A cache line is the unit of data transfer between the cache and main memory . Typically the cache line is 64 bytes. The processor will read or write an entire cache line when any location in the 64 byte region is read or written.
One additional bit is generally attached to each entry in the page table: a valid–invalid bit. When this bit is set to valid, the associated page is in the process's logical address space and is thus a legal (or valid) page. When the bit is set to invalid, the page is not in the process's logical address space.
Calculations
- Use the following information if you are told the cache is 4 MB or something similar.
- 1 KB = 210 bytes (1024 bytes)
- 1 MB = 210 KB (1024 bytes) = 210 * 210 bytes = 220 bytes (1048576 bytes)
- Block = log2 (BytesPerLine) = number of bits needed to represent the maximum number (remember to start using a '0' offset).
cache block - The basic unit for cache storage. May contain multiple bytes/words of data. cache set - A “row” in the cache. The number of blocks per set is deter- mined by the layout of the cache (e.g. direct mapped, set-associative, or fully associative). tag - A unique identifier for a group of data.
Cache Addressing. A cache in the primary storage hierarchy contains cache lines that are grouped into sets. If each set contains k lines then we say that the cache is k-way associative. An offset part identifies a particular location within a cache line. A set part identifies the set that contains the requested data.
Cache entriesWhen a cache line is copied from memory into the cache, a cache entry is created. The cache entry will include the copied data as well as the requested memory location (called a tag). When the processor needs to read or write a location in memory, it first checks for a corresponding entry in the cache.
A page fault occurs when a program attempts to access a block of memory that is not stored in the physical memory, or RAM. The fault notifies the operating system that it must locate the data in virtual memory, then transfer it from the storage device, such as an HDD or SSD, to the system RAM.
How does Caching work? The data in a cache is generally stored in fast access hardware such as RAM (Random-access memory) and may also be used in correlation with a software component. A cache's primary purpose is to increase data retrieval performance by reducing the need to access the underlying slower storage layer.
'Dirty' memory is memory representing data on disk that has been changed but has not yet been written out to disk. Regions of memory mapped files that have been updated but not written out to disk yet.
In computer operating systems, paging is a memory management scheme by which a computer stores and retrieves data from secondary storage for use in main memory. In this scheme, the operating system retrieves data from secondary storage in same-size blocks called pages.
What purpose does the "modified" bit serve in a demand paging system? Answer: If you require a page to be swapped out and must allow for the possibility of it being swapped back in, you need to make sure that the page has the latest changes when it is again swapped in.
A page fault (sometimes called #PF, PF or hard fault) is a type of exception raised by computer hardware when a running program accesses a memory page that is not currently mapped by the memory management unit (MMU) into the virtual address space of a process.
L1 is "level-1" cache memory, usually built onto the microprocessor chip itself. L2 (that is, level-2) cache memory is on a separate chip (possibly on an expansion card) that can be accessed more quickly than the larger "main" memory. A popular L2 cache memory size is 1,024 kilobytes (one megabyte).
Write Back is also known as Write Deferred. Dirty Bit : Each Block in the cache needs a bit to indicate if the data present in the cache was modified(Dirty) or not modified(Clean). If Cache fails or if System fails or power outage the modified data will be lost.
In computer operating systems, demand paging (as opposed to anticipatory paging) is a method of virtual memory management. It follows that a process begins execution with none of its pages in physical memory, and many page faults will occur until most of a process's working set of pages are located in physical memory.
A virtual address is a binary number in virtual memory that enables a process to use a location in primary storage (main memory) independently of other processes and to use more space than actually exists in primary storage by temporarily relegating some contents to a hard disk or internal flash drive.
Virtual memory is a section of volatile memory created temporarily on the storage drive. It is created when a computer is running many processes at once and RAM is running low.
There are two general strategies for dealing with writes to a cache: Write-through - all data written to the cache is also written to memory at the same time. Write-back - when data is written to a cache, a dirty bit is set for the affected block. The modified block is written to memory only when the block is replaced.
When a computer processor does not gets data item that it requires in cache, then the problem is known as cache miss. The problem of cache miss is the state when the data requested for processing cannot be found in the system specifically in the cache memory.
Answer. Cache in the primary storage stack consists cache lines that are grouped into sets. If each set consists of k lines then that cache is k-way associative cache. Data request with address specifying the location of requested data.
The dirty bit is set to 1 when the processor writes data to this memory. The bit also indicates the associated block of memory that has been modified and has not been saved to storage yet. Hence, if a piece of data in cache needs to be written back to cache the dirty bit has to be set 0. Dirty bit=0 is the answer.
read-through/write-through (rt/wt): this is where the application treats cache as the main data store and reads data from it and writes data to it. the cache is responsible for reading and writing this data to the database, thereby relieving the application of this responsibility.
Answer. 'Least recently used' Cache Eviction techniques consider Locality.
