Full Answer
How many entries are there in page table?
The page table needs one entry per page. Assuming a 4GB (2^32 byte) virtual and physical address space and a page size of 4kB (2^12 bytes), we see that the the 2^32 byte address space must be split into 2^20 pages. This means the page table must have 2^20 entries.
How is page table entry calculated?
Number of bits in a page = Logical Address - Page Offset = 24 - 12 = 12 bits. Number of pages = 2 ^ 12 = 2 X 2 X 10 ^ 10 = 4 KB. Let's say, Page table entry = 1 Byte. Therefore, the size of the page table = 4 KB X 1 Byte = 4 KB.
What is in a page table entry?
Page table entry. Each page table entry (PTE) holds the mapping between a virtual address of a page and the address of a physical frame. There is also auxiliary information about the page such as a present bit, a dirty or modified bit, address space or process ID information, amongst others.
How many bits are in a page table entry?
So, the entry refers to page table entry (PTE). The data stored in each entry is the physical memory address (PFN). The underlying assumption here is the physical memory also uses a 32-bit address space. Therefore, PTE will be at least 4 bytes (4 * 8 = 32 bits).
What is a page table entry size?
Page Directory and Page Table entries are each 4 bytes long, so the Page Directory and Page Tables are a maximum of 4 Kbytes, which also happens to be the Page Frame size. The high-order 20 bits point to the base of a Page Table or Page Frame. Bits 9 to 11 are available to the operating system for its own use.
How many entries are there in the first level page table?
256 entriesUsing the subdivision above, the 1st level page table contains 256 entries, each entry pointing to a 2nd level page table. A 2nd level page table contains 16 entries, each entry pointing to a 3rd page table. A 3rd page table contains 256 entries, each entry pointing to a page.
What is stored in page table?
The page table stores all the Frame numbers corresponding to the page numbers of the page table. In other words, the page table maps the page number to its actual location (frame number) in the memory. In the image given below shows, how the required word of the frame is accessed with the help of offset.
What is a page entry?
As the name suggests, the entry page is the point where Internet users arrive on your website or mobile site – it is the first page someone views on your site. The entry page is not necessarily your home page.
Where are page table entries stored?
Typically, page tables are said to be stored in the kernel-owned physical memory.
How many bytes is a page?
How many bytes for anything?Information objectHow many bytes?A typical text word10 to 20 bytesA line of text70 bytesTwo or three paragraphs of text1 kilobyte (KB)A page of plain text2 KB25 more rows
Is page offset 12 bits?
Question: Why is the offset 12 bits? Answer: We define the size of a page to be 4096 = 212 bytes. We want to be able to index into any byte of a destination physical page. So, we need 12 bits to represent every possible offset.
Where is the page table?
kernel spaceThe page table of the process is held in the kernel space. The kernel may have several page tables in RAM, but only one is the active page table. In x86 CPUs, it's the page table pointed by register CR3.
How is page table offset calculated?
2. offset = A mod page_sizethis is the distance from the beginning of the page.e.g. address in the process, A = 10,000.page size = 4k.page offset = 10000 mod 4k = 10,000 mod 4096 = 1908.this calculation is done quickly on the computer since the page size is power of 2, e.g., 4k = 2^12.More items...
How is paging calculated?
For Main Memory-Physical Address Space = Size of main memory.Size of main memory = Total number of frames x Page size.Frame size = Page size.If number of frames in main memory = 2X, then number of bits in frame number = X bits.If Page size = 2X Bytes, then number of bits in page offset = X bits.More items...
How is page file calculated?
Your paging file size should be 1.5 times your physical memory at a minimum and up to 4 times the physical memory at most to ensure system stability. Your minimum paging file size can be calculated by 8 GB x 1.5, and your maximum paging file size by 8 GB x 4.
How are page frames calculated?
The size of a frame is the same as that of a page, so the size of a frame is 1024 bytes (210 bytes). If the physical memory is 32MB (225 bytes), the number of frames is 225 / 210 = 215 and this is also the maximum number of pages that can be present in memory at the same time.
What happens if you put a page in the cache?
Now if you try to put that page in the cache, that cache will show the old information.
What does "modified" mean in a page?
Modified means sometimes you might try to write something on to the page. If a page is modified, then whenever you should replace that page with some other page, then the modified information should be kept on the hard disk or it has to be written back or it has to be saved back.
What is frame number?
Frame Number – It gives the frame number in which the current page you are looking for is present. The number of bits required depends on the number of frames.Frame bit is also known as address translation bit. Number of bits for frame = Size of physical memory/frame size.
What is page table?
The page table is a key component of virtual address translation which is necessary to access data in memory.
How many rows are in an inverted page table?
At its core is a fixed-size table with the number of rows equal to the number of frames in memory. If there are 4,000 frames, the inverted page table has 4,000 rows. For each row there is an entry for the virtual page number (VPN), the physical page number (not the physical address), some other data and a means for creating a collision chain, as we will see later.
How to cause page fault?
Some MMUs trigger a page fault for other reasons, whether or not the page is currently resident in physical memory and mapped into the virtual address space of a process: 1 Attempting to write when the page table has the read-only bit set causes a page fault. This is a normal part of many operating system's implementation of copy-on-write; it may also occur when a write is done to a location from which the process is allowed to read but to which it is not allowed to write, in which case a signal is delivered to the process. 2 Attempting to execute code when the page table has the NX bit (no-execute bit) set in the page table causes a page fault. This can be used by an operating system, in combination with the read-only bit, to provide a Write XOR Execute feature that stops some kinds of exploits.
Why do we use nested page tables?
Nested page tables can be implemented to increase the performance of hardware virtualization. By providing hardware support for page-table virtualization, the need to emulate is greatly reduced. For x86 virtualization the current choices are Intel 's Extended Page Table feature and AMD 's Rapid Virtualization Indexing feature.
Why is a per-process hash table used?
A per-process identifier is used to disambiguate the pages of different processes from each other. It is somewhat slow to remove the page table entries of a given process; the OS may avoid reusing per-process identifier values to delay facing this. Alternatively, per-process hash tables may be used, but they are impractical because of memory fragmentation , which requires the tables to be pre-allocated.
How many pages are virtual addresses?
A virtual address in this schema could be split into two, the first half being a virtual page number and the second half being the offset in that page.
Why do linear page tables stay in physical memory?
However, part of this linear page table structure must always stay resident in physical memory in order to prevent circular page faults and look for a key part of the page table that is not present in the page table.
How many pages does a page table need?
The page table needs one entry per page . Assuming a 4GB (2^32 byte) virtual and physical address space and a page size of 4kB (2^12 bytes), we see that the the 2^32 byte address space must be split into 2^20 pages.
How does the size of a page table change?
The size of the page table (hierarchical or otherwise) grows with the size of the virtual address space. If we have a large virtual address space (such as in a 64 bit architecture), the page table will become huge. Hierarchical paging will allow us to keep most of that out of main memory, but would require a 6-level hierarchy (why?). That means to look up an address you need to read at least 6 frame numbers, which is expensive.
How to look up an address in paging?
To look up an address in a hierarchical paging scheme, we use the first 10 bits to index into the top level page table. This tells us which POPT to use. The next 10 bits tell us which entry on that POPT points to the data page we are interested in. The remaining 12 bits gives us the offset into that page.
How many bits are in a frame number?
Since there are 2^32 physical addresses divided into frames of size 2^12 (frame size = page size), we see that there are 2^20 frames, so we need 20 bit s to store the frame number.
Why do we need a dirty bit in a page table?
We also need some extra bits: since pages may be swapped out to disk, we need a valid bit in the page table entry to indicate whether the page is valid (i.e. currently present in memory) or not. We can also store a dirty bit so that we can avoid writing pages back to disk if we need to swap them out.
Can a hash table map page numbers to frame numbers?
Instead of making a very large sparse array, we can instead use a hash table with one entry per frame. The hash table maps page numbers to frame numbers.
Can you make the page table the same size as the address space?
Just as with the address space, we can solve these problems by paging the page table itself. For convenience, we can make the pages of the page table (POPTs) the same size as the pages of the process's address space. This allows us to use the same set of frames to store either process data or POPTs.
How big is a page table?
The size of the page table depends upon the number of entries in the table and the bytes stored in one entry.
Why must the page table be in the main memory?
The page table must also be present in the main memory all the time because it has the entry for all the pages.
Is the page table the same size as the frame?
However, in some cases, the page table size and the frame size might not be same. In those cases, the page table is considered as the collection of frames and will be stored in the different frames.
Overview
Page table data
The page table is an array of page table entries.
Each page table entry (PTE) holds the mapping between a virtual address of a page and the address of a physical frame. There is also auxiliary information about the page such as a present bit, a dirty or modified bit, address space or process ID information, amongst others.
Secondary storage, such as a hard disk drive, can be used to augment physical memory. Pages …
Role of the page table
In operating systems that use virtual memory, every process is given the impression that it is working with large, contiguous sections of memory. Physically, the memory of each process may be dispersed across different areas of physical memory, or may have been moved (paged out) to secondary storage, typically to a hard disk drive (HDD) or solid-state drive (SSD).
When a process requests access to data in its memory, it is the responsibility of the operating s…
The translation process
The memory management unit (MMU) inside the CPU stores a cache of recently used mappings from the operating system's page table. This is called the translation lookaside buffer (TLB), which is an associative cache.
When a virtual address needs to be translated into a physical address, the TLB is searched first. If a match is found, which is known as a TLB hit, the physical ad…
Translation failures
The page table lookup may fail, triggering a page fault, for two reasons:
• The lookup may fail if there is no translation available for the virtual address, meaning that virtual address is invalid. This will typically occur because of a programming error, and the operating system must take some action to deal with the problem. On modern operating systems, it will cause a segmentation fault signal being sent to the offending program.
Frame table data
The simplest page table systems often maintain a frame table and a page table. The frame table holds information about which frames are mapped. In more advanced systems, the frame table can also hold information about which address space a page belongs to, statistics information, or other background information.
Page table types
There are several types of page tables, which are optimized for different requirements. Essentially, a bare-bones page table must store the virtual address, the physical address that is "under" this virtual address, and possibly some address space information.
An inverted page table (IPT) is best thought of as an off-chip extension of the T…
See also
• Translation lookaside buffer
• Page replacement algorithm
• Pointer (computer programming)
• W^X