Ciro Santilli @cirosantilli 40

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x86 Paging Tutorial / Kernel vs process memory layout Updated 2025-07-16

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The Linux Kernel reserves two zones of virtual memory:

one for kernel memory
one for programs

The exact split is configured by CONFIG_VMSPLIT_.... By default:

on 32-bit:

the bottom 3/4 is program space: 00000000 to BFFFFFFF

the top 1/4 is kernel memory: C0000000 to FFFFFFFF, like this:

------------------ FFFFFFFF
Kernel
------------------ C0000000
------------------ BFFFFFFF


Process


------------------ 00000000

on 64-bit: currently only 48-bits are actually used, split into two equally sized disjoint spaces. The Linux kernel just assigns:
- the bottom part to processes 00000000 00000000 to 008FFFFF FFFFFFFF
- the top part to the kernel: FFFF8000 00000000 to FFFFFFFF FFFFFFFF, like this:
  ------------------ FFFFFFFF Kernel ------------------ C0000000 (not addressable) ------------------ BFFFFFFF Process ------------------ 00000000

Kernel memory is also paged.

In previous versions, the paging was continuous, but with HIGHMEM this changed.

There is no clear physical memory split: stackoverflow.com/questions/30471742/physical-memory-userspace-kernel-split-on-linux-x86-64

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x86 Paging Tutorial / Memory management unit Updated 2025-07-16

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Paging is done by the Memory Management Unit (MMU) part of the CPU.

Like many others (e.g. x87 co-processor, APIC), this used to be by separate chip on early days.

It was later integrated into the CPU, but the term MMU still used.

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1965 Nobel Prize in Physics laureate Updated 2025-07-16

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How to use an Oxford Nanopore MinION to extract DNA from river water and determine which bacteria live in it / Sequencing Updated 2025-07-16

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Once we had the amplified 16S DNA, we were almost ready to start sequencing!

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PyZX Updated 2025-07-16

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github.com/Quantomatic/pyzx

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Turing complete Updated 2025-07-16

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A computer model that is as powerful as the most powerful computer model we have: Turing machine!

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Closed source is less bad on online services Updated 2025-07-16

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Ciro Santilli can accept closed source on server products more easily than offline, because the servers have to be paid for somehow (by stealing your private data).

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Euler-Mascheroni constant Updated 2025-07-16

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Convergence: math.stackexchange.com/questions/629630/simple-proof-euler-mascheroni-gamma-constant

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Impenetrable Bose Gas Updated 2025-07-16

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Linear polynomial Updated 2025-07-16

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A polynomial of degree 1, i.e. of form

a x + b

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Linux distribution buildable from source Updated 2025-07-16

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As of 2020, no one knows how to build the major desktop distros fully from source into the ISO, and especially so in a reproducible build way. Everything is done in build servers somewhere with complicated layers of prebuilds. It's crap.

Yes:

No:

Ubuntu: askubuntu.com/questions/82302/how-to-compile-ubuntu-from-source-code source: sausage factory

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List of Nobel Prizes in Chemistry Updated 2025-07-16

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Non-relativistic quantum mechanics Updated 2025-07-16

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The first quantum mechanics theories developed.

Their most popular formulation has been the Schrödinger equation.

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NoSQL Updated 2025-07-16

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x86 Paging Tutorial / PAE Updated 2025-07-16

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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.

To relieve the problem, Intel added 4 new address lines, so that 64GB could be addressed.

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.

PAE is turned on and off via the PAE bit of cr4.

Even if the total addressable memory is 64GB, individual process are still only able to use up to 4GB. The OS can however put different processes on different 4GB chunks.

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x86 Paging Tutorial / Page size choice 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