C A N A D I A N I N T E R N A T I O N A L S C H O O L O F H O N G K O N G 5.1 Introduction 5.2 Components of a Computer System Algorithm The Von Neumann architecture is based on the following three characteristics: four major subsystems called memory, input/output, the arithmetic/ logic unit (ALU), and the control unit. the stored program concept, in which the instructions to be executed by the computer are represented as binary values and stored in memory the sequential execution of instructions, in which one instruction at a time is fetched from memory and passed to the control unit, where it is decoded and executed block diagram for a computer subsystem STORAGE Data Input Unit Secondary Storage Primary Storage Output Unit Information Control Unit Arithmetic / Logic Unit CPU Data Flow Control Flow 1
stores and retrieves the instructions and the data being executed information stored in memory is represented internally using the binary numbering system standardized memory width is 8-bit (or 1 byte) the largest unsigned integer value that can be stored in a byte is 11111111 (binary) or 255 (decimal) the value 2 N is the maximum (theoretical) memory size N Maximum Memory Size Industry Name 10 2 10 = 1K kilobyte 20 2 20 = 1M megabyte 30 2 30 = 1G gigabyte 40 2 40 = 1T terabyte 50 2 50 = 1P petabyte 2
A computer with a 16-bit address would have 64KB of memory. 2 16 = 2 6 x 2 10 = 64 x 2 10 = 64K basic memory operations are fetching and storing fetching (non-destructive) retrieve a copy of the contents of the memory cell with the specified address and return those contents as the result of the operation storing (destructive) assign the specified value into the memory cell specified by address RAM (random access memory) memory is divided into fixed-size units called cells, and each cell is associated with a unique identifier called an address all accesses to memory are to a specified address the time it takes to fetch or store the contents of a cell is the same for all the cells in memory 3
ROM (read-only memory) information has been prerecorded during manufacture information cannot be modified or removed, only fetched used to hold important system instructions and data in a place where a user cannot accidentally or intentionally overwrite them Memory Registers used to implement fetch and store operations requires two operands the address of the cell being accessed the value being accessed (fetched) or assigned (stored) Memory Registers Memory Address Register (MAR) holds the address of the cell to be fetched or stored must be at least N bits wide Memory Data Register (MDR) contains the data value being fetched or stored size of MDR is usually a multiple of 8 4
An example of operations in RAM Fetch (address) 1. Load the address into the MAR. 2. Decode the address in the MAR. 3. Copy the contents of that memory location into the MDR. Store (address, value) 1. Load the address into the MAR. 2. Load the value into the MDR. 3. Decode the address in the MAR. 4. Store the contents of the MDR into that memory location. Cache Memory the first time that the computer references a piece of data, it should move that data from regular RAM memory to a special, high-speed memory unit called cache memory a cache is typically 5 to 10 times faster than RAM but much smaller When the computer needs a piece of information: 1. look first in cache memory to see whether the information is there 2. if the desired information is not in the cache, then access it from RAM using the fetch operation 3. copy the data just fetched into the cache along, if the cache is full, then discard some of the older items that have not recently been accessed 5
SUBSYSTEM: input/output the devices that allow a computer system to communicate and interact with the outside world as well as store information RAM is volatile memory mass (archival) storage is nonvolatile memory SUBSYSTEM: input/output input/output devices come in two basic types those that represent information in human-readable form for human consumption keyboards, screens, printers those that store information in machine-readable form for access by a computer system mass storage systems: flash memory, hard-discs, optical discs SUBSYSTEM: input/output two forms of mass storage systems direct access storage device (DASD) every unit of information has unique addresses, time needed to access that information depends on its physical location and the current state of the device flash memory, hard-discs, optical discs sequential access storage devices (SASD) not all units of data be identifiable via unique addresses audio cassette tapes 6
SUBSYSTEM: arithmetic/logic unit (ALU) performs mathematical and logical operations and comparison for equality modern machines have the ALU and the control unit integrated into a single component called the processor SUBSYSTEM: arithmetic/logic unit (ALU) ALU is made up of three parts: the registers a storage cell that holds the operands of an arithmetic operation and when the operation is complete it holds its result the interconnections between components a bus is a path for electrical signals the ALU circuitry the circuits that carry out the operations SUBSYSTEM: control unit (CU) the unit to fetch from memory the next instruction to be executed, to decode it, and to execute it by issuing the appropriate command to the ALU, memory, or I/O controllers 7
SUBSYSTEM: control unit (CU) machine language instructions the instructions that can be decoded and executed by the control unit of a computer are represented in machine language the instructions in this language are expressed in binary SUBSYSTEM: control unit (CU) control unit registers and circuits to fetch and execute instructions, the control unit relies on the program counter (PC) and the instruction register (IR) and on an instruction decoder circuit Homework Pg. 238 # 8 Do you think that human memory is or is not a random access memory? Give an argument why or why not. 8
Homework Pg. 244 # 5 Defragmenting a disk means to reorganize files on the disk so that as many pieces of the file as possible are stored in sectors on the same track, regardless of the surface it is on. Explain why defragmentation can be beneficial. 9