alter (change/modify) its contents. PROM is used to program instructions into Read Only Memory. Also, PROM is a form of ROM, which is manufactured without program instructions stored on it. Program instructions can be read into PROM and therefore allowing it to become Read Only Memory. PROM is a variation (type) of ROM into which you, the user, can load read only programs and data. Generally, once a program is loaded into PROM, it is seldom, if ever, changed. It may also be seen as a blank memory chip on which you can place data items permanently. Erasable Programmable Read Only Memory (EPROM) This is a type of Read Only Memory that allows the user to erase or update PROM. They are used in robots and other devices in which the program instructions stored will only be changed or may only be changed periodically. Write Once Read Many (WORM) Program A program that erases data and/or program instructions from the computer system's memory. It is usually used with malicious or purposeful intent to destroy and/or erase information and program instructions stored on a computer. BIT This is a binary digit. Examples of binary digits are 0 or 1. A bit may also be described as the smallest unit of data storage. BYTE This is a group of 8 adjacent bits joined to represent a character. KILOBYTE (KB) This is 1024 bytes or little over 1000 bytes of storage capacity. MEGABYTE (MB) A Megabyte is one million (1, 048,576 – 1024 x 1024) bytes of storage capacity. GIGABYTE (GB) A Gigabyte is one billion bytes of storage capacity. TERABYTE (TB) A Terabyte is one trillion bytes of storage capacity. WORD An established number of bits that the computer handles as one unit. ADDRESS Name, numeral or label that designates (tells) a particular location in memory. LOCATION This is where data is stored. WORD SIZE This is the number of bits in each word. MEMORY LOCATION/ADDRESS This is an address in memory. BUFFER This is an intermediate memory that temporarily stores data that is being taken from Main Memory and sent to an output device. Buffer might also be described as a special area of memory from which data can be accessed. REGISTERS A high-speed storage area built directly into the ALU and is used to hold instructions and data currently being used by the CPU. It may also be described as a high-speed storage area in which data pertaining to the execution of a particular instruction is stored. CMOS - Complementary Metal Oxide Semiconductor A type of semiconductor often used in computers for batter-powered circuits that store the date, time and system configuration information. Additional Content: Synchronous Dynamic Random Access Memory (SDRAM), Single In-line Memory Module (SIMM), Dual In-line Memory Module (DIMM), Flash Memory and Cache. Memory Bit Internal Storage Volatile Memory - SDRAM, Rambus DRAM, Cache and Registers Nonvolatile Memory - ROM, ROM and Flash Memory 64 – 128 MB (Minimum) 128 – 256 MB (Minimum) 512 and up (Minimum) USE Home and business users managing personal finances, using standard application software such as Word Processing; using educational or entertainment CD-ROMs, communicating with others on the Web. Users requiring more advanced multimedia capabilities; running, number intensive accounting, financial or spreadsheet programs; using voice recognition; working with videos, music and digital imaging; creating Web sites; participating in video conferences; playing Internet guides. Power users creating professional Web sites; running sophisticated CAD, 3 D design, or graphics intensive software. Fig. 2 RAM Guidelines Processor 600 – 750 MHz Celeron Processor 866 MHz – 933 MHz Pentium 4 Processor 1.2 GHz – 1.5 GHz Pentium 4 Processor Memory 64 – 128 MB RAM 128 – 256 MB RAM Greater than256 MB RAM Fig. 3 Determining how much RAM you need depends on the applications you intend to run on your computer. Advertisements for computers normally list the type of processor, the speed of the computer measured in MHz or GHz, as well as the amount of RAM installed. Assignment to do: Collect several advertisements for computers that are being sold and determine whether the size of memory stated is sufficient for the given speed of the CPU. Make your recommendations.