在修改程序集中编写bootsector代码之后,我想知道我是否可以在C中执行相同操作.到目前为止,空函数中的代码生成如下:
在C:
void _start() {
halt:
goto halt;
}
随后的asm(由GCC生成):
7c00: 55 push %bp
7c01: 89 e5 mov %sp,%bp
7c03: eb fe jmp 0x7c03
但是,是否可以指定对于此(入口)函数,我不希望初始化基本和堆栈指针？BIOS将控制权直接传输到0x7c00,因此设置堆栈指针的前两条指令是冗余的.
我已经尝试添加__attribute__ ((always_inline, noreturn, regparm(0)))到函数声明,但似乎没有做任何事情.
1> Craig Estey..：
我讨厌劝阻一个令人钦佩的想法,但是用C语言编写PC MBR引导块是非常困难的(如果不是不可能的话),原因有几个,主要与PC /传统BIOS引导的怪癖有关:
BIOS将加载您的块,但然后将在精确位置写入它以提供信息.这是"引导参数块"[BPB].除此之外,它还有从中引导的驱动器号.
BPB从偏移量0x03开始.所以,前3个字节必须有一个jmpasm指令来跳过这个.
扇区的最后两个字节必须具有BIOS引导签名0x55AA
分区表的精确位置必须有空间[将由分区编辑器覆盖 fdisk
剩下宝贵的空间,代码很棘手,并且与BIOS密切相关.您必须能够使用intasm指令来查询BIOS并发出磁盘读取调用.它们具有精确的序列,对于某些人,您必须能够修改x86 段寄存器以及通过字节加载控制通用寄存器(例如,您必须能够%ah独立设置值%al).
大多数靴子[几乎都是]在装配中写入引导扇区.为了节省空间,一些引导块执行一些代码然后用数据覆盖该代码,因为代码被执行一次并且空间被重用.这在C中很难做到.
当您调整C程序时,您可能会发现它不适合512字节.
在这种情况下grub,其引导块在引导块之后从该区域加载一个或两个扇区并转移控制.使用引导块设置的参数,可以读取更多内容grub. grub从第二个扇区到第一个分区[起始于1MB],"共同选择""无人区"的一部分.然后,传输控制,然后加载剩余部分并决定如何引导操作系统.后期阶段用C语言写成.
对于Windows,MBR只会加载标记为可引导的分区的第一个扇区,然后转移控制.第二阶段是NT的启动加载程序.它知道在NTFS文件系统中它之后的前N个块包含引导的其余部分,因此它只是按顺序加载它们并传输控制.
请注意,第二阶段引导有一些组装,但在加载和探测一点之后,它们可以切换到C代码.这种方法是我推荐的.
举个例子,这里是引导块的来源grub2:
/* -*-Asm-*- */
/*
* GRUB -- GRand Unified Bootloader
* Copyright (C) 1999,2000,2001,2002,2005,2006,2007,2008,2009 Free Software Foundation, Inc.
*
* GRUB is free software: you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation, either version 3 of the License, or
* (at your option) any later version.
*
* GRUB is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with GRUB. If not, see .
*/
#include
#include
/*
* defines for the code go here
*/
/* Print message string */
#define MSG(x) movw $x, %si; call LOCAL(message)
#define ERR(x) movw $x, %si; jmp LOCAL(error_message)
.file "boot.S"
.text
/* Tell GAS to generate 16-bit instructions so that this code works
in real mode. */
.code16
.globl _start, start;
_start:
start:
/*
* _start is loaded at 0x7c00 and is jumped to with CS:IP 0:0x7c00
*/
/*
* Beginning of the sector is compatible with the FAT/HPFS BIOS
* parameter block.
*/
jmp LOCAL(after_BPB)
nop /* do I care about this ??? */
/*
* This space is for the BIOS parameter block!!!! Don't change
* the first jump, nor start the code anywhere but right after
* this area.
*/
. = _start + GRUB_BOOT_MACHINE_BPB_START
. = _start + 4
/* scratch space */
mode:
.byte 0
disk_address_packet:
sectors:
.long 0
heads:
.long 0
cylinders:
.word 0
sector_start:
.byte 0
head_start:
.byte 0
cylinder_start:
.word 0
/* more space... */
. = _start + GRUB_BOOT_MACHINE_BPB_END
/*
* End of BIOS parameter block.
*/
kernel_address:
.word GRUB_BOOT_MACHINE_KERNEL_ADDR
. = _start + GRUB_BOOT_MACHINE_KERNEL_SECTOR
kernel_sector:
.long 1, 0
. = _start + GRUB_BOOT_MACHINE_BOOT_DRIVE
boot_drive:
.byte 0xff /* the disk to load kernel from */
/* 0xff means use the boot drive */
LOCAL(after_BPB):
/* general setup */
cli /* we're not safe here! */
/*
* This is a workaround for buggy BIOSes which don't pass boot
* drive correctly. If GRUB is installed into a HDD, check if
* DL is masked correctly. If not, assume that the BIOS passed
* a bogus value and set DL to 0x80, since this is the only
* possible boot drive. If GRUB is installed into a floppy,
* this does nothing (only jump).
*/
. = _start + GRUB_BOOT_MACHINE_DRIVE_CHECK
boot_drive_check:
jmp 3f /* grub-setup may overwrite this jump */
testb $0x80, %dl
jz 2f
3:
/* Ignore %dl different from 0-0x0f and 0x80-0x8f. */
testb $0x70, %dl
jz 1f
2:
movb $0x80, %dl
1:
/*
* ljmp to the next instruction because some bogus BIOSes
* jump to 07C0:0000 instead of 0000:7C00.
*/
ljmp $0, $real_start
real_start:
/* set up %ds and %ss as offset from 0 */
xorw %ax, %ax
movw %ax, %ds
movw %ax, %ss
/* set up the REAL stack */
movw $GRUB_BOOT_MACHINE_STACK_SEG, %sp
sti /* we're safe again */
/*
* Check if we have a forced disk reference here
*/
movb boot_drive, %al
cmpb $0xff, %al
je 1f
movb %al, %dl
1:
/* save drive reference first thing! */
pushw %dx
/* print a notification message on the screen */
MSG(notification_string)
/* set %si to the disk address packet */
movw $disk_address_packet, %si
/* check if LBA is supported */
movb $0x41, %ah
movw $0x55aa, %bx
int $0x13
/*
* %dl may have been clobbered by INT 13, AH=41H.
* This happens, for example, with AST BIOS 1.04.
*/
popw %dx
pushw %dx
/* use CHS if fails */
jc LOCAL(chs_mode)
cmpw $0xaa55, %bx
jne LOCAL(chs_mode)
andw $1, %cx
jz LOCAL(chs_mode)
lba_mode:
xorw %ax, %ax
movw %ax, 4(%si)
incw %ax
/* set the mode to non-zero */
movb %al, -1(%si)
/* the blocks */
movw %ax, 2(%si)
/* the size and the reserved byte */
movw $0x0010, (%si)
/* the absolute address */
movl kernel_sector, %ebx
movl %ebx, 8(%si)
movl kernel_sector + 4, %ebx
movl %ebx, 12(%si)
/* the segment of buffer address */
movw $GRUB_BOOT_MACHINE_BUFFER_SEG, 6(%si)
/*
* BIOS call "INT 0x13 Function 0x42" to read sectors from disk into memory
* Call with %ah = 0x42
* %dl = drive number
* %ds:%si = segment:offset of disk address packet
* Return:
* %al = 0x0 on success; err code on failure
*/
movb $0x42, %ah
int $0x13
/* LBA read is not supported, so fallback to CHS. */
jc LOCAL(chs_mode)
movw $GRUB_BOOT_MACHINE_BUFFER_SEG, %bx
jmp LOCAL(copy_buffer)
LOCAL(chs_mode):
/*
* Determine the hard disk geometry from the BIOS!
* We do this first, so that LS-120 IDE floppies work correctly.
*/
movb $8, %ah
int $0x13
jnc LOCAL(final_init)
/*
* The call failed, so maybe use the floppy probe instead.
*/
testb $GRUB_BOOT_MACHINE_BIOS_HD_FLAG, %dl
jz LOCAL(floppy_probe)
/* Nope, we definitely have a hard disk, and we're screwed. */
ERR(hd_probe_error_string)
LOCAL(final_init):
/* set the mode to zero */
movzbl %dh, %eax
movb %ah, -1(%si)
/* save number of heads */
incw %ax
movl %eax, 4(%si)
movzbw %cl, %dx
shlw $2, %dx
movb %ch, %al
movb %dh, %ah
/* save number of cylinders */
incw %ax
movw %ax, 8(%si)
movzbw %dl, %ax
shrb $2, %al
/* save number of sectors */
movl %eax, (%si)
setup_sectors:
/* load logical sector start (top half) */
movl kernel_sector + 4, %eax
orl %eax, %eax
jnz LOCAL(geometry_error)
/* load logical sector start (bottom half) */
movl kernel_sector, %eax
/* zero %edx */
xorl %edx, %edx
/* divide by number of sectors */
divl (%si)
/* save sector start */
movb %dl, %cl
xorw %dx, %dx /* zero %edx */
divl 4(%si) /* divide by number of heads */
/* do we need too many cylinders? */
cmpw 8(%si), %ax
jge LOCAL(geometry_error)
/* normalize sector start (1-based) */
incb %cl
/* low bits of cylinder start */
movb %al, %ch
/* high bits of cylinder start */
xorb %al, %al
shrw $2, %ax
orb %al, %cl
/* save head start */
movb %dl, %al
/* restore %dl */
popw %dx
/* head start */
movb %al, %dh
/*
* BIOS call "INT 0x13 Function 0x2" to read sectors from disk into memory
* Call with %ah = 0x2
* %al = number of sectors
* %ch = cylinder
* %cl = sector (bits 6-7 are high bits of "cylinder")
* %dh = head
* %dl = drive (0x80 for hard disk, 0x0 for floppy disk)
* %es:%bx = segment:offset of buffer
* Return:
* %al = 0x0 on success; err code on failure
*/
movw $GRUB_BOOT_MACHINE_BUFFER_SEG, %bx
movw %bx, %es /* load %es segment with disk buffer */
xorw %bx, %bx /* %bx = 0, put it at 0 in the segment */
movw $0x0201, %ax /* function 2 */
int $0x13
jc LOCAL(read_error)
movw %es, %bx
LOCAL(copy_buffer):
/*
* We need to save %cx and %si because the startup code in
* kernel uses them without initializing them.
*/
pusha
pushw %ds
movw $0x100, %cx
movw %bx, %ds
xorw %si, %si
movw $GRUB_BOOT_MACHINE_KERNEL_ADDR, %di
movw %si, %es
cld
rep
movsw
popw %ds
popa
/* boot kernel */
jmp *(kernel_address)
/* END OF MAIN LOOP */
/*
* BIOS Geometry translation error (past the end of the disk geometry!).
*/
LOCAL(geometry_error):
ERR(geometry_error_string)
/*
* Read error on the disk.
*/
LOCAL(read_error):
movw $read_error_string, %si
LOCAL(error_message):
call LOCAL(message)
LOCAL(general_error):
MSG(general_error_string)
/* go here when you need to stop the machine hard after an error condition */
/* tell the BIOS a boot failure, which may result in no effect */
int $0x18
LOCAL(stop):
jmp LOCAL(stop)
notification_string: .asciz "GRUB "
geometry_error_string: .asciz "Geom"
hd_probe_error_string: .asciz "Hard Disk"
read_error_string: .asciz "Read"
general_error_string: .asciz " Error\r\n"
/*
* message: write the string pointed to by %si
*
* WARNING: trashes %si, %ax, and %bx
*/
/*
* Use BIOS "int 10H Function 0Eh" to write character in teletype mode
* %ah = 0xe %al = character
* %bh = page %bl = foreground color (graphics modes)
*/
1:
movw $0x0001, %bx
movb $0xe, %ah
int $0x10 /* display a byte */
LOCAL(message):
lodsb
cmpb $0, %al
jne 1b /* if not end of string, jmp to display */
ret
/*
* Windows NT breaks compatibility by embedding a magic
* number here.
*/
. = _start + GRUB_BOOT_MACHINE_WINDOWS_NT_MAGIC
nt_magic:
.long 0
.word 0
/*
* This is where an MBR would go if on a hard disk. The code
* here isn't even referenced unless we're on a floppy. Kinda
* sneaky, huh?
*/
. = _start + GRUB_BOOT_MACHINE_PART_START
part_start:
probe_values:
.byte 36, 18, 15, 9, 0
LOCAL(floppy_probe):
/*
* Perform floppy probe.
*/
movw $probe_values - 1, %si
LOCAL(probe_loop):
/* reset floppy controller INT 13h AH=0 */
xorw %ax, %ax
int $0x13
incw %si
movb (%si), %cl
/* if number of sectors is 0, display error and die */
cmpb $0, %cl
jne 1f
/*
* Floppy disk probe failure.
*/
MSG(fd_probe_error_string)
jmp LOCAL(general_error)
/* "Floppy" */
fd_probe_error_string: .asciz "Floppy"
1:
/* perform read */
movw $GRUB_BOOT_MACHINE_BUFFER_SEG, %bx
movw %bx, %es
xorw %bx, %bx
movw $0x201, %ax
movb $0, %ch
movb $0, %dh
int $0x13
/* if error, jump to "LOCAL(probe_loop)" */
jc LOCAL(probe_loop)
/* %cl is already the correct value! */
movb $1, %dh
movb $79, %ch
jmp LOCAL(final_init)
. = _start + GRUB_BOOT_MACHINE_PART_END
/* the last 2 bytes in the sector 0 contain the signature */
.word GRUB_BOOT_MACHINE_SIGNATURE
@MichaelPetch显然,我并没有提倡它,只是提到了gcc中存在的选项.找到仍然有效的386前主板有点困难.从1996年开始,我的386主板在2010年左右放弃了[死气].更难找到OP86仍然可以使用的8086仍然可以访问或渴望.即使对于二手/多余的电脑插座,它们也太古老了(例如weirdstuff.com)
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