 Prof. Dr. M. H. Assal Introduction to Computer AS 26/10/2014

 Memory refers to the physical devices used to store programs (sequences of instructions) or data (e.g. program state information) on a temporary or permanent basis for use in a computer or other digital electronic device.  Types of Memory o Volatile memory o Non-volatile memory Introduction to Computer 2

 Volatile memory is computer memory that requires power to maintain the stored information.  Most modern semiconductor volatile memory is either: o Static RAM o Dynamic RAM  Random Access Memory (RAM) o Random access memory (RAM) is a form of computer data storage. o A random access device allows stored data to be accessed in very nearly the same amount of time for any storage location, so data can be accessed quickly in any random order. Introduction to Computer 3

 Non-volatile memory is computer memory that can retain the stored information even when not powered.  Examples of non-volatile memory include read-only memory (ROM), flash memory, most types of magnetic computer storage devices (e.g. hard disks, floppy discs and magnetic tape), optical discs.  Read Only Memory (ROM) o Read-only memory (ROM) is a class of storage medium used in computers and other electronic devices. o Data stored in ROM cannot be modified, or can be modified only slowly or with difficulty. Introduction to Computer 4

 The amount of RAM in a PC has a direct effect on the system's speed.  The more RAM a PC has, the more program instructions and data can be held in memory, which is faster than storage on disk.  The goal of the virtual memory technique is to make an application think that it has more memory than actually exists.  Memory Swapping o Swapping is exchanging data between the hard disk and the RAM. o The purpose of swapping, is to access data being stored in hard disk and to bring it into the RAM so that it can be used by the application program. o Remember that swapping is only necessary when that data is not already in the RAM. o Swapping can greatly slow a PC's performance. Introduction to Computer 5

 Memory Swapping Introduction to Computer 6

1. Registers size 2. RAM size 3. System Clock (how many cycle per second) 4. Bus size 5. Cache Memory size Introduction to Computer 7

 Processor Components o Control Unit o Arithmetic Logic Unit (ALU) o Prefetch Unit o Decode Unit o Bus Interface Unit o Registers o Cache Memory Introduction to Computer 8

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 Every instruction that you issue to a computer by either typing a command or clicking something with the mouse is converted into machine language.  In turn, each machine language instruction in a CPU’s instruction set (the collection of basic machine language commands that the CPU can understand) is broken down into several smaller, machine-level instructions called microcode.  Microcode instructions, such as moving a single piece of data from one part of the computer system to another or adding the numbers located in two specific registers, are built into the CPU and are the basic instructions used by the CPU. Introduction to Computer 10

1. Fetch The program instruction is fetched 2. Decode The instructions are decoded so the control unit, ALU, and FPU can understand them 3. Execute The instructions are carried out 4. Store The original data or the result from the ALU or FPU execution is stored in the CPU’s registers Introduction to Computer 11

Introduction to Computer 12

Example: add 5 and 6 and show the answer on the screen requires the following steps: 1. Fetch instruction: "Get number at address 0x3A15412" 2. Decode instruction. 3. Execute: ALU finds the number. (which happens to be 5) 4. Store: The number 5 is stored in a temporary spot in Main Memory Repeat steps for another number (= 6) 9. Fetch instruction: "Add those two numbers" 10. Decode instruction. 11. Execute: ALU adds the numbers. 12. Store: The answer is stored in a temporary spot. 13. Fetch instruction: "Display answer on screen." 14. Decode instruction. 15. Execute: Display answer on screen. Introduction to Computer 13