New articles - OurBigBook.com

Ciro Santilli 40 Updated 2025-07-16

Let's break down a minimal runnable Linux x86-64 example:

hello_world.asm

section .data
    hello_world db "Hello world!", 10
    hello_world_len  equ $ - hello_world
section .text
    global _start
    _start:
        mov rax, 1
        mov rdi, 1
        mov rsi, hello_world
        mov rdx, hello_world_len
        syscall
        mov rax, 60
        mov rdi, 0
        syscall

Compiled with:

nasm -w+all -f elf64 -o 'hello_world.o' 'hello_world.asm'
ld -o 'hello_world.out' 'hello_world.o'

TODO: use a minimal linker script with -T to be more precise and minimal.

Versions:

NASM 2.10.09
Binutils version 2.24 (contains ld)
Ubuntu 14.04

We don't use a C program as that would complicate the analysis, that will be level 2 :-)

 Read the full article

Chinese garden by

Ciro Santilli 40 Updated 2025-07-16

 View more

 Read the full article

Macintosh by

Ciro Santilli 40 Updated 2025-07-16

 Read the full article

ELF Hello World Tutorial / Object hd by

Ciro Santilli 40 Updated 2025-07-16

 View more

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

 Read the full article

ELF Hello World Tutorial / Global file structure by

Ciro Santilli 40 Updated 2025-07-16

 View more

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 :-)

 Read the full article

ELF Hello World Tutorial / Section vs segment by

Ciro Santilli 40 Updated 2025-07-16

 View more

We will get into more detail later, but it is good to have it in mind now:

section: exists before linking, in object files.
One ore more sections will be put inside a single segment by the linker.
Major information sections contain for the linker: is this section:
- raw data to be loaded into memory, e.g. .data, .text, etc.
- or metadata about other sections, that will be used by the linker, but disappear at runtime e.g. .symtab, .srttab, .rela.text
segment: exists after linking, in the executable file.
Contains information about how each segment should be loaded into memory by the OS, notably location and permissions.

See also:

 Read the full article

ELF Hello World Tutorial / Sections by

Ciro Santilli 40 Updated 2025-07-16

 Read the full article

ELF Hello World Tutorial / Index 0 section by

Ciro Santilli 40 Updated 2025-07-16

 View more

Contained in bytes 0x40 to 0x7F.

The first section is always magic: www.sco.com/developers/gabi/2003-12-17/ch4.sheader.html says:

If the number of sections is greater than or equal to SHN_LORESERVE (0xff00), e_shnum has the value SHN_UNDEF (0) and the actual number of section header table entries is contained in the sh_size field of the section header at index 0 (otherwise, the sh_size member of the initial entry contains 0).

There are also other magic sections detailed in Figure 4-7: Special Section Indexes.

 Read the full article

Synthetic chromosome by

Ciro Santilli 40 Updated 2025-07-16

 View more

Basically a synonym for doing a large chunk of de novo DNA synthesis.

 Read the full article

Four Gentlemen by

Ciro Santilli 40 Updated 2025-07-16

 View more

www.pinterest.co.uk/pin/315744623848820315

Figure 1.
Plum, orchid, bamboo, chrysanthemum Chinese painting by Zheng Xie
. Source. This is an example of bird-and-flower painting, of which the Four Gentlemen are a common theme. TODO date.

 Read the full article

Stone garden by

Ciro Santilli 40 Updated 2025-07-16

 View more

zh.wikipedia.org/wiki/假山

The literal Chinese name says it all: "Fake Mountain". The stones evoke the feeling of the beautiful rock mountains of China.

The term "奇石假山" (qi2 shi2 jia3 shan1, lit. "weird shaped stone fake mountain") is also used, almost as a synonym by many people, since the stones are often chose in interesting shapes. Choosing the right stone is basically an art form in itself.

The stones used are generally limestone, which as a sedimentary rock is weaker, and more likely to be eroded into interesting shapes.

 Read the full article

ELF Hello World Tutorial / SHT_NULL by

Ciro Santilli 40 Updated 2025-07-16

 View more

In index 0, SHT_NULL is mandatory. Are there any other uses for it: stackoverflow.com/questions/26812142/what-is-the-use-of-the-sht-null-section-in-elf ?

 Read the full article

ELF Hello World Tutorial / SHT_STRTAB by

Ciro Santilli 40 Updated 2025-07-16

 View more

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

 Read the full article

ELF Hello World Tutorial / .shstrtab by

Ciro Santilli 40 Updated 2025-07-16

 View more

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.

 Read the full article

Chinese history by

Ciro Santilli 40 Updated 2025-07-16

 Read the full article

ELF Hello World Tutorial / STT_FILE by

Ciro Santilli 40 Updated 2025-07-16

 View more

Entry 1 has ELF64_R_TYPE == STT_FILE. ELF64_R_TYPE is continued inside of st_info.

Byte analysis:

10 8: st_name = 01000000 = character 1 in the .strtab, which until the following \0 makes hello_world.asm
This piece of information file may be used by the linker to decide on which segment sections go: e.g. in ld linker script we write:
```
segment_name :
{
    file(section)
}
```
to pick a section from a given file.
Most of the time however, we will just dump all sections with a given name together with:
```
segment_name :
{
    *(section)
}
```
10 12: st_info = 04
Bits 0-3 = ELF64_R_TYPE = Type = 4 = STT_FILE: the main purpose of this entry is to use st_name to indicate the name of the file which generated this object file.
Bits 4-7 = ELF64_ST_BIND = Binding = 0 = STB_LOCAL. Required value for STT_FILE.
10 13: st_shndx = Symbol Table Section header Index = f1ff = SHN_ABS. Required for STT_FILE.
20 0: st_value = 8x 00: required for value for STT_FILE
20 8: st_size = 8x 00: no allocated size

Now from the readelf, we interpret the others quickly.

 Read the full article

Chinese dynasty by

Ciro Santilli 40 Updated 2025-07-16

 Read the full article

CCP dynasty by

Ciro Santilli 40 Updated 2025-07-16

 Read the full article

ELF Hello World Tutorial / STT_SECTION by

Ciro Santilli 40 Updated 2025-07-16

 View more

There are two such entries, one pointing to .data and the other to .text (section indexes 1 and 2).

Num:    Value          Size Type    Bind   Vis      Ndx Name
  2: 0000000000000000     0 SECTION LOCAL  DEFAULT    1
  3: 0000000000000000     0 SECTION LOCAL  DEFAULT    2

TODO what is their purpose?

 Read the full article

ELF Hello World Tutorial / STT_NOTYPE by

Ciro Santilli 40 Updated 2025-07-16

 View more

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.

 Read the full article

 Pinned article: Introduction to the OurBigBook Project

Welcome to the OurBigBook Project! Our goal is to create the perfect publishing platform for STEM subjects, and get university-level students to write the best free STEM tutorials ever.

Everyone is welcome to create an account and play with the site: ourbigbook.com/go/register. We belive that students themselves can write amazing tutorials, but teachers are welcome too. You can write about anything you want, it doesn't have to be STEM or even educational. Silly test content is very welcome and you won't be penalized in any way. Just keep it legal!

Video 1.

Intro to OurBigBook

. Source.

We have two killer features:

topics: topics group articles by different users with the same title, e.g. here is the topic for the "Fundamental Theorem of Calculus" ourbigbook.com/go/topic/fundamental-theorem-of-calculus
Articles of different users are sorted by upvote within each article page. This feature is a bit like:
- a Wikipedia where each user can have their own version of each article
- a Q&A website like Stack Overflow, where multiple people can give their views on a given topic, and the best ones are sorted by upvote. Except you don't need to wait for someone to ask first, and any topic goes, no matter how narrow or broad
This feature makes it possible for readers to find better explanations of any topic created by other writers. And it allows writers to create an explanation in a place that readers might actually find it.
Figure 1.
Screenshot of the "Derivative" topic page
. View it live at: ourbigbook.com/go/topic/derivative
Video 2.
OurBigBook Web topics demo
. Source.
local editing: you can store all your personal knowledge base content locally in a plaintext markup format that can be edited locally and published either:
- to OurBigBook.com to get awesome multi-user features like topics and likes
- as HTML files to a static website, which you can host yourself for free on many external providers like GitHub Pages, and remain in full control
This way you can be sure that even if OurBigBook.com were to go down one day (which we have no plans to do as it is quite cheap to host!), your content will still be perfectly readable as a static site.
Figure 2.
You can publish local OurBigBook lightweight markup files to either https://OurBigBook.com or as a static website
.
Figure 3.
Visual Studio Code extension installation
.
Figure 4.
Visual Studio Code extension tree navigation
.
Figure 5.
Web editor
. You can also edit articles on the Web editor without installing anything locally.
Video 3.
Edit locally and publish demo
. Source. This shows editing OurBigBook Markup and publishing it using the Visual Studio Code extension.
Video 4.
OurBigBook Visual Studio Code extension editing and navigation demo
. Source.
Internal cross file references done right:
Infinitely deep tables of contents:
Figure 6.
Dynamic article tree with infinitely deep table of contents
.
Live URL: ourbigbook.com/cirosantilli/chordate
Descendant pages can also show up as toplevel e.g.: ourbigbook.com/cirosantilli/chordate-subclade