Then come the most important symbols:
Num: Value Size Type Bind Vis Ndx Name
4: 0000000000000000 0 NOTYPE LOCAL DEFAULT 1 hello_world
5: 000000000000000d 0 NOTYPE LOCAL DEFAULT ABS hello_world_len
6: 0000000000000000 0 NOTYPE GLOBAL DEFAULT 2 _starthello_world string is in the .data section (index 1). It's value is 0: it points to the first byte of that section._start is marked with GLOBAL visibility since we wrote:global _startin NASM. This is necessary since it must be seen as the entry point. Unlike in C, by default NASM labels are local.
Contains a lot of different flag masks.
E. Coli K-12 MG1655 origin of replication by 
Ciro Santilli 35 Updated 2025-01-10 +Created 1970-01-01
Ciro Santilli 35 Updated 2025-01-10 +Created 1970-01-01Note that this is not the conventional starting point for gene numbering: Section "E. Coli genome starting point".
gothinkster/django-realworld-example-app by Ciro Santilli 35 Updated 2025-01-10 +Created 1970-01-01
Ciro Santilli 35 Updated 2025-01-10 +Created 1970-01-01As of 2021, last updated 2016, and python 3.5 appears to be mandatory or else:which apparently broke in 3.6: stackoverflow.com/questions/41343263/provide-classcell-example-for-python-3-6-metaclass and
RuntimeError: __class__ not set defining 'AbstractBaseUser' as <class 'django.contrib.auth.base_user.AbstractBaseUser'>. Was __classcell__ propagated to type.__new__?pyenv install fails on Ubuntu 20.10, so... fuck. Workarounds at:but am I in the mood considering that the ancient Django version would require an immediate port anyways? Repo is at Django 1.0, while newest is now already Django 3. The Rails one is broken for the same reason. Fuck 2.Often just called collimated light due to the collimator being the main procedure to obtain it.
However, you move very far away from the source, e.g. the Sun, you also get essentially parallel light.
Exactly 1033 somatic cells on male, 959 on hermaphrodite, every time, counted as of 2020. A beauty.
Exactly 131 commit apoptosis in the hermaphrodite.
www.wormatlas.org/celllineages.html contains the full lineage as some huge and impossible to view images. This image was taken directly from The embryonic cell lineage of the nematode Caenorhabditis elegans where it is split across many pages, it is a thing of beauty on the PDF.
www.wormatlas.org/celllistsulston.htm contains a non-hierarchical table with the cells and their names.
We will sometimes just write them without superscript, as it saves typing and is useless.
One of the most beautiful molecular machines known!
The first one with such complexity that was uncovered.
The thing rotates like a water wheel for God's sake, except it uses protons instead of water.
The ATP synthase complex is so large that Power, Sex, Suicide by Nick Lane (2006) page 123 contains a cryoEM image of several ATP synthases on small membrane vesicles, this is the paper: pubs.acs.org/doi/abs/10.1021/bi00437a031# under a fucking paywall.
ATP synthase in action by HarvardX (2017)
Source. Gaussian path integrals.
Using the TLB makes translation faster, because the initial translation takes one access _per TLB level_, which means 2 on a simple 32 bit scheme, but 3 or 4 on 64 bit architectures.
The TLB is usually implemented as an expensive type of RAM called content-addressable memory (CAM). CAM implements an associative map on hardware, that is, a structure that given a key (linear address), retrieves a value.
Mappings could also be implemented on RAM addresses, but CAM mappings may required much less entries than a RAM mapping.
For example, a map in which:could be stored in a TLB with 4 entries:
- both keys and values have 20 bits (the case of a simple paging schemes)
- at most 4 values need to be stored at each time
linear physical
------ --------
00000 00001
00001 00010
00010 00011
FFFFF 00000However, to implement this with RAM, _it would be necessary to have 2^20 addresses_:which would be even more expensive than using a TLB.
linear physical
------ --------
00000 00001
00001 00010
00010 00011
... (from 00011 to FFFFE)
FFFFF 00000For each process, the virtual address space looks like this:
------------------ 2^32 - 1
Stack (grows down)
v v v v v v v v v
------------------
(unmapped)
------------------ Maximum stack size.
(unmapped)
-------------------
mmap
-------------------
(unmapped)
-------------------
^^^^^^^^^^^^^^^^^^^
brk (grows up)
-------------------
BSS
-------------------
Data
-------------------
Text
-------------------
------------------- 0The kernel maintains a list of pages that belong to each process, and synchronizes that with the paging.
If the program accesses memory that does not belong to it, the kernel handles a page-fault, and decides what to do:
- if it is above the maximum stack size, allocate those pages to the process
- otherwise, send a SIGSEGV to the process, which usually kills it
When an ELF file is loaded by the kernel to start a program with the
exec system call, the kernel automatically registers text, data, BSS and stack for the program.The
brk and mmap areas can be modified by request of the program through the brk and mmap system calls. But the kernel can also deny the program those areas if there is not enough memory.brk and mmap can be used to implement malloc, or the so called "heap".mmap is also used to load dynamically loaded libraries into the program's memory so that it can access and run it.Stack allocation: stackoverflow.com/questions/17671423/stack-allocation-for-process
Calculating exact addresses Things are complicated by:
- Address Space Layout Randomization.
- the fact that environment variables, CLI arguments, and some ELF header data take up initial stack space: unix.stackexchange.com/questions/145557/how-does-stack-allocation-work-in-linux/239323#239323
Why the text does not start at 0: stackoverflow.com/questions/14795164/why-do-linux-program-text-sections-start-at-0x0804800-and-stack-tops-start-at-0
Single level paging scheme numerical translation example by Ciro Santilli 35 Updated 2025-01-10 +Created 1970-01-01
Ciro Santilli 35 Updated 2025-01-10 +Created 1970-01-01Suppose that the OS has setup the following page tables for process 1:and for process 2:
entry index entry address page address present
----------- ------------------ ------------ -------
0 CR3_1 + 0 * 4 0x00001 1
1 CR3_1 + 1 * 4 0x00000 1
2 CR3_1 + 2 * 4 0x00003 1
3 CR3_1 + 3 * 4 0
...
2^20-1 CR3_1 + 2^20-1 * 4 0x00005 1entry index entry address page address present
----------- ----------------- ------------ -------
0 CR3_2 + 0 * 4 0x0000A 1
1 CR3_2 + 1 * 4 0x12345 1
2 CR3_2 + 2 * 4 0
3 CR3_2 + 3 * 4 0x00003 1
...
2^20-1 CR3_2 + 2^20-1 * 4 0xFFFFF 1Before process 1 starts running, the OS sets its
cr3 to point to the page table 1 at CR3_1.When process 1 tries to access a linear address, this is the physical addresses that will be actually accessed:
linear physical
--------- ---------
00000 001 00001 001
00000 002 00001 002
00000 003 00001 003
00000 FFF 00001 FFF
00001 000 00000 000
00001 001 00000 001
00001 FFF 00000 FFF
00002 000 00003 000
FFFFF 000 00005 000To switch to process 2, the OS simply sets
cr3 to CR3_2, and now the following translations would happen:linear physical
--------- ---------
00000 002 0000A 002
00000 003 0000A 003
00000 FFF 0000A FFF
00001 000 12345 000
00001 001 12345 001
00001 FFF 12345 FFF
00004 000 00003 000
FFFFF 000 FFFFF 000Step-by-step translation for process 1 of logical address
0x00000001 to physical address 0x00001001:- split the linear address into two parts:
| page (20 bits) | offset (12 bits) |So in this case we would have:
*page = 0x00000. This part must be translated to a physical location.
*offset = 0x001. This part is added directly to the page address, and is not translated: it contains the position _within_ the page. - look into Page table 1 because
cr3points to it. - The hardware knows that this entry is located at RAM address
CR3 + 0x00000 * 4 = CR3:
*0x00000because the page part of the logical address is0x00000
*4because that is the fixed size in bytes of every page table entry - since it is present, the access is valid
- by the page table, the location of page number
0x00000is at0x00001 * 4K = 0x00001000. - to find the final physical address we just need to add the offset:
00001 000 + 00000 001 --------- 00001 001because00001is the physical address of the page looked up on the table and001is the offset.We shift00001by 12 bits because the pages are always aligned to 4KiB.The offset is always simply added the physical address of the page. - the hardware then gets the memory at that physical location and puts it in a register.
Another example: for logical address
0x00001001:- the page part is
00001, and the offset part is001 - the hardware knows that its page table entry is located at RAM address:
CR3 + 1 * 4(1because of the page part), and that is where it will look for it - it finds the page address
0x00000there - so the final address is
0x00000 * 4k + 0x001 = 0x00000001
The problem with single-level paging by Ciro Santilli 35 Updated 2025-01-10 +Created 1970-01-01
Ciro Santilli 35 Updated 2025-01-10 +Created 1970-01-01The problem with a single-level paging scheme is that it would take up too much RAM: 4G / 4K = 1M entries _per_ process.
If each entry is 4 bytes long, that would make 4M _per process_, which is too much even for a desktop computer:
ps -A | wc -l says that I am running 244 processes right now, so that would take around 1GB of my RAM!For this reason, x86 developers decided to use a multi-level scheme that reduces RAM usage.
The downside of this system is that is has a slightly higher access time, as we need to access RAM more times for each translation.
The Quantum Story by Jim Baggott (2011) page 2 mentions how newton's support for the corpuscular theory of light led it to be held for a very long time, even when evidence of the wave theory of light was becoming overwhelming.
Kavli Institute for Theoretical Physics by Ciro Santilli 35 Updated 2025-01-10 +Created 1970-01-01
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