f453 revision booklet

Advanced Computing Theory

OCR F453 Written Exam (January 2011)

:: Jonathan Ashworth

The Function of Operating Systems (3.1)

Features of an Operating System (3.1.1)

An operating system provides…

o Management of hardware resources

o An interface between the user and the machine

o An interface between applications software and the machine

o Security for the data on the system

o Utility software to allow maintenance to be done

o Error handling

Interrupt Handling (3.1.2) A new process is added to the queue with a certain priority

o If the priority is higher than the current process, the interrupt will be serviced

immediately…

o All data concerning the current process is placed upon a stack

Highest priority interrupt is serviced first to avoid data loss/hardware damage

The previous process may resume when all higher priority processes have finished (fetch

data from the stack)

Process A

Process B

Process C

Process D

(empty)

(empty)

Process E

Stack

Process A




Scheduling, Job Queues & Priorities (3.1.3) Many operating systems need to run many processes at the same time

A multi-tasking operating system holds many jobs in its memory

o The OS must switch between different jobs in a particular order

The switching between jobs is defined by a set of rules/algorithms (known

as scheduling)

Sorting rules/algorithms include…

o Round Robin (RR): each job in the READY queue is allocated an equal

time slice, allowing all jobs to gain processor time. If a job completes

during its time slice, it will leave the system

o First Come First Served (FCFS): the first job runs until completion, then allowing the

next job to run until completion... and so on. If a long job gets onto the processor, it

will not leave until completed

o Shortest Job First (SJF): the OS places each job depending on the time expected to

complete into the READY queue

o Shortest Remaining Time (SRT): sorts the jobs in the READY queue into shortest time

remaining to complete. Long/large jobs may never run as they never get to the top

of the queue

o Multi-level Feedback Queue (MFQ): uses many levels of priority. The job with the

highest priority is allowed to run once, before being sent back to the READY queue

with a new priority

If the new priority is still the highest, it will run again

If the new priority is lower than another job, that job will run

When a job has the lowest priority, Round Robin is applied




Scheduling must…

o Maximise the use of the whole computer

o Allow fair running for each job/user

o Provide a reasonable response time to jobs/users (both active

and batch jobs/users)

o Prevent the system failing if it is becoming overloaded

o Be consistent by providing similar response times for similar tasks

A scheduler decides when a job may enter and must leave the processor…

o A pre-emptive scheme allows the Low Level Scheduler to remove a job from the

running state so another job may be serviced

o A non-pre-emptive scheme will allow a job to continue running until it no longer

needs the processor

Useful in a small system where there are few jobs

Not useful where there are many jobs, as there is a danger of a job

monopolising the processor waiting on an input/output

o High Level Scheduler (HLS): Places a job into the READY queue, whilst ensuring the

system is not overloaded

o Medium Level Scheduler (MLS): Swaps jobs from the IAS onto the backing store and

visa versa (see paging)

o Low Level Scheduler (LLS): Places and removes jobs in/from the running state. It

places the job back in the READY queue in the correct position

A pre-emptive system




Input/Output bound jobs are jobs that use a peripheral a lot, e.g…

o Waiting for a user input

o Output to a printer

Processor bound jobs are jobs that use the processor a lot, e.g…

o Manipulating large images

o Calculating huge numbers

Memory Management (3.1.4) Requirements of memory management are…

o Allocate memory to multiple jobs to allow for multi-tasking

o Allow jobs larger than the total size of memory to run

o Relocate un-essential pages when needed

o Protect jobs from each other

o Protect the operating system

o Allow memory to be shared by swapping pages

One way of holding jobs and data in main memory, is to split them into equal sized

wordspaces called pages…

o Splitting a (large) program into pages

o Only part of program needs to be in memory

o Some of program can stay on disk (virtual memory)

o Allows program to run when there is insufficient memory

A = I/O Bound job

B = CPU bound job




o Splitting memory into pages

o Both need indexing using a table

o Sections of program need not be all together

This system is subject to disk threshing, which will slow the entire system down and possibly

cause failure or data loss

o Occurs when moving pages between memory & disk

o Disk is relatively slow

o High rate of disk access

o More time spent transferring pages than on processing

Direct/Absolute Addressing: a command is sent with the real

address (e.g. ADD 10 means Add the data in address location 10)

o The amount of memory that can be addressed is limited

If using a 16-bit machine and the op-code ADD was 01101, this would leave

2048 address that could be addressed

o To execute the same operation on a number of addresses, we would have to write

the same instruction many times

Indirect Addressing: a command is sent with an indirect address (e.g. ADD 10 means the

address 10 contains the address of the data to be used)

o This allows all 16/32/64 bits to be used to address data (access more data)

Indexed Addressing: if we wanted to add together 100 items in an array, we would need

100 instructions (ADD 10, ADD 11, ADD12…ADD100.

o Indexed addressing includes a special register called the index register (IR) which is

initially set to 0

o When the first value is taken from 10, the IR becomes 0+10

o For every other item, the IR is simply incremented

o This allows only one instruction to be executed with information it should be done

100 times




Relative Addressing: when a program is compiled, it is not always clear where the program

and data will be stored in memory, so the instruction ADD 10 is useless as the data may not

be in address 10

o Instead, the start address of a programs block (or page) is recorded in a table

o Now ADD 10 means ADD (location: [START ADDRESS]+10)

Every address is relative to the start address

Immediate Addressing: The real value is in the command to be executed (e.g. ADD 10 means

add the value 10)

Spooling (3.1.5) When a job needs to output to a slow peripheral, a spool will be used to make efficient use

of the resources…

o The CPU outputs to the spool queue the address of the data needing to be output

Allowing the CPU to service other jobs, whilst the output is dealt with

The spool queue normally uses First in First out ordering, however jobs can be added with

different priorities

When a peripheral is ready for the next job in the spool queue, it will read the address, fetch

the data and begin to output

Spooling allows many jobs/users to use a single output peripheral in turn

(Remember: a spool is different from a buffer, because a buffer allows only one job/user to access it at

a time, and it requires the CPU to keep filling it up)

Output Peripheral

0x01 0x02 0x03 0x04 0x05 0x06 0x07




Modern PC Operating System (3.1.6) There are two main types of operating system used today…

o Command-based (like MSDOS)

o Graphical-User-Interface (like Windows)

The OS provides the user and programs with facilities to handle inputs and outputs, copy

and move files, handle memory allocation and any other basic task

Steps needed to boot a PC…

o Power-On-Self-Test (POST) routine is executed

Found in ROM

Clears all registries and CPU of any left-over data

Loads the first value of the boot program into the program counter

o Boot program executes

Stored in ROM

Simplified version of the BIOS (Basic I/O System)

Checks the POST routine and Boot program

o The CPU now sends signals to hardware to check it is functioning properly

System checks: Busses, System clock, RAM, Disk drive and Keyboard

o The boot program now checks drive A (often a CD/Floppy Drive) for an OS. If there is

no disk, it will check drive C (often a Hard Drive)

Once found, the operating system’s files are loaded




The File allocation table (FAT) is a computer file system architecture that…

o Maps the location of files on the backing store (sectors grouped in clusters)

o Provides addresses/pointers to start of files

o Has a linked list

o Holds files names

o Identifies free space

o Stores access rights

o is updated by the OS (file saving/deletion)

o Stored in IAS reduce disk access and to increase the time taken to find a file

The operating system manages the FAT table…

o Allocation of space on disk

o Deleting files

o Copying files

o Moving files

o Control file access rights

o Map logical file addresses to physical addresses on disk

o Creating shortcuts




The Function and Purpose of Translators (3.2)

Types of Translators & their Purposes (3.2.1)

A translator converts…

o Human readable commands (high level language) into…

o Machine readable commands (binary/object code)

Low level languages (close to the language used by the computer)…

o Represent binary with mnemonics, such as…

ADD = 001 (a command)

NUM1 = 00101 (an address)

o The use of mnemonics in place of binary is called Assembly Language

o An Assembler translates the mnemonics (using a table) to produce machine code

High level languages (close to the language used by a human)…

o Because they are close the human language and far from machine code, a more

complex translator is needed…

A Complier takes a high level language (source code) and translates it into a

machine code (object code) equivalent

A disadvantage of using a compiler is that it if very memory

hungry…

o Source code needs to be in IAS

o Compiler needs to be in IAS

o Resultant object code is in IAS (to be copied to backing

store)

Another disadvantage is that it is hard to pin-point an error in the

source code

A Interpreter reads the source code line by line, translating it and then

executing

Errors are easy to pin-point

Less demanding on the system resources




Lexical & Syntax Analysis, Code Generation & Optimisation (3.2.2) Lexical analysis…

o Each statement will contain a keyword telling the compiler what to do

o The compiler has a table of accepted keywords and will look up the statement’s

keywords to ensure they exists

If the keyword does not exist, an error will be produced

o All keywords are replaced by a set of binary numbers called tokens

o It removes any white space and comments inserted by the programmer

o It will spot all names (variables/constants) it does not recognise and create a lookup

table called a symbol table

The symbol table contains the name, data type, memory location and any

restrictions for the variable/constant

Syntax analysis…

o This parse checks the tokens generated by the lexical analyser to see if it is

grammatically correct

o This is done by using Backus-Naur Form rules (see 3.7.2)

o If a token breaks a grammatical rule, it can output an error for that line of code

Code generation…

o The compiler now takes the valid strings of tokens and changes them into low-

level/intermediate code

o This process is One-to-Many as there are many low-level instructions needed to

represent a single instruction in high-level

Resulting in a larger resultant file than the original high-level file

Code optimisation…

o Simply removes the unnecessary code such as (A := A +0) to result in more efficient

(faster) code




Library Routines (3.2.3) Are pre-written, pre-compiled and pre-tested routines that recur in many larger programs

o This saves time for development and compilation

The routines are loaded into memory when a utility program called a loader needs them

and they are linked to the relevant places in the existing code by a utility routine called a

linker

Loader

Linker

HDD IAS




Computer Architectures (3.3)

Von Neumann Architecture (3.3.1)

Uses a single processor

Following a linear sequence (fetch-decode-execute)

Uses special registers to control this sequence

Instructions and data stored in the same place

Registers & Fetch-execute Cycle (3.3.2) The special registers that carry out the fetch-decode-execute are…

o PC > MAR > IAS > MDR > CIR > Accumulator

The control unit fetches instructions from memory, decodes and synchronises all operations

The ALU is where data is processed both arithmetically and logically

o Arithmetical processing involves mathematical operations (add, sub…)

o Logical processing involves logical operations (AND, OR, XOR…)

Data Bus

Address

Bus




Fetching…

o The PC holds the address of the next instruction to be executed

As instructions are held sequentially in memory, this can often just be

incremented after each cycle (this is not the case when a JUMP instruction is

executed)

o When the instruction is needed, the content of the PC is copied to the MAR…

o Causing the MDR to fetch the instruction from the IAS at the address specified by

the MAR

o The content of the MDR is then placed in the CIR, ready to be executed

Decoding…

o The instruction is now split into two parts…

The operation code

The address of the data to be used

o The address is then placed in the MAR…

o Causing the MDR to fetch the data

o The operation is now decoded by referring to a table

This table defines the binary for all operational codes (RISC & CISC)

Refined Instruction Set Computer (RISC): is not able to recognise a

lot of mnemonics, but greatly decreases process time as it is not

spending as much time looking up a table

o Some instructions will require a combination of simpler

instructions to run

Complex Instruction Set Computer (CISC): can recognise any

mnemonic which can make processing faster if there are a great

range of different mnemonics being used

Op code Address

0101 100110101001

ADD #9A9




Executing…

o Now the decoded instruction and the data in the MDR are sent to the necessary

locations…

If the instruction is to store the data, then the data (in the MDR) is placed

back into the IAS at the location stored in the MAR

If the instruction is a JUMP, the address in the MAR is used to change the PC

If the instruction is arithmetical/logical, the data is sent to the ALU to be

processed

The result being stored in the Accumulator

Other Architectures (3.3.3) Co-Processors (sometimes called floating point units)

o Floating point binary representation requires a long string of bits

A maths co-processor is a device which has a number of registers that are

long enough to handle the length of a floating point binary number in one

go, making processing faster

o Allow more complex calculations/processing to be given to a dedicated co-

processor which only processes these similar calculations

Such as a GPU (graphical processing unit)

Advantage of allowing the CPU to complete other instructions




Parallel Processors

o Different parts of a program can be executed at the same time (providing the

program has been designed to have different parts executed at the same time)

o Advantages…

Greatly increases the speed programs can execute

Decreases time taken for complex mathematical processing

Speeds up the processing of data from many inputs

o Disadvantages…

A more complex program must be developed, compared to the traditional

sequential program, to allow execution of different parts at the same time

CPU A CPU B CPU C

IAS




Array Processors

o Has many ALUs (arithmetic logic unit) and one processor

Useful for manipulating 1 dimensional arrays with the same calculation,

where the same calculation can be applied to every element in the array in

one cycle

Pipelining

o Uses the standard fetch-decode-execute cycle

An instruction can be in any one of these 3 stages

o Pipelining allows the system resources to be used more effectively by making sure

there is an instruction at each stage of the cycle:

o If a JUMP instruction appears, the pipe is broken and a new instruction must be

added




Data Representation (3.4)

Floating Point Binary (3.4.1)

Uses a mantissa and exponent to represent a fractional number in binary

o Mantissa is the numerical value

o Exponent is the multiplier constant

Uses normal binary numbering with additional fractional numbers (4, 2, 1, 0.5, 0.25, 0.125…)

The greater the size of the mantissa, the larger the number you can represent

The greater the size of the exponent, the greater degree of accuracy you can achieve

Normalisation (3.4.2) A number must be normalised before it can be stored as a floating point binary number

o This means there should be only one leading 0 on the mantissa (unless it’s a

negative number, where it leads with 1 followed by a 0)

o The exponent is found by moving the decimal place to normalise the mantissa

Mantissa Exponent

01001 010

Denary: 2.25

Binary: 4 2 1 0.5 0.25 0.125 0.625

0 1 0 0 1 0 0




Data Structures and Data Manipulation (3.5)

Implementation of Data Structures (3.5.1)

Static data structures…

o Do not change size during execution (e.g. an array)

o Advantages:

Compiler can allocate memory during compilation

Easy to program

Easy to check of overflow

Allows random access (indexing)

o Disadvantages:

The programmer must estimate the amount of memory needed to store the

structure

It can waste a lot of memory

Dynamic data structures…

o May change during execution (e.g. a linked list)

o Advantages:

Only uses the memory needed at any time

Makes efficient use of memory

o Disadvantages:

Difficult to program

Can be slow to implement searches

Only allows serial access




A stack is a (LIFO) data structure …

o To push (add) data, the stack must first be checked it is not full…

Next, the pointer (points to the highest data item) is incremented

The data item can now be pushed onto the stack

o To pop (delete) data, the pointer is decremented. This allows new data to overwrite

the current data

A Queue is a (FIFO) data structure…

o To insert data into a queue, it must first be checked it is not full

If it is not full, the head pointer in incremented and data inserted at this

location

If it is full, the data may not be inserted

o Before deleting, the queue must first be checked it is not empty

If it is empty, the head pointer = the tail pointer

If it is not empty, the data at the tail pointer is copied and the tail pointer is

incremented

A Tree is a structure where each data item points to two other items depending on a rule

David

Barbra Ed




Searching, Merging & Sorting (3.5.2) Binary searching…

o While list is not empty do

Find mid-point cell in current list

If the value in this cell is the required value, return cell position

If the value found is less than the mid-point then

Change the search list into the first half of the current list

Else change the search list into the second half of the current list

o Report Error (Item not in list)

o Using this method on a large list, greatly increases search time

However, the time taken to search a small list is increased compared to a

serial search

Serial searching…

o Simply compares each item in the array/file with a known value

o This is useful in a small file/array

File merging is very simple…

o Consider the two lists:

12 54 24 34 and 24 10 56 32

o To merge, we compare each item. The item with the least value is inserted to the

new list first: 12 24 10 54 24 56 32 34




Insertion sort (shuttle sort): sorts a file depending upon a set of rules

o Compare the first and second number of the list. If the second number is less than

the first, swap the items

o Now compare the first, second and third items. If the third item is less than both the

first and second, move the third value to the first position

Quick sort also sorts files, but if very efficient at sorting large files

o 2 pointers are used: a left pointer and a right pointer

o The left and right item values are compared, if the right item is less than the left,

they swap

Move the left pointer +1 and repeat first step

o If the left item value is less than the right item value, the right pointer moves -1 and

repeats step 1




High-level Language Programming Paradigms (3.6)

Declarative Programing (3.6.1)

Declarative languages are an alternative paradigm (method of programming) to sequential

languages

4th generation language (closer to human language and built with data in mind)

Given a set of facts (e.g. DOG(Max), DOG(Baz) & CAT(Bennett))

Uses rules to define the relationship between facts

An objective/goal is given (such as: DOG(x))

o Searches first instance

If it fails, backtrack to the start and search the next instance

If it succeeds, return value (e.g. Max)

o The computer now backtracks to find any other instances – returning any other true

instances (e.g. Baz)

o Once all instances of DOG(x) have been found (true and false) the goal has been

satisfied

Useful in knowledge based systems (e.g. medical diagnosis)

Object-orientated Programming (3.6.2) Object oriented languages have classes and derived classes

o A class contains data about similar entities and methods used to manipulate this

data

o A derived class contains data about similar entities derived from the super class’s

entities. It too contains methods used to manipulate this data

o An object is an instance of a class and is an actual real-world entity

Data Encapsulation is the concept that data/variables can only be accessed via the methods

provided

Inheritance is another concept that allows the re-use of variables and methods of a super

class of which a new class is derived




To plan and explain OOP structures, a method called Unified Modelling Language is used…

o Use case diagrams show how users are related by a process

o State diagrams show how an object may behave through the various processes of a

system (cross between a DFD and a flow diagram)

o Sequence diagrams show how the objects in the classes interact with one another

o Activity diagrams are like flow charts, but with the difference of showing the steps

needed to get the object into a particular state

Super Class

Derived Classes

Class name

Data

Methods




o Communication diagrams are used to show how different object combine to pursue

a common purpose

Programming Techniques (3.7)

Standard Programming Techniques (3.7.1) Jackson Structured Diagrams (Top-down & Step-wise refinement)

A parameter is a variable/value passed to a sub-routine (function)

Local variables only exist inside a single sub-routine

Global variables are accessible throughout the entire system

Every time a function is called, the return address must be stored in a stack to be able to

resume the main program

Methods of Defining Syntax (3.7.2) Almost everything used in a computer language needs to be defined using some fixed

‘definites’ that are not subject to rules (called terminal symbols – things like the letter A)…

o Backus-Naur Form (BNF): Uses angle brackets to define meta variables

<LETTER>::=A | B | C | D

<WORD>::=<LETTER> | <LETTER><WORD>

o Syntax Diagrams: Uses terminal symbols in the form of a diagram




Reverse Polish Notation provides unambiguous expressions, which do not need brackets

and can be solved using stacks

o An infix notation has an operator between the two operands (A + B)

The problem with infix notation is when an expression such as A + B * C is

used, we need to have brackets in order to decide which operation to do

first

The order as to which order to operate is ambiguous

o Polish notation (Prefix) puts the operand at the start of an expression (e.g. + A B)

This is unambiguous as there is no longer a need for brackets

o Reverse Polish notation (Postfix) puts the operand at the end of the expression

This too is unambiguous, but also allows the result to be worked out using

techniques with stacks

2 * (A + B) in reverse polish is A B + 2 *




Databases (3.8)

Design and Normalisation (3.8.1)

A database is a series of related tables…

o When files are part of a database they are called tracks

A table is a collection of sets of similar data called records

o An Attribute is a column containing a single item concerning multiple entities

o A Tuple is a row containing multiple items concerning a single entity

An Attribute that holds an item that is unique from every other entity is called a primary key

A table can be ‘linked’ together to relate the data

o A foreign key maintains the relationship between two tables

If all the data is held in one large table with data redundancy, then it is in UnNormalised

Form (UNF)

Data in the database can be arranged in 3 different ways…

o First Normal Form (1NF): all repeated data items are removed (data redundancy)

o Second Normal Form (2NF): ensures all data items are dependent on a primary key

o Third Normal Form (3NF): removes part key dependencies (an attribute that takes a

certain value because of another attribute’s value)

Data Redundancy is the repeating the same data item for multiple tuples

Data Integrity is the likeliness of a data item being correct (more likely if there is no data

redundancy)

Attribute A Attribute B Attribute C Attribute D

Tuple 1 Item 1A Item 1B Item 1C Item 1D






Data Consistency is about how reliable the data is (consistently correct or wrong)

Methods & Tools for Analysing & Implementing Databases (3.8.2) An example database bellow shows a Many to Many relationship

The following shows the database in 3NF with a new link table (Ticket)

o A link table normally consists of the key attributes of the tables on both sides

Database Management System (DBMS) (3.8.3) DBMS’s controls the database and access to the data, it must control…

o The amendment of data

o The addition of data

o The deletion of data

… to ensure the rules of the database are not broken

A DBMS includes software called Data Description Language

(DLL) which defines…

o Tables in the database

o Data types in the table

o Constrains on the data

o The structure within the database

The data dictionary is a file of descriptions, structure and storage of the data

A database design is called a Schema

o Each user is given a subschema which defines the rules about how they may access

the data

User Film

User Film Ticket




The DBMS also provides a Data Manipulation Language (DLL)… (see 3.9.4)

Use of Structured Query Language (SQL) (3.8.4) SQL is a 4th generation language that allows a user to query a database

o 4th generation languages are created with the data in mind

To delete a row in SQL...

o DELETE FROM Users WHERE Formname = ‘7CG’

To insert a row in SQL…

o INSERT INTO Users VALUES (‘7BC’, ‘Mr Campbell’, 21)

To select a row in SQL…

o SELECT * FROM Users WHERE Age = 21

Column

name

Table

name

Value

name

Operation

Table

name

Values Operation

Column

name

Table

name

Value

name

Operation All

columns




Index

The Function of Operating Systems (3.1) ............................................................................................... 1

Features of an Operating System (3.1.1) ............................................................................................ 1

Interrupt Handling (3.1.2) ................................................................................................................... 1

Scheduling, Job Queues & Priorities (3.1.3) ........................................................................................ 1

Memory Management (3.1.4)............................................................................................................. 4

Spooling (3.1.5) ................................................................................................................................... 6

Modern PC Operating System (3.1.6) ................................................................................................. 6

The Function and Purpose of Translators (3.2) ....................................................................................... 9

Types of Translators & their Purposes (3.2.1) .................................................................................... 9

Lexical & Syntax Analysis, Code Generation & Optimisation (3.2.2) ................................................ 10

Library Routines (3.2.3) ..................................................................................................................... 11

Computer Architectures (3.3) ............................................................................................................... 12

Von Neumann Architecture (3.3.1) ................................................................................................... 12

Registers & Fetch-execute Cycle (3.3.2) ........................................................................................... 12

Other Architectures (3.3.3) ............................................................................................................... 14

Data Representation (3.4) ..................................................................................................................... 17

Floating Point Binary (3.4.1) ............................................................................................................. 17

Normalisation (3.4.2) ........................................................................................................................ 17

Data Structures and Data Manipulation (3.5) ....................................................................................... 18

Implementation of Data Structures (3.5.1) ....................................................................................... 18

Searching, Merging & Sorting (3.5.2) ................................................................................................ 19

High-level Language Programming Paradigms (3.6) ............................................................................. 22

Declarative Programing (3.6.1) ......................................................................................................... 22

Object-orientated Programming (3.6.2) ........................................................................................... 22

Programming Techniques (3.7) ............................................................................................................. 24




Standard Programming Techniques (3.7.1) ...................................................................................... 24

Methods of Defining Syntax (3.7.2) .................................................................................................. 24

Databases (3.8) ..................................................................................................................................... 25

Design and Normalisation (3.8.1) ..................................................................................................... 26

Methods & Tools for Analysing & Implementing Databases (3.8.2) ................................................. 27

Database Management System (DBMS) (3.8.3) ............................................................................... 27

Use of Structured Query Language (SQL) (3.8.4) .............................................................................. 28

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