Assembly

Introduction

Assembly is the lowest level of language (just above binary).

It is platform specific (different for each CPU architecture: x86, x86-64, ARM, ...).

It was used to write code close to the hardware, but is now mostly used for reverse engineering (understanding how malwares work).

It is one of fastest language, with C, but also the most difficult to write & maintain. Today, some C compilers can compile C code directly to binary, without using assembly. That's why assembly is less used than before.

Types

Type	Declaration	Size
Byte	db	1 byte
Word	dw	2 bytes
Double word	dd	4 bytes
Quadruple word	dq	8 bytes
Ten bytes	dt	10 bytes

Example:

choice      DB    'y'
number      DW    65535
negative    DD    -2147483648
big         DQ    9223372036854775807
very_big    DT    18446744073709551615

Note:

• Characters are stored as its ASCII value in hexadecimal;

• There is no signed & unsigned types differentiation;

• Negative values are automatically converted to its 2's complement representation;

• There is no floating point & integer types differentiation;

• float & double are represented using 32 bits & 64 bits respectively.

Arrays

• Manual definition:

name: type value1, value2, ...

• Automatic definition:

name: times number_of_elements type value

Note: ALL elements are initialized to value.

• Uninitialized definition:

name: res_type number_of_elements

Example:

    array1: db 1, 2, 3, 4, 5 ; Define entire array manually
    array2: times 5 db 0     ; 5 elements of type byte initialized to 0
    array3: resb 5           ; 5 elements of type byte uninitialized

Format

Format	Data type
%d	DWORD
%hd	WORD
%hhd	BYTE

Input

• rdi is used to store the format of the input (e.g. %d).

• rsi is used to store the address of the variable where the input will be stored.

• rax is equal to 1 if we scan a float, 0 otherwise.

extern scanf

global main

section .data
    fmt_scan: db '%hd', 0 ; 0 is the null terminator (end of string)

section .bss
    user_input: resb 2 ; resb = reserve bytes for uninitialized data

section .text
main:
    mov rdi, fmt_scan 
    mov rsi, user_input
    mov rax, 0
    call scanf

Output

• rdi is used to store the format of the output (e.g. %d).

• rsi is used to store the value to be printed.

• rax is equal to 1 if we print a float, 0 otherwise.

extern printf

global main

section .data
    fmt_print db 'The number is %hhd', 10, 0 ; 10 is the newline character
    number db 10

main:
    mov rdi, fmt_print
    mov rsi, number
    mov rax, 0
    call printf

Conditions

cmp is used to compare two values. It subtracts the second value from the first one and sets the flags ZF, SF, CF & OF. Then we can use j instructions to jump to a label if a condition is true.

Condition	Instruction	Sign
Equal	je	signed or unsigned
Not equal	jne	signed or unsigned
Above	ja	unsigned
Above or equal	jae	unsigned
Below	jb	unsigned
Below or equal	jbe	unsigned
Greater	jg	signed
Greater or equal	jge	signed
Lower	jl	signed
Lower or equal	jle	signed

greater:
    mov     eax, 1

main:
    cmp     eax, ebx
    jg      greater

Compilation

First, we need to assemble the assembly code to get the object file:

nasm -f elf64 -o file.o file.asm

• -f elf64 is used to specify the format of the object file. elf64 is used for x86-64.

• -o file.o is used to specify the object file.

• The last argument is the assembly code file.

Then, we need to link the object file to get the executable:

gcc -m64 -no-pie -o file file.o

• -m64 is used to specify the architecture of the executable. 64 is used for x86-64.

• -no-pie is used to specify that we don't want a position independent executable.

• -o file is used to specify the executable name we will create.

• The last argument is the object file.

Notes

• If [rax] is used but no valid memory address is stored in rax, there is a segmentation fault.

• [rax + 5] means that we take the value stored 5 bytes after the address stored in rax. If rax is 0x1000, then [rax + 5] is the value stored at 0x1005. Same for -.

References

• Tutorials point

• x86_64 Linux Assembly #2 - "Hello, World!" Breakdown