Topics (119k) Articles (119k) Users (287) Discussions (237) Comments (383) Files (715) New article
In x86 systems, there may actually be 2 address translation steps:like this:
- first segmentation
- then paging
(logical) ------------------> (linear) ------------> (physical)
segmentation pagingThe major difference between paging and segmentation is that:
- paging splits RAM into equal sized chunks called pages
- segmentation splits memory into chunks of arbitrary sizes
This is the main advantage of paging, since equal sized chunks make things more manageable by reducing memory fragmentation problems. See also:
Paging came after segmentation historically, and largely replaced it for the implementation of virtual memory in modern OSs.
Paging has become so much more popular that support for segmentation was dropped in x86-64 in 64-bit mode, the main mode of operation for new software, where it only exists in compatibility mode, which emulates IA-32.
x86 Paging Tutorial Example: simplified single-level paging scheme by 
Ciro Santilli 40 Updated 2025-07-16
Ciro Santilli 40 Updated 2025-07-16x86_64 uses 48 bits (256 TiB), and legacy mode's PAE already allows 52-bit addresses (4 PiB). 56-bits is a likely future candidate.
But that would mean that the page directory would have
2^18 = 256K entries, which would take too much RAM: close to a single-level paging for 32 bit architectures!x86_64 uses 4 levels in a
9 | 9 | 9 | 9 scheme, so that the upper level only takes up only 2^9 higher level entries.The 48 bits are split equally into two disjoint parts:
----------------- FFFFFFFF FFFFFFFF
Top half
----------------- FFFF8000 00000000
Not addressable
----------------- 00007FFF FFFFFFFF
Bottom half
----------------- 00000000 00000000A 5-level scheme is emerging in 2016: software.intel.com/sites/default/files/managed/2b/80/5-level_paging_white_paper.pdf which allows 52-bit addresses with 4k pagetables.
Physical address extension.
With 32 bits, only 4GB RAM can be addressed.
This started becoming a limitation for large servers, so Intel introduced the PAE mechanism to Pentium Pro.
Page table structure is also altered if PAE is on. The exact way in which it is altered depends on weather PSE is on or off.
If either PAE and PSE are active, different paging level schemes are used:
- no PAE and no PSE:
10 | 10 | 12 - no PAE and PSE:
10 | 22.22 is the offset within the 4Mb page, since 22 bits address 4Mb. - PAE and no PSE:
2 | 9 | 9 | 12The design reason why 9 is used twice instead of 10 is that now entries cannot fit anymore into 32 bits, which were all filled up by 20 address bits and 12 meaningful or reserved flag bits.The reason is that 20 bits are not enough anymore to represent the address of page tables: 24 bits are now needed because of the 4 extra wires added to the processor.Therefore, the designers decided to increase entry size to 64 bits, and to make them fit into a single page table it is necessary reduce the number of entries to 2^9 instead of 2^10. - PAE and PSE:
2 | 9 | 21
The Translation Lookahead Buffer (TLB) is a cache for paging addresses.
The Linux kernel makes extensive usage of the paging features of x86 to allow fast process switches with small data fragmentation.
There are also however some features that the Linux kernel might not use, either because they are only for backwards compatibility, or because the Linux devs didn't feel it was worth it yet.
Paging is done by the Memory Management Unit (MMU) part of the CPU.
It was later integrated into the CPU, but the term MMU still used.
Two level address translation to make OS emulation more efficient.
Peter Cordes mentions that some architectures like MIPS leave paging almost completely in the hands of software: a TLB miss runs an OS-supplied function to walk the page tables, and insert the new mapping into the TLB. In such architectures, the OS can use whatever data structure it wants.
Free:
- rutgers-pxk-416 chapter "Memory management: lecture notes"
It is mindblowing that the sale wasn't cancelled due to anti trust.
The outcome of this is that WhatApp will remain with the same feature set forever, while other competitors have been growing, notably Discord and Slack.
It seems that there is a case looming 10 years after the fact: www.cityam.com/facebook-fails-to-block-antitrust-lawsuit-over-whatsapp-and-instagram-ownership/ Wake up???
This means that all secret services in the world have alrady scraped this information for everyone that uses WhatsApp!!!
They just have to go incrementally through the list of all phone numbers... 001 0000 0000, 001 0000 0001, 001 0000 0002, etc. and then you can deduce who has which phone number.
OMG... it is analogous to the Facebook profile face dump.
Sure, it is forbidden in theory: faq.whatsapp.com/general/security-and-privacy/about-harvesting-personal-information/.
It is quite mind blowing that this is polyphyletic on mammals and birds, what can't parallel evolution achieve??
Phylogenetic tree of the vertebrates
. Source. Highlights how birds should obviously be classified as reptiles. Pinned article: Introduction to the OurBigBook Project
Welcome to the OurBigBook Project! Our goal is to create the perfect publishing platform for STEM subjects, and get university-level students to write the best free STEM tutorials ever.
Everyone is welcome to create an account and play with the site: ourbigbook.com/go/register. We belive that students themselves can write amazing tutorials, but teachers are welcome too. You can write about anything you want, it doesn't have to be STEM or even educational. Silly test content is very welcome and you won't be penalized in any way. Just keep it legal!
Intro to OurBigBook
. Source. We have two killer features:
- topics: topics group articles by different users with the same title, e.g. here is the topic for the "Fundamental Theorem of Calculus" ourbigbook.com/go/topic/fundamental-theorem-of-calculusArticles of different users are sorted by upvote within each article page. This feature is a bit like:
- a Wikipedia where each user can have their own version of each article
- a Q&A website like Stack Overflow, where multiple people can give their views on a given topic, and the best ones are sorted by upvote. Except you don't need to wait for someone to ask first, and any topic goes, no matter how narrow or broad
This feature makes it possible for readers to find better explanations of any topic created by other writers. And it allows writers to create an explanation in a place that readers might actually find it.
Figure 1. Screenshot of the "Derivative" topic page. View it live at: ourbigbook.com/go/topic/derivativeVideo 2. OurBigBook Web topics demo. Source. - local editing: you can store all your personal knowledge base content locally in a plaintext markup format that can be edited locally and published either:This way you can be sure that even if OurBigBook.com were to go down one day (which we have no plans to do as it is quite cheap to host!), your content will still be perfectly readable as a static site.
- to OurBigBook.com to get awesome multi-user features like topics and likes
- as HTML files to a static website, which you can host yourself for free on many external providers like GitHub Pages, and remain in full control
Figure 2. You can publish local OurBigBook lightweight markup files to either OurBigBook.com or as a static website.
Figure 3. Visual Studio Code extension installation.
Figure 5. . You can also edit articles on the Web editor without installing anything locally. Video 3. Edit locally and publish demo. Source. This shows editing OurBigBook Markup and publishing it using the Visual Studio Code extension. 
- Infinitely deep tables of contents:
All our software is open source and hosted at: github.com/ourbigbook/ourbigbook
Further documentation can be found at: docs.ourbigbook.com
Feel free to reach our to us for any help or suggestions: docs.ourbigbook.com/#contact