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* This software and ancillary information (herein called SOFTWARE)
* called LinuxBIOS is made available under the terms described here.
* The SOFTWARE has been approved for release with associated
* LA-CC Number 00-34. Unless otherwise indicated, this SOFTWARE has
* been authored by an employee or employees of the University of
* California, operator of the Los Alamos National Laboratory under
* Contract No. W-7405-ENG-36 with the U.S. Department of Energy.
* The U.S. Government has rights to use, reproduce, and distribute this
* SOFTWARE. The public may copy, distribute, prepare derivative works
* and publicly display this SOFTWARE without charge, provided that this
* Notice and any statement of authorship are reproduced on all copies.
* Neither the Government nor the University makes any warranty, express
* or implied, or assumes any liability or responsibility for the use of
* this SOFTWARE. If SOFTWARE is modified to produce derivative works,
* such modified SOFTWARE should be clearly marked, so as not to confuse
* it with the version available from LANL.
* Copyright (C) 2000, Ron Minnich
* Advanced Computing Lab, LANL
/* Start code to put an i386 or later processor into 32-bit protected mode.
#include <arch/rom_segs.h>
.globl _start16bit
.type _start16bit, @function
/* Save the BIST result */
movl %eax, %ebp
/* IMMEDIATELY invalidate the translation lookaside buffer (TLB) before
* executing any further code. Even though paging is disabled we
* could still get false address translations due to the TLB if we
* didn't invalidate it. Thanks to for this TLB fix.
xorl %eax, %eax
movl %eax, %cr3 /* Invalidate TLB*/
/* Invalidating the cache here seems to be a bad idea on
* modern processors. Don't.
* If we are hyperthreaded or we have multiple cores it is bad,
* for SMP startup. On Opterons it causes a 5 second delay.
* Invalidating the cache was pure paranoia in any event.
* If you cpu needs it you can write a cpu dependent version of
/* Note: gas handles memory addresses in 16 bit code very poorly.
* In particular it doesn't appear to have a directive allowing you
* associate a section or even an absolute offset with a segment register.
* This means that anything except cs:ip relative offsets are
* a real pain in 16 bit mode. And explains why it is almost
* impossible to get gas to do lgdt correctly.
* One way to work around this is to have the linker do the
* math instead of the assembler. This solves the very
* pratical problem of being able to write code that can
* be relocated.
* An lgdt call before we have memory enabled cannot be
* position independent, as we cannot execute a call
* instruction to get our current instruction pointer.
* So while this code is relocateable it isn't arbitrarily
* relocatable.
* The criteria for relocation have been relaxed to their
* utmost, so that we can use the same code for both
* our initial entry point and startup of the second cpu.
* The code assumes when executing at _start16bit that:
* (((cs & 0xfff) == 0) and (ip == _start16bit & 0xffff))
* or
* ((cs == anything) and (ip == 0)).
* The restrictions in mean that _start16bit initially
* must be loaded at or above 0xffff0000 or below 0x100000.
* The linker scripts computes gdtptr16_offset by simply returning
* the low 16 bits. This means that the intial segment used
* when start is called must be 64K aligned. This should not
* restrict the address as the ip address can be anything.
* Also load an IDT with NULL limit to prevent the 16bit IDT being used
* in protected mode before c_start.S sets up a 32bit IDT when entering
* ram stage. In practise: CPU will shutdown on any exception.
* See IA32 manual Vol 3A 19.26 Interrupts.
movw %cs, %ax
shlw $4, %ax
movw $nullidt_offset, %bx
subw %ax, %bx
lidt %cs:(%bx)
movw $gdtptr16_offset, %bx
subw %ax, %bx
lgdtl %cs:(%bx)
movl %cr0, %eax
andl $0x7FFAFFD1, %eax /* PG,AM,WP,NE,TS,EM,MP = 0 */
orl $0x60000001, %eax /* CD, NW, PE = 1 */
movl %eax, %cr0
/* Restore BIST to %eax */
movl %ebp, %eax
/* Now that we are in protected mode jump to a 32 bit code segment. */
ljmpl $ROM_CODE_SEG, $__protected_start
* The gdt is defined in, it has a 4 Gb code segment
* at 0x08, and a 4 GB data segment at 0x10;
.align 4
.globl gdtptr16
.word gdt_end - gdt -1 /* compute the table limit */
.long gdt /* we know the offset */
.align 4
.globl nullidt
.word 0 /* limit */
.long 0
.word 0
.globl _estart16bit