Welcome to FoolOS

=================
Copyright M.Idziorek 2014 <m.i@gmx.at>

Disclaimer
----------

This is a simple and useless "operating system", with no features,
sorry. Currently it is just the rotting fruit of my explorations of 
the 32-bit protected mode and interrupt handling

NOTES
=====

cli; disable interrupts
lgdt
lidt
sti; enable interrupts

Linux Startup x86
=================

1. arch/x86/boot/header.S

Contains the header and linux 16 bit code.

This code should be entered with a bootloader at the address specified 
within the header (as _start) which will put us at 'start_of_setup'

Direct loading will put us at 'start2' at the very start which will
show an error message

If everyhing goes right we will enter the 16-bit real mode C module 
with:

call main ; should not return

2. arch/x86/boot/main.c

void main(void) will do some checks and detections (cpu,mem,..) and
invoke: go_to_protected_mode();

3. arch/x86/boot/pm.c

go_to_protected_mode() - will disable interrupts and set up the initial 
idt and gdt descriptor tables before calling: protected_mode_jump() and 
passing the address of code32_start.

4. arch/x86/boot/pmjump.S

back in assembly-world the actual transition is made inside 
'protected_mode_jump' and we move on to 'in_pm32' where the data segment
is set up and we jmpl to the 32-bit entry point of the kernel.

5. arch/x86/kernel/head32.c (assumption!?)

void i386_start_kernel(void)

6. init/main.c

start_kernel(void)

setup_arch()!!

Interrupts
==========

arch/x86/include/asm/irq_vectors.h each entry is 8 bytes

ISA interrupts:

0x30 0x0000 0x0000 0x0000 0x0000
0x31
0x32
..
0x3f