Dirty bit
Encyclopedia
The computer hardware
concept of a Dirty Bit (Modify Bit) can be used when discussing the Cache
or Page replacement algorithm
s of an operating system
.
in the page is written into, indicating that the page has been modified. When we select a page for replacement, we examine its modify bit. If the bit is set, we know that the page has been modified since it was read in from the disk. In this case, we must write that page to the disk. If the dirty bit is not set, however, the page has not been modified since it was read into memory
. Therefore, if the copy of the page on the disk has not been overwitten (by some other page, for example), then we can avoid writing the memory page to the disk: it is already there.
Computer hardware
Personal computer hardware are component devices which are typically installed into or peripheral to a computer case to create a personal computer upon which system software is installed including a firmware interface such as a BIOS and an operating system which supports application software that...
concept of a Dirty Bit (Modify Bit) can be used when discussing the Cache
Cache
In computer engineering, a cache is a component that transparently stores data so that future requests for that data can be served faster. The data that is stored within a cache might be values that have been computed earlier or duplicates of original values that are stored elsewhere...
or Page replacement algorithm
Page replacement algorithm
In a computer operating system that uses paging for virtual memory management, page replacement algorithms decide which memory pages to page out when a page of memory needs to be allocated...
s of an operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
.
Page Replacement
When speaking about page replacement, each page (frame) may have a modify bit associated with it in the hardware. The dirty bit for a page is set by the hardware whenever any word or byteByte
The byte is a unit of digital information in computing and telecommunications that most commonly consists of eight bits. Historically, a byte was the number of bits used to encode a single character of text in a computer and for this reason it is the basic addressable element in many computer...
in the page is written into, indicating that the page has been modified. When we select a page for replacement, we examine its modify bit. If the bit is set, we know that the page has been modified since it was read in from the disk. In this case, we must write that page to the disk. If the dirty bit is not set, however, the page has not been modified since it was read into memory
Random-access memory
Random access memory is a form of computer data storage. Today, it takes the form of integrated circuits that allow stored data to be accessed in any order with a worst case performance of constant time. Strictly speaking, modern types of DRAM are therefore not random access, as data is read in...
. Therefore, if the copy of the page on the disk has not been overwitten (by some other page, for example), then we can avoid writing the memory page to the disk: it is already there.