Ciro Santilli @cirosantilli 37

Ciro Santilli 37 Updated 2025-07-16

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Why are pages 4 KiB anyways?

There is a trade-off between memory wasted in:

page tables
extra padding memory within pages

This can be seen with the extreme cases:

if the page size were 1 byte:
- granularity would be great, and the OS would never have to allocate unneeded padding memory
- but the page table would have 2^32 entries, and take up the entire memory!
if the page size were 4 GiB:
- we would need to swap 4 GiB to disk every time a new process becomes active
- the page size would be a single entry, so it would take almost no memory at all

x86 designers have found that 4 KiB pages are a good middle ground.

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x86 Paging Tutorial / Multi-level paging scheme numerical translation example by

Ciro Santilli 37 Updated 2025-07-16

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Page directory given to process by the OS:

entry index   entry address      page table address  present
-----------   ----------------   ------------------  --------
0             CR3 + 0      * 4   0x10000             1
1             CR3 + 1      * 4                       0
2             CR3 + 2      * 4   0x80000             1
3             CR3 + 3      * 4                       0
...
2^10-1        CR3 + 2^10-1 * 4                       0

Page tables given to process by the OS at PT1 = 0x10000000 (0x10000 * 4K):

entry index   entry address      page address  present
-----------   ----------------   ------------  -------
0             PT1 + 0      * 4   0x00001       1
1             PT1 + 1      * 4                 0
2             PT1 + 2      * 4   0x0000D       1
...                                  ...
2^10-1        PT1 + 2^10-1 * 4   0x00005       1

Page tables given to process by the OS at PT2 = 0x80000000 (0x80000 * 4K):

entry index   entry address     page address  present
-----------   ---------------   ------------  ------------
0             PT2 + 0     * 4   0x0000A       1
1             PT2 + 1     * 4   0x0000C       1
2             PT2 + 2     * 4                 0
...
2^10-1        PT2 + 0x3FF * 4   0x00003       1

where PT1 and PT2: initial position of page table 1 and page table 2 for process 1 on RAM.

With that setup, the following translations would happen:

linear    10 10 12 split  physical
--------  --------------  ----------
00000001  000 000 001     00001001
00001001  000 001 001     page fault
003FF001  000 3FF 001     00005001
00400000  001 000 000     page fault
00800001  002 000 001     0000A001
00801004  002 001 004     0000C004
00802004  002 002 004     page fault
00B00001  003 000 000     page fault

Let's translate the linear address 0x00801004 step by step:

In binary the linear address is:

0    0    8    0    1    0    0    4
0000 0000 1000 0000 0001 0000 0000 0100

Grouping as 10 | 10 | 12 gives:

0000000010 0000000001 000000000100
0x2        0x1        0x4

which gives:

page directory entry = 0x2
page table     entry = 0x1
offset               = 0x4

So the hardware looks for entry 2 of the page directory.

The page directory table says that the page table is located at 0x80000 * 4K = 0x80000000. This is the first RAM access of the process.
Since the page table entry is 0x1, the hardware looks at entry 1 of the page table at 0x80000000, which tells it that the physical page is located at address 0x0000C * 4K = 0x0000C000. This is the second RAM access of the process.
Finally, the paging hardware adds the offset, and the final address is 0x0000C004.

Page faults occur if either a page directory entry or a page table entry is not present.

The Intel manual gives a picture of this translation process in the image "Linear-Address Translation to a 4-KByte Page using 32-Bit Paging": Figure 1. "x86 page translation process"

Figure 1.
x86 page translation process
.

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Why do the up ad down quarks have different masses? by

Ciro Santilli 37 Updated 2025-07-16

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www.quora.com/Why-do-up-quarks-and-down-quarks-have-different-charges

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Tangible asset by

Ciro Santilli 37 Updated 2025-07-16

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Stars nearest to the Sun by

Ciro Santilli 37 Updated 2025-07-16

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Figure 2.
Distance of stars nearest to the Sun as function of time
. Source.

upload.wikimedia.org/wikipedia/commons/e/e9/Near-stars-past-future-en.svg

Some notable ones:

Proxima Centauri, the nearest one, at 4 ly. It is part of the Alpha Centauri star system, which contains two other stars at very similar distances as well, and their relative distances to earth will change positions in a few tens of thousands of years.
Sirius, the brightest star in the sky at 9 ly

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SQL transaction isolation level by

Ciro Santilli 37 Updated 2025-07-16

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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.

Good summary on the PostgreSQL page: www.postgresql.org/docs/14/transaction-iso.html

Implementation specifics:

SQLite isolation levels

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JOIN (SQL) by

Ciro Santilli 37 Updated 2025-07-16

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PostgreSQL by

Ciro Santilli 37 Updated 2025-07-16

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PostgreSQL feels good.

Its feature set is insanely large! Just look at stuff like: stackoverflow.com/questions/1986491/sql-split-string-by-space-into-table-in-postgresql/1993058#1993058

Had a look at the source tree, and also felt good.

If Oracle is the Microsoft of database, Postgres is the Linux, and MySQL (or more precisely MariaDB) is the FreeBSD (i.e. the one that got delayed by legal issues). Except that their software licenses were accidentally swapped.

The only problem with Postgres is its name. PostgreSQL is so unpronounceable and so untypeable that you should just call it "Postgres" like everyone else.

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Tunnel magnetoresistance by

Ciro Santilli 37 Updated 2025-07-16

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Video 1.

What is spintronics and how is it useful? by SciToons (2019)

Source. Gives a good 1 minute explanation of tunnel magnetoresistance.

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Emission theory (vision) by

Ciro Santilli 37 Updated 2025-07-16

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It is so mind blowing that people believed in this theory. How can you think that, when you turn on a lamp and then you see? Obviously, the lamp must be emitting something!!!

Then comes along this epic 2002 paper: pubmed.ncbi.nlm.nih.gov/12094435/ "Fundamentally misunderstanding visual perception. Adults' belief in visual emissions". TODO review methods...

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Metastable electron by

Ciro Santilli 37 Updated 2025-07-16

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A fundamental component of three-level lasers.

As mentioned at youtu.be/_JOchLyNO_w?t=581 from Video "How Lasers Work by Scientized (2017)", they stay in that state for a long time compared to non spontaneous emission of metastable states!

phys.libretexts.org/Courses/University_of_California_Davis/UCD%3A_Physics_9HE_-_Modern_Physics/06%3A_Emission_and_Absorption_of_Photons/6.3%3A_Lasers mentions that they are kept in that excited state due to selection rules.

This is also one of mechanisms behind phosphorescence with triplet states.

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SARS-CoV-2 structural protein by

Ciro Santilli 37 Updated 2025-07-16

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