#![no_std] Rust, Paging & Hardware Interrupts (IDT)

The Philosophy of Freedom OS

Freedom OS is an open-source bare-metal operating system kernel written from scratch in pure Rust without relying on the standard library (#![no_std]). In this environment, there is no heap allocator, no thread scheduler, no file system, and no runtime support. You are programming directly against bare-metal silicon.

Disabling the Runtime & Handling Panics

#![no_std]
#![no_main]

use core::panic::PanicInfo;

#[panic_handler]
fn panic(info: &PanicInfo) -> ! {
    // Print panic message directly to serial port or VGA memory
    unsafe {
        write_vga_error("KERNEL PANIC: Unrecoverable CPU fault");
    }
    loop {
        // Halt CPU to prevent execution runaway
        x86_64::instructions::hlt();
    }
}

#[no_mangle]
pub extern "C" fn _start() -> ! {
    // Entry point invoked by bootloader
    init_gdt();
    init_idt();
    unsafe { init_page_tables(); }
    
    loop {
        x86_64::instructions::hlt();
    }
}

Interrupt Descriptor Table (IDT) Configuration

When a hardware timer fires or a page fault occurs, the CPU jumps to a vector table indexed by the IDT. Rust handles this cleanly through the x86-interrupt ABI:

extern "x86-interrupt" fn page_fault_handler(
    stack_frame: InterruptStackFrame,
    error_code: PageFaultErrorCode,
) {
    let fault_address = x86_64::registers::control::Cr2::read();
    serial_println!("PAGE FAULT at {:#x}, error: {:?}", fault_address, error_code);
    loop { x86_64::instructions::hlt(); }
}