Assembly - Getting Started

About

An Assembly language script is the input expected by an assembler (compiler). The assembler defines then the precise syntax of the script but every assembler share a common set of syntax rule.

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Step by Step

Install the assembler

In this tutorial, we are using nasm.

With apt-get on ubuntu.

apt-get install -y nasm

Windows: http://www.nasm.us/pub/nasm/releasebuilds/2.09.04/win32/

Create a assembly file

With Vim - Vim, gvim, vi Executable Binary

vim script.asm

Content

jmp eax

Compile

Bin

The bin format is a flat binary format without any extra information (metadata) such as source file name, …. It's pure machine instruction.

Create a bin file

nasm -f bin script.asm -o script.bin

With hexdump, show the content as an hex_dump representation of the script.bin file

# hd (short for hexdump)
hd script.bin

00000000  66 ff e0                                          |f..|
00000003

We can see that the file only consists of 3 bytes: 66 ff e0, which is equivalent to the assembly instruction jmp eax.

Elf

Create a ELF file

nasm -f elf script.asm -o script.elf

Show the content with hexdump in a hex_dump output representation

# hd (short for hexdump)
hd script

00000000  7f 45 4c 46 01 01 01 00  00 00 00 00 00 00 00 00  |.ELF............|
00000010  01 00 03 00 01 00 00 00  00 00 00 00 00 00 00 00  |................|
00000020  40 00 00 00 00 00 00 00  34 00 00 00 00 00 28 00  |@.......4.....(.|
00000030  05 00 02 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000040  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00000060  00 00 00 00 00 00 00 00  01 00 00 00 01 00 00 00  |................|
00000070  06 00 00 00 00 00 00 00  10 01 00 00 02 00 00 00  |................|
00000080  00 00 00 00 00 00 00 00  10 00 00 00 00 00 00 00  |................|
00000090  07 00 00 00 03 00 00 00  00 00 00 00 00 00 00 00  |................|
000000a0  20 01 00 00 21 00 00 00  00 00 00 00 00 00 00 00  | ...!...........|
000000b0  01 00 00 00 00 00 00 00  11 00 00 00 02 00 00 00  |................|
000000c0  00 00 00 00 00 00 00 00  50 01 00 00 30 00 00 00  |........P...0...|
000000d0  04 00 00 00 03 00 00 00  04 00 00 00 10 00 00 00  |................|
000000e0  19 00 00 00 03 00 00 00  00 00 00 00 00 00 00 00  |................|
000000f0  80 01 00 00 0c 00 00 00  00 00 00 00 00 00 00 00  |................|
00000100  01 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000110  ff e0 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000120  00 2e 74 65 78 74 00 2e  73 68 73 74 72 74 61 62  |..text..shstrtab|
00000130  00 2e 73 79 6d 74 61 62  00 2e 73 74 72 74 61 62  |..symtab..strtab|
00000140  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00000160  01 00 00 00 00 00 00 00  00 00 00 00 04 00 f1 ff  |................|
00000170  00 00 00 00 00 00 00 00  00 00 00 00 03 00 01 00  |................|
00000180  00 73 63 72 69 70 74 2e  61 73 6d 00 00 00 00 00  |.script.asm.....|
00000190