Mid-Term Flashcards
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
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
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
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.
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).
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]
Data structure where OS keeps administrative & status information about a process:
PCB
Number of pages of a process that may appear in a frame of memory at a time
1
Part of the OS always resident in memory
kernel
What you get when you cannot find a page table entry for a page
page fault
An executable image of a program suitable for loading into memory to be run is a
load module
The time between a user submitting a request and the start of some response is
response time
Special associative cache memory used to speed up page table look-ups
TLB (translate lookaside buffer)
Fixed partition allocation schemes suffer from ______ fragmentation
internal
The only tool that comes standard with Windows for pure plain text editing
notepad
A synchronization technique that works in all combinations of single/multiple CPU with single/multiple memories
message passing
The set of pages of a process present in real memory is the ______ set.
resident
Part of computer that mediates movement of data between CPU and other system components like memory and peripherals:
bus
To create a new process in any Unix style system, first use a _____ system call
fork
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:
0
Peripheral devices may be operated using a polling technique, or using _______
interrupts
Suppose a system uses 512-byte pages: how many address bits are needed for the offset onto a page (hint: 1024 = 2^10):
9
An area of code that must not be simultaneously accessed by more than one process is a _______
critical section
A hardware instruction to support mutual exclusion ______
one of tx, exch
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?
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
- Fetch word from memory pointed at by PC
- Decode word as instruction
- Fetch any operand(s) instruction needs
- Execute the instruction
- Update status flags as consequence (e.g., NZVC)
- Store any result(s)
Dealing with errors
trap. Instant, must be dealt with as they occur
OS
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
