System Software Questions Flashcards
Explain how the processes are affected when the following events take place.
(i) The running process needs to read a file from a disk
-Running process is halted
-Process moves to blocked state
State the conditions that are necessary for a process to move from the ready to the running state.
-Current process no longer running // processor is available
-Process was at the head of the ready queue // process has highest priority
State the conditions that are necessary for a process to move from the blocked to the ready state.
The only Required resource becomes available // event is complete
Explain how the processes are affected when the following events take place.
(ii) The running process uses up its time slice
Running process is halted // another process has use of the processor
-Process moves to ready state
-Until next time slice allocated
Give three reasons why process scheduling is needed.
-to allow multiprogramming
-to give each process a fair share of the CPU time
-to keep the CPU busy all the time
-to allow highest priority jobs to be executed first
Identify and describe the Ready state.
-The process is not being executed
-The process is in the queue waiting for the processor’s attention / time slice
Identify and describe the Running state.
The process is being executed by the processor
The process is currently using its allocated processor time / time slice
Identify and describe the Blocked state.
-The process is waiting for an event
- SO it cannot be executed at the moment
…@.g. input/output
Describe how an operating system can maximise the use of resources.
“Primary Memory”
-Moving frequently accessed instructions to cache for faster recall
-Making use of virtual memory with paging or segmentation to swap memory to and from a disk
-Removing unused items/tasks from RAM by marking a partition as available as soon as the process using it has terminated
Explain what is meant by the following terms.
-Page
-Page Frame
-Page Table
Page:
Virtual Memory is divided into blocks of a fixed size
Page frame:
-the main memory is divided into page frames of the same size as
page
Page table:
the Page (Map) table shows the mapping of pages to page frames
Explain why an operating system needs to use scheduling algorithms.
-To allow multiprogramming.
-To give each process a fair share of CPU time.
-To ensure higher priority tasks are executed first.
-To keep CPU busy at all times.
State what is meant by an interrupt.
A signal from a software source or hardware device seeking the attention of the processer
Explain how paging is used to manage virtual memory.
-RAM is divided into blocks of fixed size called pages.
-Virtual Memory is divided into blocks of fixed sizes called frames
-Page table is set-up to Map physical address to logical address.
Describe what is meant by disk Thrashing
Occurs when CPU spends more time on swapping data between Disk and Ram instead of performing any useful processing
Pages are required back in RAM as soon as they are moved to disk
Describe what is meant my Virtual Memory
Secondary storage is used to extend the RAM
So that cpu can access more memory space than available ram.
Data is swaped between Ram and Disk.
Describe how an operating system can maximise the use of resources.
“Disk”
Disk Caching: A disc cache holds data that is frequently transferend to and from disk
Compression Utility: Reducing the size of a file stored on disk
Defragmentation Utility
What is the difference between a Program and a Process?
Program is written code (static)
Process is executing code (dynamic)
Explain why a process cannot be moved from blocked state to running state [ 3 ]
In blocked state process waits for I/O opperation, when its done, process is put in ready state and moved to ready queue,
The OS decides which process to allocate proccessor from ready queue.
Explain the role of the high-level scheduler in a multiprogramming operating system.
Decides which process are to be loaded from backing store (disk) to into memory.
Explain why a process cannot move directly from the ready state to the blocked state. [ 3 ]
To be in blocked state process must initiate some I/O opperation
To initiate oppration, the process must b e executing.
So the process must be in running state.
Explain how the contents of the keyword and symbol tables are used to translate the source code program.
Keywords are looked up in the keyword table
Identifiers are looked up in the symbol table
Keywords are represented by tokens.
Identifiers are converted to addresses.
State one additional task completed at the lexical analysis stage that does not involve the use of a keyword or a symbol table.
White space is removed
Comments are removed
The final stage of compilation can be code optimisation.
Explain why code is optimised.
Code will have fewer instructions.
Code reorganized to make it more efficient.
Program will require less space.
Program will complete task in a shorter time.
Explain how RPN is used by an Interpreter to evaluate expressions.
Expressions are always evaluated from left to right
If element is a number, push it onto the stack
If element is an operator, then POP the first two numbers on stack, perform the operation and push the result
Explain the advantages of using RPN for the evaluation of expressions.
No need for rules of precedence
No need for Brackets.
The evaluation is always left to right
Identify with reasons , a data structure that could be
used to evaluate an expression in RPN ?
Structure: Stack
Reason: Operands are popped from the stack in the Reverse order to how they were pushed.
Explain how an in interpreter executes a program
without producng a complete translated version of it.
The interpreter examines the source code, one statement at a time.
It checks each statement for errors.
Executes the statement if no error is found.
If an error is found, this is reported and the interpreter Halts.
