Assembly Language: 3 System Calls - X86 (32 BIT) Arch #assembly #assemblylanguage

A system call in assembly language on x86 processors is a mechanism that allows user-level programs to request services and interact with the operating system's kernel. These services can include operations such as file I/O, process management, memory management, and other low-level functionalities that are typically restricted to privileged code.

In x86 assembly, system calls are invoked using the "int 0x80" instruction or the more modern "syscall" instruction (available on newer x86-64 processors). The process of making a system call involves setting up specific registers with the desired service number and any required arguments, triggering a software interrupt (or "trap") to transition from user mode to kernel mode, where the operating system's kernel handles the request, and then returning the result back to the user-level program.

The typical steps involved in making a system call in x86 assembly are as follows:

Load the system call number into the "eax" register: The system call number represents the specific service or operation that the program wants to request from the kernel. The exact value of the system call number is architecture and operating system-dependent, and it is defined by the operating system's API (Application Programming Interface).

Load any required arguments into other registers: Depending on the specific system call, certain arguments might need to be passed to the kernel to provide additional information about the request. The number and placement of these arguments are also defined by the operating system's API.

Execute the "int 0x80" or "syscall" instruction: The "int 0x80" instruction is used in 32-bit mode, while the "syscall" instruction is used in 64-bit mode. When this instruction is executed, it triggers a software interrupt that causes the CPU to switch from user mode to kernel mode, allowing the operating system's kernel to handle the system call.

Handle the system call in kernel mode: Upon receiving the software interrupt, the kernel takes control and processes the system call. It validates the system call number, extracts the arguments from the appropriate registers, performs the requested operation, and returns any results to the user-level program.

Return to user mode: Once the kernel has completed the system call, it returns the control back to the user-level program, and the program can continue its execution with the result of the system call (if any) now available in the appropriate register.

It's important to note that making system calls directly in assembly language is usually avoided in higher-level programming languages, as they provide more user-friendly abstractions through system call wrappers and libraries. However, understanding how system calls work at the assembly level is crucial for low-level programming and operating system development. Additionally, modern compilers can automatically generate the necessary code for system calls when writing code in languages like C or C++.

I hope you enjoy this tutorial, stay tunned for next Assembly tutorial where we are going to discuss addressing modes.

Timestamp:
System Calls presentation: 00:00
Writing the program: 03:15

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[rkroy]

Please continue sir....Thank you so much.

[Aluna]

I love use linux but for my work I have to use windows, do you know something like bash + vim + tmux that could use with windowpowershell?, and thanks for your videos, this is so damn good info!!!