currently  system  is atmost  8*65536  bytes long.   This
should be no problem even  in the future.  I want to keep
it simple.This  512 size kernel kB kernel  size should be
enough -  infact more would  mean we should have  to move
not just  these startup  routines, but also  do something
about  the cache memory(block  I/O devices.the  area left
over in the lower 640kB is meant for these.No oher memory
is assumed to  be "physical", ie all memory  above 1Mb is
demand paging. All addresses  under 1Mb are guaranteed to
match their	physical addresses.

$grep -r setup *
boot/head.s:	call setup_idt
boot/head.s:	call setup_gdt
boot/head.s:	mov %ax,%es		# reloaded in 'setup_gdt'
boot/head.s: *  setup_idt
boot/head.s:setup_idt:
boot/head.s: *  setup_gdt
boot/head.s:setup_gdt:
boot/head.s:	jmp setup_paging
boot/head.s:setup_paging:
include/linux/sys.h:extern int sys_setup();
include/linux/sys.h:fn_ptr sys_call_table[] = { sys_setup, sys_exit, sys_fork, sys_read,
include/unistd.h:#define __NR_setup	0	/* used only by init, to get system going */
init/main.c:static inline _syscall0(int,setup)
init/main.c: * Interrupts are still disabled. Do necessary setups, then
init/main.c:	setup();
kernel/hd.c:int sys_setup(void)

The  Interrupt Descriptor Table  (IDT) is  an array  of 8
byte   interrupt  descriptors   in   memory  devoted   to
specifying (at most)  256 interrupt service routines. The
first 32  entries are reserved  for processor exceptions,
and  any 16  of the  remaining  entries can  be used  for
hardware interrupts. The  rest are available for software
interrupts.

source : http://www.acm.uiuc.edu/sigops/roll_your_own/i386/idt.html


setup_idt:
lea ignore_int,%edx
movl$ox00080000,%eax
movw %dx,%ax
movw $0x8E00,%dx