Class : BsCS 4th -B
Subject : Operating System
Muhammad Shakeeb ( IS/17122-Aut -FO7 M )Syed Zeeshan Nasir (
IS/17133-Aut F 07 M )Faisal Muhammad ( IS/17122-Aut F07 M)Group
Members :
Unix Memory Management
1
Handling Memory
2
Process A's virtual address space Physical memory Process B's
virtual address space
3
Memory ManagementThe initial memory management schemes were
constrained in size by the relatively small memory resources of
UNIX was developed on the PDP machines Pre 3BSD system use swapping
exclusively to handle memory contention among processes If there is
too much contention, processes are swapped out until enough memory
is availableAllocation of both main memory and swap space is done
first-fit
4
Paging Berkeley UNIX systems depend primarily on paging for
memory-contention management, and depend only secondarily on
swapping Demand paging When a process needs a page and the page is
not there, a page fault tot he kernel occurs, a frame of main
memory is allocated, and the proper disk page is read into the
frame A page daemon process uses a modified second-chance
page-replacement algorithm to keep enough free frames to support
the executing processes If the scheduler decides that the paging
system is overloaded, processes will be swapped out whole until the
overload is relieved
5
PagingPage table entry
Disk block descriptors
Page frame data table
Swap use table
6
Page Table Entry and Disk Block Descriptor
Disk Block DescriptorPage Table Entry
RegionPage Table EntryPage Table Disk Block DescriptorSwap
Device Block NumberType
Page AddressAgeCp/WrtModRefValProt
Contains the physical address of page and the following
bits:Valid: whether the page content legalReference: whether the
page is referenced recentlyModify: whether the page content is
modifiedcopy on write: kernel must create a new copy when a process
modifies its content (required for fork)Age: Age of the
pageProtection: Read/ write permissionPage Table Entry
Disk Block DescriptorSwap Device number as there may be several
swap devicesBlock number that contains pageSwap Device Block
NumberType
Page Frame Data Table EntryPage StateReference countLogical
DeviceBlock NumberPfdata Pointer
Reference CountPage/Storage Unit Number
Swap use Table entry
10
11 Page Frame Data Table EntryPage State : Indicate whether this
frame is available or has an associated age. In the latter case,
the status of the page is specified on swap device, in executable
fill, or DMA in progress.Reference count: Number of process that
referncesthe page.Block Number: Bock location of the page copy on
the logical devices. Logical Devices: Logical devices that contains
a copy of the page.
Swap use table EntryReferences count: Number of page table
entries that point to page on the swap device.Page/storage unit
number: Page identifier on storage unit.
Replacement policy
12
