Three polarizers 45 degrees apart Updated 2025-07-16
x86 Paging Tutorial 64-bit architectures Updated 2025-07-16
x86_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.
Julia Vie Updated 2025-07-16
Ross Ulbricht told her about Silk Road, but then later said he had sold the website to someone else, without really doing that.
- LinkedIn profile: www.linkedin.com/in/julia-vie-35296959/
- Twitter profile: twitter.com/destenyrangel
Malus' Law Updated 2025-07-16
Matches the quantum superposition probability proportional to the square law. Poor Étienne-Louis Malus, who died so much before this was found.
Oxford Master Course in Mathematical and Theoretical Physics Updated 2025-07-16
RDFlib Updated 2025-07-16
SQL
COUNT function Updated 2025-07-16Have a look at some interesting examples under nodejs/sequelize/raw/many_to_many.js.
University of Cambridge student culture Updated 2025-07-16
Virtuoso Universal Server Updated 2025-07-16
Khan Academy Updated 2025-07-16
Kudos for being a not-for-profit. Also, anyone can create content: e-learning websites must allow students to create learning content. Oh, but TODO is possible for anyone to make content publicly visible? Course join links lik: www.khanacademy.org/join/MJZ6NSV7 require login. webapps.stackexchange.com/questions/165132/how-to-create-a-course-that-is-publicly-visible-without-the-need-to-login-on-kha If that's the case, it is a fatal flaw not shared by OurBigBook.com.
Kilogram Updated 2025-07-16
Unit of mass.
Defined in the 2019 redefinition of the SI base units via the Planck constant. This was possible due to the development of the Kibble balance.
NuNET Updated 2025-07-16
Relativistic particle in a box thought experiment Updated 2025-07-16
Described for example in lecture 1.
SQL
genenerate_series Updated 2025-07-16 SQL transaction isolation level Updated 2025-07-16
Each transaction isolation level specifies what can or cannot happen when two queries are being run in parallel, i.e.: the memory semantics of the system.
Remember that queries can affects thousands of rows, and database systems like PostgreSQL can run multiple such queries at the same time.
Implementation specifics:
Type-II superconductor Updated 2025-07-16
Particle decay Updated 2025-07-16
Can produce two entangled particles.
x86 Paging Tutorial Application Updated 2025-07-16
Paging makes it easier to compile and run two programs or threads at the same time on a single computer.
For example, when you compile two programs, the compiler does not know if they are going to be running at the same time or not.
And thread stacks, that must be contiguous and keep growing down until they overwrite each other, are an even bigger issue!
But if two programs use the same address and run at the same time, this is obviously going to break them!
Paging solves this problem beautifully by adding one degree of indirection:
(logical) ------------> (physical)
pagingWhere:
As far as programs are concerned, they think they can use any address between 0 and 4 GiB (2^32,
FFFFFFFF) on 32-bit systems.The OS then sets up paging so that identical logical addresses will go into different physical addresses and not overwrite each other.
This makes it much simpler to compile programs and run them at the same time.
Paging achieves that goal, and in addition:
- the switch between programs is very fast, because it is implemented by hardware
- the memory of both programs can grow and shrink as needed without too much fragmentation
- one program can never access the memory of another program, even if it wanted to.This is good both for security, and to prevent bugs in one program from crashing other programs.
Or if you like non-funny jokes:
Comparison between the Linux kernel userland memory virtualization and The Matrix
. Source. Is this RAM real?
x86 Paging Tutorial ARM Updated 2025-07-16
Information about ARM paging can be found at: cirosantilli.com/linux-kernel-module-cheat#arm-paging
x86 Paging Tutorial Basic TLB operation Updated 2025-07-16
After a translation between linear and physical address happens, it is stored on the TLB. For example, a 4 entry TLB starts in the following state:
valid linear physical
----- ------ --------
> 0 00000 00000
0 00000 00000
0 00000 00000
0 00000 00000The
> indicates the current entry to be replaced.And after a page linear address and after a second translation of
00003 is translated to a physical address 00005, the TLB becomes: valid linear physical
----- ------ --------
1 00003 00005
> 0 00000 00000
0 00000 00000
0 00000 0000000007 to 00009 it becomes: valid linear physical
----- ------ --------
1 00003 00005
1 00007 00009
> 0 00000 00000
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