/*-
* Copyright (c) 2003 The NetBSD Foundation, Inc.
* All rights reserved.
*
* This code is derived from software contributed to The NetBSD Foundation
* by David Laight.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
* ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
* TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
* PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
* BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
* CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
* SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
* INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
* CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
* ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
* POSSIBILITY OF SUCH DAMAGE.
*/
/*
* Loosely based on code from stand/lib/libcrt/bootsect/start_bootsect.S
*/
/*
* real_to_prot()
*
* Switch CPU to 32bit protected mode to execute C.
*
* NB: Call with the 32bit calll instruction so that a 32 bit
* return address is pushed.
*
* All registers are preserved, %ss:%esp will point to the same
* place as %ss:%sp did, although the actual value of %esp might
* be changed.
*
* Interrupts are disabled while we are in 32bit mode to save us
* having to setup a different IDT. This code is only used during
* the boot process and it doesn't use any interrupts.
*/
ENTRY(real_to_prot)
.code16
pushl %eax
cli
lgdt %cs:gdtarg /* Global descriptor table */
movl %cr0, %eax
or $CR0_PE, %ax
movl %eax, %cr0 /* Enter 'protected mode' */
ljmp $bootcodeseg, $1f /* Jump into a 32bit segment */
1:
.code32
/* Set all the segment registers to map the same area as the code */
mov $bootdataseg, %eax
mov %ax, %ds
mov %ax, %es
mov %ax, %ss
addl stkdif, %esp /* Allow for real %ss != %ds */
popl %eax
ret
/*
* prot_to_real()
*
* Switch CPU back to 16bit real mode in order to call system bios functions.
*
* All registers are preserved, except that %sp may be changed so that
* %ss:%sp points to the same memory.
* Note that %ebp is preserved and will not reference the correct part
* of the stack.
*
* Interrupts are enabled while in real mode.
*
* Based on the description in section 14.5 of the 80386 Programmer's
* reference book.
*/
/*
* EPIA_HACK
*
* VIA C3 processors (Eden, Samuel 2) don't seem to correctly switch back to
* executing 16 bit code after the switch to real mode and subsequent jump.
*
* It is speculated that the CPU is prefetching and decoding branch
* targets and not invalidating this buffer on the long jump.
* Further investigation indicates that the caching of return addresses
* is most likely the problem.
*
* Previous versions just used some extra call/ret and a few NOPs, these
* only helped a bit, but booting compressed kernels would still fail.
*
* Trashing the return address stack (by doing 'call' without matched 'ret')
* Seems to fix things completely. 1 iteration isn't enough, 16 is plenty.
*/
ENTRY(prot_to_real)
.code32
pushl %eax
#ifdef EPIA_HACK
push %ecx
push $0x10
pop %ecx
1: call trash_return_cache
loop 1b
pop %ecx
#endif
/*
* Load the segment registers while still in protected mode.
* Otherwise the control bits don't get changed.
* The correct base addresses are loaded later.
*/
movw $bootrealdata, %ax
movw %ax, %ds
movw %ax, %es
movw %ax, %ss
/*
* Load %cs with a segment that has the correct attributes for
* 16bit operation.
*/
ljmp $bootrealseg, $1f
1:
/* Jump far indirect to load real mode %cs */
ljmp *%cs:toreal
xreal:
/*
* CPU is now in real mode, load the other segment registers
* with their correct base addresses.
*/
mov %cs, %ax
mov %ax, %ds
mov %ax, %es
/*
* If stack was above 64k, 16bit %ss needs to be different from
* 32bit %ss (and the other segment registers).
*/
mov stkseg, %ax
mov %ax, %ss
subl stkdif, %esp
/* Check we are returning to an address below 64k */
push %bp
movw %sp, %bp
movw 2/*bp*/ + 4/*eax*/ + 2(%bp), %ax /* high bits ret addr */
test %ax, %ax
jne 1f
pop %bp
