Computer Hardware: Basic Definitions

Transcription

1 Computer Hardware: Basic Definitions Programming: The planning, scheduling, or performing of a task or event Computer program: A sequence of instructions for a computer to execute Computer: An electronic, programmable device that can store, retrieve, and process data Modern computers based on the von Neumann architecture Information: General, abstract knowledge Data: Information to be processed by a computer Must be in form computer can understand Computer converts data to information Computer system consists of 2 general components: 1. Hardware 2. Software Units Unit Quantity symbol Bit (binary digit) atomic piece of storage/memory b Byte: 8 bits smallest standard size of storage/data B Second s Hertz Speed H Common prefixes Prefix Value Symbol Quantity Tera 2 40 = 1, 099, 511, 627, 776 t memory, speed Giga 2 30 = 1, 073, 741, 824 g memory, speed Mega 2 20 = 1, 048, 576 m memory, speed Kilo 2 10 = 1, 024 k memory, speed Milli 10 3 (0.001) m time Micro 10 6 ( ) µ time Nano 10 9 ( ) n time 1

2 Computer Hardware: Basic Definitions (2) Approximations Prefix Approximate Value Tera = 1, 000, 000, 000, 000 Giga 10 9 = 1, 000, 000, 000 Mega 10 6 = 1, 000, 000 Kilo 10 3 = 1, 000 2

3 Computer Hardware: Basic Components Represents physical components 2 general types Computer proper 1. Main memory 2. Central Processing Unit (CPU) 3. Bus Peripherals Add-ons E.g., printer, DVD drive, hard disk 3

4 Computer Hardware: Memory - Intro Refers to data storage components of computer Hierarchy of memory types: Type Speed Cost Nature Register fastest most expensive volatile Cache volatile RAM slowest least expensive volatile ROM nonvolatile Volatile memory loses its contents when computer is powered off Stores everything used by the computer: programs, data,... Stored electronically Standard approach uses bi-stable (two-state) device Transistors used - electronic switches When voltage applied to base, electricity flows from collector to emitter Switch is on No voltage to base, no flow Switch is off Transistor represents a bit(binary digit) of data Each transistor can represent 2 values : on, and off Values represented as 1 and 0 Byte considered basic unit of storage 4

5 Computer Hardware: Memory - RAM RAM stands for Random Access Memory Consists of a linear sequence of storage locations (cells, words,...) Each byte has a unique address, numbered from 0 to total amount of memory - 1 Often, bytes grouped together into words Memory generally measured in terms of mega or gigabytes RAM is primary storage area for computer during operation Often referred to as primary storage, main memory Stores both programs and data When power turned off, all switches open Memory becomes Contents must be loaded during operation Physically exists as an array of cells Each row and column has a wired connection To access a particular memory cell, activate the connections for its row and column 5

6 Computer Hardware: Memory - RAM (2) Types of RAM Dynamic (DRAM) Electricity stored in a capacitor Transistor controls amount (voltage) stored If greater than about 50% capacity 1 If less than about 50% capacity 0 Capacitors lose charge over time (msec) Computer must refresh value in each bit on a regular basis Done thousands of times per second Hence: dynamic RAM Static (SRAM) Uses multiple transistors per bit Value does not need to be refreshed Compared to DRAM 1. Faster (no need to refresh) 2. More expensive 3. Larger Hence, not used for main memory Typical time to access data: 50 nsec 6

7 Computer Hardware: Memory - ROM ROM stands for Read Only Memory Often called BIOS (Basic Input Output System) Data permanently stored - cannot be changed Hardwired using diodes Stores parts of operating system needed to start computer 7

8 Computer Hardware: Seconday Storage - Intro Secondary storage refers to memory that is not part of the computer proper Examples: Disks Tapes CDR DVD Flash memory These types of memory are nonvolatile: They retain their contents when unpowered Without secondary storage, programs, data, etc. computer every time it was turned on These devices generally are magnetic or optical in nature would need to be typed into a 8

9 Computer Hardware: Secondary Storage - Magnetic Devices 1. Disks Typical disk structure: Bits stored as alignments of magnetic particles Microscopic magnetic particles have north and south poles One alignment corresponds to a 1 bit, the other to a 0 bit Data bits appear sequentially along a track Disk addresses consist of track, block, and byte offset Block size multiple of 512 bytes Data transfer is a whole block at a time 9

10 Computer Hardware: Secondary Storage - Magnetic Devices (2) Read/write process: Given a disk address to access Disk spins Typically 7200 rpm 8.33 msec per 1 rotation Position arm over appropriate track Called seek time E.g., 0.02 msec to move to adjacent track Wait until appropriate block rotates under head - latency time (fast) Called (rotational) latency E.g., 0.02 msec to move to adjacent track Best case: 0 msec Worst case: 8.33 msec Read the block Called read time Requires 8.33/n msec, where n is number of blocks per track 10

11 Disk pack Computer Hardware: Secondary Storage - Magnetic Devices (3) Set of disks packaged as a rigid unit Each arm has 2 read/write heads - one for each adjacent surface Actuator moves arms as a unit Disk address will include surface Cylinder: set of tracks that can be read/written for a given position of the actuator Data transfers flow through a buffer Part of memory reserved for data transfers Usually can hold several disk blocks Want efficient data transfer Factors that affect transfer speed: Physical record structure File organization Buffer management 11

12 2. Tape Computer Hardware: Secondary Storage - Magnetic Devices (4) Disk is a random access device Can access disk address independently of others Tape has 8 tracks Each track stores one bit Bytes stored sequentially along tape Tape is sequential access device Given an address, must pass over all data that precedes it Significantly slower than disk Used for long-term storage of files 12

13 Computer Hardware: Secondary Storage - Optical Devices CDR and DVD basically the same Structure is similar to disk structure, except have continuous spiral instead of separate tracks Bits stored as differences of reflectivity Laser shines on disk Light is reflected into a sensor Bright interpreted as 1, dull as 0 CDROM Plastic substrate has bumps burned in with laser Shiny aluminum coated over the substrate Laser light shining on a bump reflected away from sensor, so appears dark (representing a 0 ) Data cannot be changed 13

14 Computer Hardware: Secondary Storage - Optical Devices (2) CD-R Uses a heat-sensitive, translucent dye instead of bumps When heated, dye becomes opaque To write a 0, heat spot on CD with write laser More powerful than read laser Read laser not strong enough to affect dye Can only be written once 14

15 Computer Hardware: Secondary Storage - Optical Devices (3) CD-RW Uses a phase-change chemical In crystalline form, is translucent In amorphous form, is opaque When melted and cooled quickly, becomes amorphous When melted and cooled slowly, becomes crystalline Initially, compound in crystalline form To write a 0, melt and cool quickly To overwrite a 0 with a 1, melt and cool slowly 15

16 Computer Hardware: Flash Memory Flash memory is like ROM on the motherboard However, it uses an EEPROM - Eelectronically Erasable Programmable ROM Using transistors and quantum effects at the atomic level, a charge can be semipermanantly stored in a memory cell The charge can be removed at a later date 16

17 1. Integers (whole numbers) Computer Hardware: Representation Each digit of a binary integer represents a power of 2 In decimal (base 10), each digit represents a power of 10: = In binary (base 2), each digit represents a power of 2: = = = Note that the largest value that can be represented is limited by the amount of storage allocated If 1 byte is used for integers, the largest integer that can be represented is = = 255 If 2 bytes are used for integers, the largest integer that can be represented is = = To convert decimal representation to binary: Cast out 2 s Steps: (a) Divide decimal representation by 2 (b) Write down the remainder (c) Repeat the previous 2 steps, using the quotient for the dividend (d) Continue until quotient is 0 (e) Write the remainders in the reverse order in which they were calculated Example: (a) 53/2 = 26R1 (b) 26/2 = 13R0 (c) 13/2 = 6R1 (d) 6/2 = 3R0 (e) 3/2 = 1R1 (f) 1/2 = 0R1 (g) Answer =

18 Computer Hardware: Representation (2) Negative integers are represented using a leading 1 One s complement A leading 1 means a negative number A leading 0 means a positive number Remaining digits interpreted as above Example: = ( ) = ( ) = There is a problem with this representation: = = Two s complement Developed to eliminate above problem To represent an integer in two s complement representation: (a) Write the binary representation as usual (b) Flip all the digits (1 0, 0 1) (c) Add 1 Example: (a) Binary representation of +53: (b) Flip all the digits: (c) Add 1: Note that using either approach cuts the size of the largest integer that can be represented in half One s complement: Largest positive integer (1 byte): = Smallest negative integer (1 byte): = Two s complement: Largest positive integer (1 byte): = Smallest negative integer (1 byte): =

19 2. Floating point (real) numbers Computer Hardware: Representation (3) Represented using scientific notation: = 327, 460, 000 In the computer, only the values are stored: sign mantissa sign exponent Since storage is of fixed size, a fixed set of bits are used for the mantissa and exponent In addition, the decimal point is assumed to be in a specific location For example (base 10): (a) Assuming decimal is in front of first digit of mantissa (b) 4 digits for mantissa (c) 2 digits for exponent 327, 460, 000 = This leads to loss of precision (inability to represent all digits in original) It increases the range of values that can be represented: Largest value: (As opposed to in decimal using 8 digits) Binary representation is used for mantissa, exponent, and signs in the computer 19

20 Computer Hardware: Representation (4) 3. Characters There is no direct correlation between printable characters and binary representation Arbitrary codes are used ASCII (American Standard Code for Information Interchange) Standard code for representing characters Uses 7 bits A represented by = a represented by = represented by = Note: Character 1 not the same as the integer 1 Unicode Extension of ASCII Uses up to 4 bytes EBCDIC 4. Parity bits IBM standard for mainframes Extra bit to help insure data integrity Odd parity means the number of 1 s in a byte will add to an odd number The parity bit is set to insure this is the case For example: If an extended byte does not have the proper parity, an error has occured 20

21 Computer Hardware: Representation (5) 5. Hexadecimal Used to simplify representation of binary values Hexadecimal is base 16 Requires 16 digits Uses letters in addition to the digits 0-9 hex digit A B C D E F value Every byte can be represented by a pair of hex digits Example: is represented as 1101 = = D, 0010 = 2 10 = 2, or D

22 Computer Hardware: Central Processing Unit (CPU) Brains of computer Components: 1. Registers Special high-speed memory cells local to the CPU 2. Arithmetic Logic Unit (ALU) Performs computations 3. Control unit (CU) Controls operation of the system 22

23 Computer Hardware: Central Processing Unit (CPU) (2) Operation based on von Neumann architecture 1. Computer composed of following systems (a) Memory (b) CPU (c) IO system 2. Programs are stored in memory 3. Program statements are executed sequentially 23

24 Storage local to CPU Computer Hardware: CPU - Registers High-speed memory directly accessible to CPU components Examples: 1. General purpose registers to hold operands and results of computations 2. Accumulator - Specially designated register for operands and results Memory Address Register (MAR) Stores address of memory location to be accessed Must be large enough to store largest address in memory address space E.g., 4GB = 2 30 memory requires 31 bits 7. Memory Data Register (MDR) Stores data to be retrieved from or saved to memory One or more bytes in width 8. Instruction Register (IR) Holds instruction being executed Must be large enough to hold largest instruction E.g., 4, 8, 16 bytes 9. Program Counter (PC) Holds address (in memory) of next instruction to be executed Must be large enough to store largest address of memory reserved for holding programs 24

25 Types of operations performed: Arithmetic Addition Subtraction Logic AND OR NOT EXCLUSIVE OR Shifts Shift left Shift right Rotate left Rotate right Computer Hardware: CPU - ALU 25

26 Computer Hardware: CPU - ALU (2) Operands can be loaded from registers, or from elsewhere Results can be stored into a register, or elsewhere The control line selects the operation to be performed Can be designed to perform more complex operations But requires more circuitry 26

27 Manages operation of computer Operates using following cycle 1. Retrieve instruction from memory Computer Hardware: CPU - CU (a) This step referred to as a fetch (b) A copy of the instruction is loaded into the IR (c) Increment the PC It now holds the address of the next instruction to be executed 2. Decode the instruction Each instruction is in binary format Instructions of form opcode operands There is one opcode for each specific operation the CPU can perform Operands may take the form of actual values (e.g., 105, c ) or addresses (where values are stored) Instructions may have from 0 to 3 operands For example: bit opcode, 8 bit operand Performs x = y

28 Computer Hardware: CPU - CU (2) The instruction decoder circuit reads the opcode of the instruction The circuitry of this decoder converts the opcode into signals that carry out the execution of the instruction; e.g. To add the contents of 2 registers and store the results into another register To load the value at a particular memory address into the MDR The signals enable various bus lines and devices needed for the task 3. Execute instruction The circuitry carries out the task 28

29 Computer Hardware: CPU - Memory Access Execution may require transfer of data between memory and CU Fetch (read) 1. Load address into MAR 2. Copy contents of address into MDR 3. A fetch is non-destructive - contents of address are unaffected 29

30 Store (write) Computer Hardware: CPU - Memory Access (2) 1. Load address into MAR 2. Load value into MDR 3. Copy contents of MDR into address 4. A write is destructive - original contents of address are lost 30

31 Computer Hardware: CPU - Cache Memory To speed up processing, CPU may have cache memory This is high-speed memory local to CPU Access time about 10 ns Generally a few KB in size Operation based on Principle of Locality: What has been recently accessed is likely to be accessed again in near future Used as follows: When CPU needs something from memory Search cache for item if not found in cache retrieve from main memory store in cache To see how it improves speed: Assume memory fetch requires 50 ns Assume memory fetch requires 10 ns Assume data will be found in cache 60% of the time Average time to fetch a piece of data: ( ) + (0.4 ( )) = = 30 ns on average Why not use cache for RAM if so fast? Expensive 31

32 Speed measured in Hertz (mega, giga) Computer Hardware: CPU - Processing Speed This refers to how fast the system clock ticks MIPS (Millions of Instructions Per Second) May require more than one clock tick per instruction FLOPS (FLOating Point operations per Second) Considered a more realistic measurement of speed The system clock controls the speed at which instructions are executed In general, each major step in CPU takes one clock cycle: Fetch Decode Execute Fetch data from memory Write result to memory Bus width affects transfer speeds If bus width is 16 bits, but data is 32 bits, requires 2 memory fetches 32