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Operating Systems
Test 2
82
Computer Science
Undergraduate 4
11/30/2009

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Term
address space
Definition
The amount of memory available
Term
logical address space
Definition
A process's view of its own memory.
Term
disk head scheduling
Definition
trying to minimize head movement by organizing how the requests are queued.
Term
shortest seek time first
Definition
type of disk head scheduling, moves to the closest requested track from the current head position.
Term
SCAN
Definition
type of disk head scheduling. Moves the head in one direction until all requests have been serviced in that direction, then reverse the direction.
Term
C-SCAN
Definition
type of disk head scheduling. It moves the head in one direction until an edge of the disk is reached then reset to the opposite edge.
Term
LOOK
Definition
type of disk head scheduling. moves the head in one direction until no more requests exist between the current head position and the approaching edge of the disk, then the disk head is reset.
Term
virtual address space
Definition
refers to the logical(or virtual) view of how a process is stored in memory.
Term
physical address space
Definition
the address space supported by the hardware
Term
virtual address
Definition
The address in virtual memory where data/instruction is located
Term
physical address
Definition
the address in physical memory where data/instruction is actually located in hardware.
Term
linking
Definition
combines assembly code with the code necessary for all code to work (library routines). The address on references change due to the addition of the library routines.
Term
loading
Definition
moving linked code/data into physical memory, where references are translated into their physical values.
Term
relocation
Definition
moving something in memory to another place in memory
Term
dynamic relocation
Definition
process is moved to another part of memory, but it doesn't require the program to be edited since it keeps logical address rather than virtual or physical address
Term
base register
Definition
the lowest register in the memory allocated to the next frame.
Term
limit register
Definition
the register that determines the size of the physical address space for a process.
Term
external fragmentation
Definition
unused memory between units of allocation
Term
internal fragmentation
Definition
unused memory within a unit of allocation
Term
hole
Definition
empty-blocks
Term
fragmentation
Definition
holes in memory
Term
first-fit
Definition
place the program in the first space in memory where it fits
Term
best-fit
Definition
program is placed in the hole closest in size to the program
Term
worst fit
Definition
place the program in the biggest hole possible
Term
compaction
Definition
relocate programs to coalesce holes
Term
swapping
Definition
swapping refers to the removal of an entire process from memory when contention for memory becomes too high. It is a form of medium-term scheduling
Term
demand paging
Definition
load pages into memory only when a page fault occurs
Term
local page replacement
Definition
replace a page of the faulting process
Term
global replacement
Definition
possibly replace the page of another process
Term
page trace
Definition
the list of pages referenced as time goes by
Term
least recently used
Definition
replace the page that hasn't been referenced for the longest time
Term
clock
Definition
maintains a circular list of pages resident in memory, uses a used bit to track how often a page is accessed. The bit is set whenever a page is referenced. Replace pages that haven't been referenced for once complete revolution
Term
second chance
Definition
modify the clock algorithm to allow dirty pages to always survive one sweep of the clock hand. The dirty bit is when a page is written to.
Term
belady's anomaly
Definition
states that more memory does not necessarily lead to better performance.
Term
principle of locality
Definition
states that 90% of the execution of a program is sequential. Most iterative constructs consist of a relatively small number of instructions. When processing large data structures, the dominant cost is sequential processing on individual structure elements.
Term
working set
Definition
the number of pages in memory needed to make progress
Term
window size
Definition
the number of references that can be made before a certain page is replaces if not referenced.
Term
VMIN
Definition
replace a page that is not referenced in the next x accesses.
Term
page fault frequency
Definition
if the time between page faults is big, decrease the working set; if the time between page faults is small, increase the working set
Term
multiprogramming level
Definition
number of processes in the ready queue
Term
thrashing
Definition
Memory is fully allocated. There is a chain of page faults, creates a queue of processes at the paging device. CPU goes close to 0.
Term
platter
Definition
the individual circle in the disk
Term
block
Definition
a sector in software; a space in a disk
Term
sector
Definition
a space on a disk platter
Term
track
Definition
a certain circumference a head will go through in a disk platter
Term
cylinder
Definition
the same track on multiple platters
Term
read/write head
Definition
on each side of the platters, they read and write data on to them
Term
seek time
Definition
the time it takes for the heads to be moved to the appropriate track
Term
rotational latency
Definition
wait for the sector to appear under the head
Term
transfer time
Definition
read/write the sector to/from main memory
Term
contiguous allocation
Definition
array elements map to contiguous sectors on disk.
Term
linked allocation
Definition
files stored as a linked list of blocks. Directory entry is a pointer to the first and last file blocks
Term
indexed allocation
Definition
create a non-data block for each file called the index block. It has a list of pointers to file blocks. Directory entry is a pointer to the index block.
Term
device directory
Definition
a block(s) on disk containing data storing names, locations, lengths, owner, etc. of all files on disk. Data structures storing free block list. It is stored at a fixed location on disk. It has to search, create, delete, list, and backup files.
Term
virtual memory
Definition
hides all physical aspects of memory from users, makes memory logically unbounded.
Term
page frame
Definition
physical memory is split up to equal sections, each section is called a page frame
Term
page
Definition
a page frame in virtual memory. Same size as the physical page frame.
Term
page table
Definition
maps virtual pages to physical frames
Term
Translation lookaside buffer
Definition
a cache containing a ordered pairs, matches a page to its corresponding page frame
Term
page fault
Definition
a reference to a non-mapped page
Term
effective memory access time
Definition
= (memory access time*probability of a page hit)+(page fault service time * probability of a page fault).
Term
static partioning
Definition
once the size is set in the memory partitions, it stays that way
Term
dynamic partioning
Definition
allow the OS change the size of the partitions in memory
Term
placement strategy
Definition
determining where is the best place in memory to place a program
Term
replacement strategy
Definition
determining which program in memory to take out, in order to replace it with the next program
Term
load control strategy
Definition
-determining how many jobs can be in memory at one time
-determining how much of a job (how many pages of a job) can be in memory at one time
Term
paging
Definition
memory-management scheme that permits the physical address space of a process to be noncontiguous
Term
multi-level page table
Definition
add additional levels of indirection to the page table by sub-diving the page number into k parts, thus creating a hiearchy of page table.
Term
inverted page table
Definition
a hash table of virtual memory, matching a process's PID and page number to determine its frame number
Term
paging the page table
Definition
takes the page table and splits it up into a smaller page table, with each register in it pointing to another page table
Term
shared memory
Definition
is memory that may be simultaneously accessed by multiple programs with an intent to provide communication among them or avoid redundant copies
Term
disk controller
Definition
takes the demands for data and operates the disk-drive hardware to carry out the commands
Term
free list
Definition
records all the free, usable disk blocks.
Term
cylinder groups
Definition
a collection of a relatively small number of cylinders. File allocation is done within a single cylinder group whenever possible
Term
disk partition
Definition
separating the disk into different sections, so it appears to actually be multiple disks. Sometimes the OS doesn't have enough bits to access all the data in the disk
Term
direct access
Definition
accessing the data directly as the requests for it come in
Term
disk mirroring
Definition
split the disk into two equal partitions, when data is written to one, its is written on the other, so that data contents of the two partitions are identical
Term
disk striping
Definition
blocks are broken into sub blocks that are stored on separate disks. provides for higher disk bandwidth since the disks can transfer data in parallel
Term
RAID-3
Definition
has a parity disk. Uses block interleaved parity striping. Allows one to recover from the crash of any one disk
Term
RAID 5
Definition
block-wise striping. uses parity block interleaving. "horizontal" slices though the data.
Term
parity block interleaving
Definition
the parity block changes disk every time the block is complete.
Term
overlay
Definition
replacing a process's completed part of code in memory with the next part of its code it needs to run
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