karenn

The software which we need is

• nasm
• qemu

Assemble instruction is

nasm print_string.asm -f bin -o print_string.bin

The option -f bin is telling the assemble to produce raw machine code.

Then we will have a runnable file named print_string.bin,we can run this OS by

qemu-system-i386 print_string.bin

Switch from 16bit real mode to 32 protect mode:

1. define a GDT and it will be loaded into memory;
2. using cli to disable interrupts;
3. using instruction lgdt then we can save the address of GDT into gdtr;
4. change the first bit of CR0 to 1, now we are in 32bit protected mode;