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/* -----------------------------------------------------------------------
*
* Copyright 2007-2008 H. Peter Anvin - All Rights Reserved
*
* Permission is hereby granted, free of charge, to any person
* obtaining a copy of this software and associated documentation
* files (the "Software"), to deal in the Software without
* restriction, including without limitation the rights to use,
* copy, modify, merge, publish, distribute, sublicense, and/or
* sell copies of the Software, and to permit persons to whom
* the Software is furnished to do so, subject to the following
* conditions:
*
* The above copyright notice and this permission notice shall
* be included in all copies or substantial portions of the Software.
*
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
* EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
* OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
* NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
* HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
* WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
* FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
* OTHER DEALINGS IN THE SOFTWARE.
*
* ----------------------------------------------------------------------- */
/*
* Modified MBR code used on an ISO image in hybrid mode.
*
* This doesn't follow the El Torito spec at all -- it is just a stub
* loader of a hard-coded offset, but that's good enough to load
* ISOLINUX.
*/
.code16
.text
HYBRID_MAGIC = 0x7078c0fb
isolinux_hybrid_signature = 0x7c00+64
isolinux_start_hybrid = 0x7c00+64+4
.globl bootsec
/* Important: the top 6 words on the stack are passed to isolinux.bin */
stack = 0x7c00
driveno = (stack-6)
ebios_flag = (stack-8)
sectors = (stack-10)
heads = (stack-12)
secpercyl = (stack-16)
BIOS_page = 0x462
/* gas/ld has issues with doing this as absolute addresses... */
.section ".bootsec", "a", @nobits
.globl bootsec
bootsec:
.space 512
.text
.globl _start
_start:
cli
xorw %ax, %ax
movw %ax, %ds
movw %ax, %ss
movw $stack, %sp
movw %sp, %si
pushw %es /* es:di -> $PnP header */
pushw %di
pushw %dx /* dl -> drive number */
movw %ax, %es
sti
cld
/* Copy down to 0:0x600 */
movw $_start, %di
movw $(512/2), %cx
rep; movsw
ljmpw $0, $next
next:
/* Check to see if we have EBIOS */
pushw %dx /* drive number */
movb $0x41, %ah /* %al == 0 already */
movw $0x55aa, %bx
xorw %cx, %cx
xorb %dh, %dh
stc
int $0x13
jc 1f
cmpw $0xaa55, %bx
jne 1f
andw $1,%cx /* Bit 0 = fixed disk subset */
jz 1f
/* We have EBIOS; patch in the following code at
read_sector_cbios: movb $0x42, %ah ; jmp read_common */
movl $0xeb42b4+((read_common-read_sector_cbios-4) << 24), \
(read_sector_cbios)
1:
popw %dx
pushw %cx /* Save EBIOS flag */
/* Get (C)HS geometry */
movb $0x08, %ah
int $0x13
andw $0x3f, %cx /* Sector count */
pushw %cx /* Save sectors on the stack */
movzbw %dh, %ax /* dh = max head */
incw %ax /* From 0-based max to count */
pushw %ax /* Save heads on the stack */
mulw %cx /* Heads*sectors -> sectors per cylinder */
/* Save sectors/cylinder on the stack */
pushw %dx /* High word */
pushw %ax /* Low word */
/*
* Load sectors. We do this one at a time mostly to avoid
* pitfalls and to share code with the stock MBR code.
*/
movw $0x7c00, %bx
movw $4, %cx /* Sector count */
movl (lba_offset), %eax
2:
call read_sector
jc disk_error
incl %eax
addb $(512 >> 8), %bh
loopw 2b
/*
* Okay, that actually worked... update the stack pointer
* and jump into isolinux.bin...
*/
cmpl $HYBRID_MAGIC,(isolinux_hybrid_signature)
jne bad_signature
cli
movw $heads, %sp
jmp isolinux_start_hybrid
bad_signature:
call error
.ascii "isolinux.bin missing or corrupt.\r\n"
/*
* read_sector: read a single sector pointed to by %eax to %es:%bx.
* CF is set on error. All registers saved.
*/
read_sector:
pushal
xorl %edx, %edx
pushl %edx /* MSW of LBA */
pushl %eax /* LSW of LBA */
pushw %es /* Buffer segment */
pushw %bx /* Buffer offset */
pushw $1 /* Sector count */
pushw $16 /* Size of packet */
movw %sp, %si
/* This chunk is skipped if we have ebios */
/* Do not clobber %eax before this chunk! */
/* This also relies on %bx and %edx as set up above. */
read_sector_cbios:
divl (secpercyl)
shlb $6, %ah
movb %ah, %cl
movb %al, %ch
xchgw %dx, %ax
divb (sectors)
movb %al, %dh
orb %ah, %cl
incw %cx /* Sectors are 1-based */
movw $0x0201, %ax
read_common:
movb (driveno), %dl
int $0x13
leaw 16(%si), %sp /* Drop DAPA */
popal
ret
disk_error:
call error
.ascii "Operating system load error.\r\n"
/*
* Print error messages. This is invoked with "call", with the
* error message at the return address.
*/
error:
popw %si
2:
lodsb
movb $0x0e, %ah
movb (BIOS_page), %bh
movb $0x07, %bl
int $0x10 /* May destroy %bp */
cmpb $10, %al /* Newline? */
jne 2b
int $0x18 /* Boot failure */
die:
hlt
jmp die
/* Address of pointer to isolinux.bin */
lba_offset = _start+432
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