Ciro Santilli @cirosantilli 37

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Edward Teller Updated 2025-07-16

www.youtube.com/playlist?list=PLVV0r6CmEsFw1phnddYWXtVkRW8eUVlqx Edward Teller interview by Web of Stories (1996) Date shown at: www.webofstories.com/play/edward.teller/1. Listener: John H. Nuckolls

Video 1.

Witnessing the test explosion Edward Teller interview by Web of Stories (1996)

Source.

Video 2.

Edward Teller, An Early Time

. Source. Comissioned by the Los Alamos National Laboratory in 1979. Producer: Mario Balibreraa.

youtu.be/Goim-4MF_uE?t=338; holy fuck he almost cut his foot off on a stupid tram accident!
youtu.be/Goim-4MF_uE?t=457: he plays the piano
youtu.be/Goim-4MF_uE?t=965: he drove Szilard to propose to Einstein the Einstein-Szilard letter

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Effective field theory Updated 2025-07-16

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www.youtube.com/watch?v=WB8r7CU7clk&list=PLUl4u3cNGP60TvpbO5toEWC8y8w51dtvm by Iain Stewart. Basically starts by explaining how quantum field theory is so generic that it is hard to get any numerical results out of it :-)

But in particular, we want to describe those subtheories in a way that we can reach arbitrary precision of the full theory if desired.

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eGroups Updated 2025-07-16

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Company co-founded by Scott Hassan, early Google programmer at Stanford University, and Carl Victor Page, Jr., Larry Page's older brother.

They were an email list management website, and became Yahoo! Groups after the acquisition.

The company was sold to Yahoo! in August 2000 for $432m and became Yahoo! Groups. They managed to miraculously dodge the Dot-com bubble, which mostly poppet in 2021. After the acquisition, Yahoo started to redirect them to: groups.yahoo.com as can be seen on the Wayback Machine: web.archive.org/web/20000401000000*/egroups.com The first archive of groups.yahoo.com is from February 2001: web.archive.org/web/20010202055100/http://groups.yahoo.com/ and it unsurprisingly looks basically exactly like eGroups.

Figure 1.

eGroups logo

. From the earliest archive of their "about" page: web.archive.org/web/19991004062653/http://www.egroups.com/info/top.html in 1999.

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Electronic test equipment Updated 2025-07-16

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Electron microscope Updated 2025-07-16

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All of them need a vacuum because you can't shoot elecrons through air, as mentioned at Video "50,000,000x Magnification by AlphaPhoenix (2022)".

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Electrum Updated 2025-07-16

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electrum.org/

askubuntu.com/questions/281233/how-can-i-install-the-electrum-bitcoin-wallet/1384053#1384053

For the love of God, on Ubuntu install from the official AppImage downloaded from electrum.org/#download, not this random outdated Snap snapcraft.io/electrum:

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ELF Hello World Tutorial / Backlinks Updated 2025-07-16

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News aggregators:

2017-05: news.ycombinator.com/item?id=14359159 web.archive.org/web/20170517174951/https://news.ycombinator.com/news

Personal websites:

www.etherington.xyz/elfguide

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ELF Hello World Tutorial / .data section Updated 2025-07-16

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.data is section 1:

00000080  01 00 00 00 01 00 00 00  03 00 00 00 00 00 00 00  |................|
00000090  00 00 00 00 00 00 00 00  00 02 00 00 00 00 00 00  |................|
000000a0  0d 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
000000b0  04 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|

80 0: sh_name = 01 00 00 00: index 1 in the .shstrtab string table
Here, 1 says the name of this section starts at the first character of that section, and ends at the first NUL character, making up the string .data.
.data is one of the section names which has a predefined meaning according to www.sco.com/developers/gabi/2003-12-17/ch4.strtab.html:
These sections hold initialized data that contribute to the program's memory image.

80 4: sh_type = 01 00 00 00: SHT_PROGBITS: the section content is not specified by ELF, only by how the program interprets it. Normal since a .data section.
80 8: sh_flags = 03 7x 00: SHF_WRITE and SHF_ALLOC: www.sco.com/developers/gabi/2003-12-17/ch4.sheader.html#sh_flags, as required from a .data section
90 0: sh_addr = 8x 00: TODO: standard says:
If the section will appear in the memory image of a process, this member gives the address at which the section's first byte should reside. Otherwise, the member contains 0.
but I don't understand it very well yet.
90 8: sh_offset = 00 02 00 00 00 00 00 00 = 0x200: number of bytes from the start of the program to the first byte in this section
a0 0: sh_size = 0d 00 00 00 00 00 00 00
If we take 0xD bytes starting at sh_offset 200, we see:
```
00000200  48 65 6c 6c 6f 20 77 6f  72 6c 64 21 0a 00        |Hello world!..  |
```
AHA! So our "Hello world!" string is in the data section like we told it to be on the NASM.
Once we graduate from hd, we will look this up like:
```
readelf -x .data hello_world.o
```
which outputs:
```
Hex dump of section '.data':
  0x00000000 48656c6c 6f20776f 726c6421 0a       Hello world!.
```
NASM sets decent properties for that section because it treats .data magically: www.nasm.us/doc/nasmdoc7.html#section-7.9.2
Also note that this was a bad section choice: a good C compiler would put the string in .rodata instead, because it is read-only and it would allow for further OS optimizations.
- a0 8: sh_link and sh_info = 8x 0: do not apply to this section type. www.sco.com/developers/gabi/2003-12-17/ch4.sheader.html#special_sections
- b0 0: sh_addralign = 04 = TODO: why is this alignment necessary? Is it only for sh_addr, or also for symbols inside sh_addr?
- b0 8: sh_entsize = 00 = the section does not contain a table. If != 0, it means that the section contains a table of fixed size entries. In this file, we see from the readelf output that this is the case for the .symtab and .rela.text sections.

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ELF Hello World Tutorial / Dynamic linking sections Updated 2025-07-16

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This program did not have certain dynamic linking related sections because we linked it minimally with ld.

However, if you compile a C hello world with GCC 8.2:

gcc -o main.out main.c

some other interesting sections would appear.

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ELF Hello World Tutorial / Global file structure Updated 2025-07-16

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An ELF file contains the following parts:

ELF header. Points to the position of the section header table and the program header table.
Section header table (optional on executable). Each has e_shnum section headers, each pointing to the position of a section.
N sections, with N <= e_shnum (optional on executable)
Program header table (only on executable). Each has e_phnum program headers, each pointing to the position of a segment.
N segments, with N <= e_phnum (only on executable)

The order of those parts is not fixed: the only fixed thing is the ELF header that must be the first thing on the file: Generic docs say:

Although the figure shows the program header table immediately after the ELF header, and the section header table following the sections, actual files may differ. Moreover, sections and segments have no specified order. Only the ELF header has a fixed position in the file.

In pictures: sample object file with three sections:

            +-------------------+
            | ELF header        |---+
+---------> +-------------------+   | e_shoff
|           |                   |<--+
| Section   | Section header 0  |
|           |                   |---+ sh_offset
| Header    +-------------------+   |
|           | Section header 1  |---|--+ sh_offset
| Table     +-------------------+   |  |
|           | Section header 2  |---|--|--+
+---------> +-------------------+   |  |  |
            | Section 0         |<--+  |  |
            +-------------------+      |  | sh_offset
            | Section 1         |<-----+  |
            +-------------------+         |
            | Section 2         |<--------+
            +-------------------+

But nothing (except sanity) prevents the following topology:

            +-------------------+
            | ELF header        |---+ e_shoff
            +-------------------+   |
            | Section 1         |<--|--+
+---------> +-------------------+   |  |
|           |                   |<--+  | sh_offset
| Section   | Section header 0  |      |
|           |                   |------|---------+
| Header    +-------------------+      |         |
|           | Section header 1  |------+         |
| Table     +-------------------+                |
|           | Section header 2  |---+            | sh_offset
+---------> +-------------------+   | sh_offset  |
            | Section 2         |<--+            |
            +-------------------+                |
            | Section 0         |<---------------+
            +-------------------+

But some newbies may prefer PNGs :-)

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ELF Hello World Tutorial / Implementations Updated 2025-07-16

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Compiler toolchains generate and read ELF files.
Sane compilers should use a separate standalone library to do the dirty work. E.g., Binutils uses BFD (in-tree and canonical source).

Operating systems read and run ELF files.
Kernels cannot link to a library nor use the C stlib, so they are more likely to implement it themselves.
This is the case of the Linux kernel 4.2 which implements it in th file fs/binfmt_elf.c.

Specialized libraries. Examples:
- github.com/eliben/pyelftools. By a hardcore Googler: plus.google.com/+EliBenderskyGplus/posts
- sourceforge.net/projects/elftoolchain

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ELF Hello World Tutorial / Object hd Updated 2025-07-16

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Running:

hd hello_world.o

gives:

00000000  7f 45 4c 46 02 01 01 00  00 00 00 00 00 00 00 00  |.ELF............|
00000010  01 00 3e 00 01 00 00 00  00 00 00 00 00 00 00 00  |..>.............|
00000020  00 00 00 00 00 00 00 00  40 00 00 00 00 00 00 00  |........@.......|
00000030  00 00 00 00 40 00 00 00  00 00 40 00 07 00 03 00  |....@.....@.....|
00000040  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00000080  01 00 00 00 01 00 00 00  03 00 00 00 00 00 00 00  |................|
00000090  00 00 00 00 00 00 00 00  00 02 00 00 00 00 00 00  |................|
000000a0  0d 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
000000b0  04 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
000000c0  07 00 00 00 01 00 00 00  06 00 00 00 00 00 00 00  |................|
000000d0  00 00 00 00 00 00 00 00  10 02 00 00 00 00 00 00  |................|
000000e0  27 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |'...............|
000000f0  10 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000100  0d 00 00 00 03 00 00 00  00 00 00 00 00 00 00 00  |................|
00000110  00 00 00 00 00 00 00 00  40 02 00 00 00 00 00 00  |........@.......|
00000120  32 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |2...............|
00000130  01 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000140  17 00 00 00 02 00 00 00  00 00 00 00 00 00 00 00  |................|
00000150  00 00 00 00 00 00 00 00  80 02 00 00 00 00 00 00  |................|
00000160  a8 00 00 00 00 00 00 00  05 00 00 00 06 00 00 00  |................|
00000170  04 00 00 00 00 00 00 00  18 00 00 00 00 00 00 00  |................|
00000180  1f 00 00 00 03 00 00 00  00 00 00 00 00 00 00 00  |................|
00000190  00 00 00 00 00 00 00 00  30 03 00 00 00 00 00 00  |........0.......|
000001a0  34 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |4...............|
000001b0  01 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
000001c0  27 00 00 00 04 00 00 00  00 00 00 00 00 00 00 00  |'...............|
000001d0  00 00 00 00 00 00 00 00  70 03 00 00 00 00 00 00  |........p.......|
000001e0  18 00 00 00 00 00 00 00  04 00 00 00 02 00 00 00  |................|
000001f0  04 00 00 00 00 00 00 00  18 00 00 00 00 00 00 00  |................|
00000200  48 65 6c 6c 6f 20 77 6f  72 6c 64 21 0a 00 00 00  |Hello world!....|
00000210  b8 01 00 00 00 bf 01 00  00 00 48 be 00 00 00 00  |..........H.....|
00000220  00 00 00 00 ba 0d 00 00  00 0f 05 b8 3c 00 00 00  |............<...|
00000230  bf 00 00 00 00 0f 05 00  00 00 00 00 00 00 00 00  |................|
00000240  00 2e 64 61 74 61 00 2e  74 65 78 74 00 2e 73 68  |..data..text..sh|
00000250  73 74 72 74 61 62 00 2e  73 79 6d 74 61 62 00 2e  |strtab..symtab..|
00000260  73 74 72 74 61 62 00 2e  72 65 6c 61 2e 74 65 78  |strtab..rela.tex|
00000270  74 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |t...............|
00000280  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000290  00 00 00 00 00 00 00 00  01 00 00 00 04 00 f1 ff  |................|
000002a0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
000002b0  00 00 00 00 03 00 01 00  00 00 00 00 00 00 00 00  |................|
000002c0  00 00 00 00 00 00 00 00  00 00 00 00 03 00 02 00  |................|
000002d0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
000002e0  11 00 00 00 00 00 01 00  00 00 00 00 00 00 00 00  |................|
000002f0  00 00 00 00 00 00 00 00  1d 00 00 00 00 00 f1 ff  |................|
00000300  0d 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000310  2d 00 00 00 10 00 02 00  00 00 00 00 00 00 00 00  |-...............|
00000320  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000330  00 68 65 6c 6c 6f 5f 77  6f 72 6c 64 2e 61 73 6d  |.hello_world.asm|
00000340  00 68 65 6c 6c 6f 5f 77  6f 72 6c 64 00 68 65 6c  |.hello_world.hel|
00000350  6c 6f 5f 77 6f 72 6c 64  5f 6c 65 6e 00 5f 73 74  |lo_world_len._st|
00000360  61 72 74 00 00 00 00 00  00 00 00 00 00 00 00 00  |art.............|
00000370  0c 00 00 00 00 00 00 00  01 00 00 00 02 00 00 00  |................|
00000380  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000390

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ELF Hello World Tutorial / Program header table Updated 2025-07-16

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Only appears in the executable.

Contains information of how the executable should be put into the process virtual memory.

The executable is generated from object files by the linker. The main jobs that the linker does are:

determine which sections of the object files will go into which segments of the executable.
In Binutils, this comes down to parsing a linker script, and dealing with a bunch of defaults.
You can get the linker script used with ld --verbose, and set a custom one with ld -T.
do relocation according to the .rela.text section. This depends on how the multiple sections are put into memory.

readelf -l hello_world.out gives:

Elf file type is EXEC (Executable file)
Entry point 0x4000b0
There are 2 program headers, starting at offset 64

Program Headers:
  Type           Offset             VirtAddr           PhysAddr
                 FileSiz            MemSiz              Flags  Align
  LOAD           0x0000000000000000 0x0000000000400000 0x0000000000400000
                 0x00000000000000d7 0x00000000000000d7  R E    200000
  LOAD           0x00000000000000d8 0x00000000006000d8 0x00000000006000d8
                 0x000000000000000d 0x000000000000000d  RW     200000

 Section to Segment mapping:
  Segment Sections...
   00     .text
   01     .data

On the ELF header, e_phoff, e_phnum and e_phentsize told us that there are 2 program headers, which start at 0x40 and are 0x38 bytes long each, so they are:

00000040  01 00 00 00 05 00 00 00  00 00 00 00 00 00 00 00  |................|
00000050  00 00 40 00 00 00 00 00  00 00 40 00 00 00 00 00  |..@.......@.....|
00000060  d7 00 00 00 00 00 00 00  d7 00 00 00 00 00 00 00  |................|
00000070  00 00 20 00 00 00 00 00                           |.. .....        |

and:

00000070                           01 00 00 00 06 00 00 00  |        ........|
00000080  d8 00 00 00 00 00 00 00  d8 00 60 00 00 00 00 00  |..........`.....|
00000090  d8 00 60 00 00 00 00 00  0d 00 00 00 00 00 00 00  |..`.............|
000000a0  0d 00 00 00 00 00 00 00  00 00 20 00 00 00 00 00  |.......... .....|

Structure represented www.sco.com/developers/gabi/2003-12-17/ch5.pheader.html:

typedef struct {
    Elf64_Word  p_type;
    Elf64_Word  p_flags;
    Elf64_Off   p_offset;
    Elf64_Addr  p_vaddr;
    Elf64_Addr  p_paddr;
    Elf64_Xword p_filesz;
    Elf64_Xword p_memsz;
    Elf64_Xword p_align;
} Elf64_Phdr;

Breakdown of the first one:

40 0: p_type = 01 00 00 00 = PT_LOAD: this is a regular segment that will get loaded in memory.
40 4: p_flags = 05 00 00 00 = execute and read permissions. No write: we cannot modify the text segment. A classic way to do this in C is with string literals: stackoverflow.com/a/30662565/895245 This allows kernels to do certain optimizations, like sharing the segment amongst processes.
40 8: p_offset = 8x 00 TODO: what is this? Standard says:
This member gives the offset from the beginning of the file at which the first byte of the segment resides.
But it looks like offsets from the beginning of segments, not file?
50 0: p_vaddr = 00 00 40 00 00 00 00 00: initial virtual memory address to load this segment to
50 8: p_paddr = 00 00 40 00 00 00 00 00: unspecified effect. Intended for systems in which physical addressing matters. TODO example?
60 0: p_filesz = d7 00 00 00 00 00 00 00: size that the segment occupies in memory. If smaller than p_memsz, the OS fills it with zeroes to fit when loading the program. This is how BSS data is implemented to save space on executable files. i368 ABI says on PT_LOAD:
The bytes from the file are mapped to the beginning of the memory segment. If the segment’s memory size (p_memsz) is larger than the file size (p_filesz), the ‘‘extra’’ bytes are defined to hold the value 0 and to follow the segment’s initialized area. The file size may not be larger than the memory size.

60 8: p_memsz = d7 00 00 00 00 00 00 00: size that the segment occupies in memory
70 0: p_align = 00 00 20 00 00 00 00 00: 0 or 1 mean no alignment required. TODO why is this required? Why not just use p_addr directly, and get that right? Docs also say:
p_vaddr should equal p_offset, modulo p_align

The second segment (.data) is analogous. TODO: why use offset 0x0000d8 and address 0x00000000006000d8? Why not just use 0 and 0x00000000006000d8?

Then the:

 Section to Segment mapping:

section of the readelf tells us that:

0 is the .text segment. Aha, so this is why it is executable, and not writable
1 is the .data segment.

TODO where does this information come from? stackoverflow.com/questions/23018496/section-to-segment-mapping-in-elf-files

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ELF Hello World Tutorial / .rela.text Updated 2025-07-16

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Section type: sh_type == SHT_RELA.

Common name: "relocation section".

.rela.text holds relocation data which says how the address should be modified when the final executable is linked. This points to bytes of the text area that must be modified when linking happens to point to the correct memory locations.

Basically, it translates the object text containing the placeholder 0x0 address:

   a:       48 be 00 00 00 00 00    movabs $0x0,%rsi
  11:       00 00 00

to the actual executable code containing the final 0x6000d8:

4000ba: 48 be d8 00 60 00 00    movabs $0x6000d8,%rsi
4000c1: 00 00 00

It was pointed to by sh_info = 6 of the .symtab section.

readelf -r hello_world.o outputs:

Relocation section '.rela.text' at offset 0x3b0 contains 1 entries:
  Offset          Info           Type           Sym. Value    Sym. Name + Addend
00000000000c  000200000001 R_X86_64_64       0000000000000000 .data + 0

The section does not exist in the executable.

The actual bytes are:

00000370  0c 00 00 00 00 00 00 00  01 00 00 00 02 00 00 00  |................|
00000380  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|

The struct represented is:

typedef struct {
    Elf64_Addr  r_offset;
    Elf64_Xword r_info;
    Elf64_Sxword    r_addend;
} Elf64_Rela;

So:

370 0: r_offset = 0xC: address into the .text whose address this relocation will modify
370 8: r_info = 0x200000001. Contains 2 fields:
- ELF64_R_TYPE = 0x1: meaning depends on the exact architecture.
- ELF64_R_SYM = 0x2: index of the section to which the address points, so .data which is at index 2.
The AMD64 ABI says that type 1 is called R_X86_64_64 and that it represents the operation S + A where:
- S: the value of the symbol on the object file, here 0 because we point to the 00 00 00 00 00 00 00 00 of movabs $0x0,%rsi
- A: the addend, present in field r_added
This address is added to the section on which the relocation operates.
This relocation operation acts on a total 8 bytes.
380 0: r_addend = 0

So in our example we conclude that the new address will be: S + A = .data + 0, and thus the first thing in the data section.

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ELF Hello World Tutorial / .shstrtab Updated 2025-07-16

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Section type: sh_type == SHT_STRTAB.

Common name: "section header string table".

The section name .shstrtab is reserved. The standard says:

This section holds section names.

This section gets pointed to by the e_shstrnd field of the ELF header itself.

String indexes of this section are are pointed to by the sh_name field of section headers, which denote strings.

This section does not have SHF_ALLOC marked, so it will not appear on the executing program.

readelf -x .shstrtab hello_world.o

outputs:

Hex dump of section '.shstrtab':
  0x00000000 002e6461 7461002e 74657874 002e7368 ..data..text..sh
  0x00000010 73747274 6162002e 73796d74 6162002e strtab..symtab..
  0x00000020 73747274 6162002e 72656c61 2e746578 strtab..rela.tex
  0x00000030 7400                                t.

The data in this section has a fixed format: www.sco.com/developers/gabi/2003-12-17/ch4.strtab.html

If we look at the names of other sections, we see that they all contain numbers, e.g. the .text section is number 7.

Then each string ends when the first NUL character is found, e.g. character 12 is \0 just after .text\0.

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ELF Hello World Tutorial / SHT_STRTAB Updated 2025-07-16

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Sections with sh_type == SHT_STRTAB are called string tables.

They hold a null separated array of strings.

Such sections are used by other sections when string names are to be used. The using section says:

which string table they are using
what is the index on the target string table where the string starts

So for example, we could have a string table containing:

Data: \0 a b c \0 d e f \0
Index: 0 1 2 3  4 5 6 7  8

The first byte must be a 0. TODO rationale?

And if another section wants to use the string d e f, they have to point to index 5 of this section (letter d).

Notable string table sections:

.shstrtab
.strtab

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ELF Hello World Tutorial / SHT_SYMTAB on the executable Updated 2025-07-16

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By default, NASM places a .symtab on the executable as well.

This is only used for debugging. Without the symbols, we are completely blind, and must reverse engineer everything.

You can strip it with objcopy, and the executable will still run. Such executables are called "stripped executables".

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Entropy Updated 2025-07-16

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OK, can someone please just stop the philosophy and give numerical predictions of how entropy helps you predict the future?

The original notion of entropy, and the first one you should study, is the Clausius entropy.

For entropy in chemistry see: entropy of a chemical reaction.

Video 1.

The Unexpected Side of Entropy by Daan Frenkel

. Source. 2021.

Video 2.

The Biggest Ideas in the Universe | 20. Entropy and Information by Sean Carroll (2020)

Source. In usual Sean Carroll fashion, it glosses over the subject. This one might be worth watching. It mentions 4 possible definitions of entropy: Boltzmann, Gibbs, Shannon (information theory) and John von Neumann (quantum mechanics).

www.quantamagazine.org/what-is-entropy-a-measure-of-just-how-little-we-really-know-20241213/ What Is Entropy? A Measure of Just How Little We Really Know. on Quanta Magazine attempts to make the point that entropy is observer dependant. TODO details on that.

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ELF Hello World Tutorial / .strtab Updated 2025-07-16

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Holds strings for the symbol table.

This section has sh_type == SHT_STRTAB.

It is pointed to by sh_link == 5 of the .symtab section.

readelf -x .strtab hello_world.o

outputs:

Hex dump of section '.strtab':
  0x00000000 0068656c 6c6f5f77 6f726c64 2e61736d .hello_world.asm
  0x00000010 0068656c 6c6f5f77 6f726c64 0068656c .hello_world.hel
  0x00000020 6c6f5f77 6f726c64 5f6c656e 005f7374 lo_world_len._st
  0x00000030 61727400                            art.

This implies that it is an ELF level limitation that global variables cannot contain NUL characters.

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ELF Hello World Tutorial / STT_NOTYPE Updated 2025-07-16

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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 _start

hello_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 _start

in NASM. This is necessary since it must be seen as the entry point. Unlike in C, by default NASM labels are local.

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