1
Q
How an OS can maximise the use of resources
A
- Operating task
- maximise the use of computer resources
- Hide complexities of hardware from the users
2
Q
Why OS needs scheduling algorithm
A
- allow multiprogramming
- Ensure fair use of processor
- Ensure fair use of memory
- Ensure fair use of peripheral
- Ensure higher priority tasks are executed sooner
- Ensure all process have opportunity to finish
- Keep CPU busy all the time
2
Q
The ways in which the user interface hides the complexities of the hardware from the user
A
- Using GUI interface rather than CLI
- Using device drivers which simplifies the complexity of hardware interface
- Simplifying the saving and retrieving of data from memory and storage devices
- Carrying out background utilities, such as virus scanning
3
Q
Multitasking
A
- Allows computer to carry out more than one process at a time
- Process: A program that has started to be executed.
- Ensure best use of computer resources by monitoring the state of each process
- Processes will share common hardware resources
- Schedule is used to decide which processes should be carried out
4
Q
Role of high-level scheduler in multiprogramming OS
A
Decides which process are to be loaded from backing store into ready queue
5
Q
Role of low-level scheduler in multiprogramming OS
A
- decides which process in ready state
- Should be put into processor
- Based on priority
- Invoked by interrupt
6
Q
Ready state
A
- process is not being executed
- Process in the queue
- Process is waiting for its processor time
7
Q
Running state
A
- process is being executed by the processor
- Process is currently using its allocated processor time
8
Q
Blocked state
A
- process is waiting for an event
- e.g input/output
- So it cannot be executed at the moment
9
Q
Conditions to move from ready to running state
A
- Process is capable of using processor
- processor is available
- Process is the head of the ready queue
10
Q
Conditions to move from running to blocked state
A
- process is running when It needs to perform I/O operations
- Placed in blocked state until I/O operation completed
10
Q
Conditions to move from blocked to ready state
A
- the only
- Required resources is available
- Process wait I/O operation to be completed
10
Q
Round robin (Scheduling Routines)
A
- Each process is assigned fixed CPU time
- Starvation doesn’t occur
10
Q
Conditions to move from running to ready state
A
- The process in processor has finished its time slice
- Process can no longer use processor
- Although it is capable for further processing
11
Q
Why cannot move from blocked to running state
A
- when a process finished I/O operation
- It needs to go ready state
- OS decides which process to allocate to processor from the ready queue
11
Q
Shortest job first (Scheduling Routines)
A
- Process is executed in the ascending order of the amount of time required.
- More processes can be executed in short amount of time
- Non-preemptive scheduling algorithm
12
Q
First come first serve (Scheduling Routines)
A
- No complex logic, each process queued as it is received and executed one by one
- Starvation doesn’t occur, all process get chance to run.
13
Q
Shortest remaining time (Scheduling Routines)
A
- Same as shortest job first
- Preemptive scheduling algorithm
14
Q
Non-pre-emptive
A
- Once resources is allocated to a process, the process
retains them until it has completed its burst time or
the process moved to waiting state
Burst time: the time when a process has control of
the CPU
- Cannot be interrupted, must first finish or switch to waiting state
-More rigid form of scheduling - If a process with a long burst time is running in the
CPU, there is a risk that another process with a
shorter burst time may be starved of resources
14
Q
Pre-emptive
A
- Resources allocated to process for a limited time
- The process can be interrupted while it is running
- More flexible form of scheduling
- High priority processes arriving the ready queue
frequently can mean there is a risk that low priority
processes may be starved of resources
15
Q
How the kernel of the OS acts as an interrupt handler & how interrupt handling is used to manage low
level scheduling
A
- When interrupt is received, other interrupts are then disabled so that the process that dealt with the
interrupt cannot itself be interrupted - State of current process is saved in kernel stack
- Identify source of interrupt
- Priority of interrupt is checked
- kernel will use interrupt dispatch table (IDT)
- IDT links the device description with the appropriate interrupt routine
- Once completed, the state of current interrupted process is restored using values saved on kernel stack
- The process is continued
- The interrupts is restored so any further interrupts can be dealt with
16
Q
Types of interrupt signals
A
- Device interrupt (e.g printer out of paper, device not present)
- Exceptions (e.g instruction fault such as division by zero, undefined op code)
- Software interrupt (e.g process requesting a resource such as a disk drive)
17
Q
Virtual memory
A
- Disk used to extend RAM
- So CPU can access more memory space than available RAM
- Only part of data in use needs to be in RAM
- Data is swapped between RAM and disk
18
Q
Terms for paging
A
- page : VM is divided into blocks of fixed size
- Page frame : main memory/RAM is divided into page frames of the same size as a page
- Page table : a table that show mapping of pages on page frames, translate logical to physical address
19
Paging vs segmentation
- Paging divides memory into fixed size block, segmentation various size block
- Paging has faster access time than segmentation
20
Paging
- Faster access time
- Invisible to user
- Consists of static linking & dynamic loading
- Procedures and associated data cannot be
separated when using paging
20
Segmentation
-Slower access time
- Visible to user
- Consists of dynamic linking & dynamic loading
- Procedures and associated data can be separated
when using segmentation
21
How paging is used to manage VM
- pages are required back to RAM as soon as they are moved to disk
- There is continuous swapping (on the same page)
- No useful processing happen
- Because pages that are in RAM and on disk are inter-dependent
- Nearly all the processing time used for page swapping
22
Disk thrashing
- pages are required back to RAM as soon as they are moved to disk
- There is continuous swapping (on the same page)
- No useful processing happen
- Because pages that are in RAM and on disk are inter-dependent
- Nearly all the processing time used for page swapping
23
how an interpreter can execute programs without producing a translated version ?
- an interpreter examines source code one statement at a time
- It checks each statement for error
- If no error is found the statement is executed
- If an error is found this is reported and the interpreter halt
- Interpretation is repeated for every iteration in same section of code
24
Why code optimisation
- code has fewer instruction
- Less memory space required
- Shorter program execution time
25
Interrupt
- a signal of software source or hardware device seeking attention of processor
26
Phrases of compiler
1) lexical analysis (convert source code) into tokens)
2) Syntax analysis
3) Code generation - produce object code
4) Code optimisation - improve efficiency of object code (minimise execution time & memory
requirement)
27
How keyword table and symbol table are used to translate the source code program ?
- keyword looked up in keyword table
- Represented by tokens
- Identifier looked up in symbol table
- Convert to location
- Used to create a sequence of tokens (for the program)
27
Lexical analysis stage:
1) Keyword table
- the reserved word used
- The matching token
- The operators used
2) Symbol table
- identifier name used
- The data type
- Value of constant
- Role. e.g contant & variable
3) removal of comments
4) removal of white spaces
5) removal of redundant characters
6) identification of error
28
Syntax analysis (make use of tree data structure)
- Parsing
- Check the table of tokens to ensure grammar of language is obeyed
- producing error report
29
Why RPN ?
- no need bracket
- No need rules of precedence for operators
- Rules of precedence means operators have different priorities
- Evaluation can be read from left to right
- No need to backtrack
