Mid-Term Flashcards by Daniel Anthes

Traps and interrupts are very similar; name one thing that makes interrupts different from traps in the way they are handled.

anything from this list:

interrupts have priorities, traps do not
traps are processed immediately when they occur, interrupts are deferred until the current instruction finishes
an interrupted process often resumes, but it is rare for a process that experienced a trap to resumeDefinition

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Arrange in order from longest latency to shortest (i.e., slowest access time to fastest), making sure to indicate which is fastest:.
optical disk, register,RAM/main memory, magnetic disk, cache

from slowest (longest latency) to fastest (shortest latency):
optical disk, magnetic disk, RAM/main memory, cache, register

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Having detected deadlock, briefly describe two strategies that can be used to correct the problem.

two from:

delete all deadlocked processes
delete deadlocked processes one by one until deadlock is gone
roll deadlocked processes back to a previous checkpoint and allow them to move forward again from that point

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Briefly explain what makes a master/slave multiprocessor different from asymmetric multiprocessor

In master/slave, only the master can perform system call code: every process needing to perform a system call, no matter what cpu it runs on, must wait for the master cpu to perform the system call. Since this is a simple extension of normal single-processor systems, master/slave systems are easier to build. In a symmetric multiprocessor, any processor can perform a system call, so there is no bottleneck at a master cpu. Synchronization across processors is much harder with symmetric multiprocessors.

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Handling a page fault requires that the missing page to be located on disk and loaded into memory. The wait for this operation is overwhelmingly dominated by the time the disk takes to respond. Briefly describe a situation in which the waiting time (time from when page faultoccurs to when the page is available in memory for use) may be twice as long as what is
described here.

The waiting time for resolution of a page fault is primarily the time it takes to get the page from the disk (rotational and seek delays drive this delay). The delay may be twice as long if no memory frames were available to hold the newly sought page and no page in any frame is unmodified, meaning that a page must first be written out to disk before a frame is available to hold the new page. Recall that writing to
disks is slower than reading (but still overwhelmingly dominated by rotational and seek delays).

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What does the term cache-hit ratio tell you?

The cache hit ratio is the ratio of the number of times a memory location is referenced and is successfully found to be in the cache (a cache hit) to the total number of memory references. [remember: a ratio is a comparison of something to something else]

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Data structure where OS keeps administrative & status information about a process:

PCB

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Number of pages of a process that may appear in a frame of memory at a time

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Part of the OS always resident in memory

kernel

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What you get when you cannot find a page table entry for a page

page fault

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An executable image of a program suitable for loading into memory to be run is a

load module

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The time between a user submitting a request and the start of some response is

response time

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Special associative cache memory used to speed up page table look-ups

TLB (translate lookaside buffer)

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Fixed partition allocation schemes suffer from ______ fragmentation

internal

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The only tool that comes standard with Windows for pure plain text editing

notepad

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A synchronization technique that works in all combinations of single/multiple CPU with single/multiple memories

message passing

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The set of pages of a process present in real memory is the ______ set.

resident

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Part of computer that mediates movement of data between CPU and other system components like memory and peripherals:

bus

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To create a new process in any Unix style system, first use a _____ system call

fork

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If, on a virtual Windows machine, a user creates and saves 10 new files, how many of these new files are added as new files to what already appears on the hosting system:

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Peripheral devices may be operated using a polling technique, or using _______

interrupts

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Suppose a system uses 512-byte pages: how many address bits are needed for the offset onto a page (hint: 1024 = 2^10):

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An area of code that must not be simultaneously accessed by more than one process is a _______

critical section

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A hardware instruction to support mutual exclusion ______

one of tx, exch

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Adding more frames of memory always improves page fault behaviour (i.e., reduces number of page faults).

false

An OS only works when directly controlling real hardware

false

Semaphores fail to work as asynchronization mechanism on systemswith multiple CPUs and one shared memory.

false

Most practical OS simply ignore the possibility of deadlock.

true

Generally,the faster the memory(lower latency), the more expensive it is.

true

All resources managed by a computer are hardware resources.

false

The only resource given up by a blocked process is the CPU.

true

The use of unequal-sized fixed partitions fails to resolve the fragmentation problem

true

Every UNIX/linux process (except one) is a child of some parent process.

true

Swapping a process out of memory moves part or all of the process.

false

Round robin scheduling degrades to FIFO if the timeslice becomes too long.

true

Paging space in a virtual memory system should always be set to the same size as the amount of physical memory in the system.

false

Cache coherency on multi-processor systems sharing a single memory is much harder to maintain than on a uni-processor.

true

In providing a mutual exclusion mechanism, weassume that fast runningprocesses do not cause problems.

false

Using virtual memory allows us to put only part of a program into memory at a time.

true

Dynamic address translation does not work on systems with a cache memory.

false

A runnable process will never be swapped out while it remains runnable.

false

Disabling mutual exclusion in an OS is a normal and widely used way to prevent deadlock.

false

Virtual memorysystems should not be used for real-time applications.

true

Processes than can no longer make use of the CPU become blocked until they can again make use of the CPU.

true

Semaphores are a software mechanism designed to provide mutual exclusion. Name and briefly describe the three operations that can be performed on a semaphore.

Three operations on semaphores: (1) initialize: initialize the semaphore counter to some integer value (2) wait: decrement the semaphore counter by one: if it is now < 0, suspend the process and place it in this semaphore's queue. (3) signal: increment the semaphore counter by one: if it is now <= 0, select some process from the semaphore's queue to be resumed.

You note that a computer system is not using its CPU very much, so you arrange to run more processes on it, hoping to keep the CPU busier. As you increase the number of processes on this system you observe that performance becomes drastically worse after a certain point: all processes start taking much longer to run. Up to this point, you discovered that the CPU was,at most, busy 30% of the time running your processes.After this point, that number becomes 0.2%. What do you suspect the problem to be? A manager suggests to you that you should get a faster CPU for the computer: would this help? briefly explain why or why not.

The problem is that the system has begun thrashing: spending more time resolving page faults than advancing processes toward completion. A faster CPU will accomplish nothing: the current, presumably slower, CPU wasn't fully occupied. The problem is related to availability of real memory, not lack of CPU speed.

FIFO pre-emptive favourable unfavourable

FIFO is not pre-emptive, is favourable to long running jobs, is unfavourable for short jobs

Round Robin Pre-Emptive? Favourable? Unfavourable?

Data <> Information

Computers work on, and networks move data | Humans create information from the data they see

IT then is the study of a dynamic system consisting of: | Information and infrastructure

Hardware, Software, Firmware, and People.

Information Technology (IT)

includes hardware, software, | firmware, people and data / information / knowledge.

what is a computer

Today the term more typically refers to an electronic device for storing and processing data, typically in binary form, according to instructions given to it in a variable program

Common Computer features

``` CPU Bus(es) Memory Data store I/O Capabilities: network, user input, user output, .... Power source ```

Memory

Place to hold a value (data or instruction) while it is worked on or until it is needed to be worked on

Memory Address

Each place in memory has an address which uniquely identifies it

How high are the typically cache hit ratios

95% or better

Input devices types of interrupt

Polled | Interrupt: these have different prorities

DMA

Direct memory access, smart devices are capable of interacting with the BUS, only for short amount of time

CPU Life

1. Fetch word from memory pointed at by PC 2. Decode word as instruction 3. Fetch any operand(s) instruction needs 4. Execute the instruction 5. Update status flags as consequence (e.g., NZVC) 6. Store any result(s)

Dealing with errors

trap. Instant, must be dealt with as they occur

Manages resources of computer to provide an enviroment for processes to run

OS Needs

OS must be resident at all times, the kernel | -Load into memoery, interrupt and trap handlers

Simple OS

Runes one program at a time

Multi-tasking OS

Illusion of running mroe than 1 task at a time, - memory management - schedulig, - Communication: task may interact

Multi-User and Multi-Tasking

Single-user, multi-tasking | Multi-user, single-tasking

Dynamic partitioning

First first Best fit Next fit Worst fit

OS Non-physical resources

queues, files and caches

Process needs

At least one CPU to execute its instructions Memory to hold instructions and data (operands) Access to input/output devices

Processes and memory

I Space for instrctions, and d space for operands (data)

Dynamic Partitioning

``` Exact fit External fragmentation Compaction Placement Algorithm (Better way, virtual addresses) ```

Virtual addressing

Frames into pages | -then convert virtual to real addresses as needed

Page Fault

Asking for page not in memory

Virtual memory

Real memory + space on disk to hold pages

Thrashing

When the computer spends all of it's time swapping out memory processes

Paging

Break up memory into small partitions called frames (2 to 4kb in size). Each frame holds exactly 1 page

Dynamic Address Translation (DAT)

Basic DAT uses special hardware and some extra registers: ``` BR: Base Register: address where module really starts (real or physical address of module start) LR: Limit Register : address of end of module ```

What is word

Fixed sized piece of data handled as a unit by the instruction set or hardware of the processor (typically 32 or 64 bit)

Large Page tables

Translation look aside buffer (TLB). Uses associative memory

OS Page fault

OS must first look on the PTE (recently used pages on CPU) then on the TLB (transcation look aside buffer) before it can go searching on the disk

Virtual memory

total memory region consisting of entierty of real memory + some amount of secondary memory (recommended disk space at least size of ram

Resident memory set

the set of pages or segments, of a process currently in real memory

Page fault

the consequence of accessing a page not resident

High page fault rate

process blocked waiting for pages idle time cpu underutilized

Denning rule

Keep use of paging mechanism at 50%, which point CPU utilization is highest

OS knowledge of process

track progress | allocate allowed resources

OS needs to know

``` ‘Who’ the process is Where did it come from? Who does it belong to? What is its current status? How much of how many different resource types is it using? --Does it want --Is it allowed to have ```

Process is a program

CPU, memory, possiblty other resources

Mid-Term Flashcards

(87 cards)