head.s

LINUX 1.0 内核c源代码,是一份精简的linux内核源代码

/*
 *  linux/boot/head.S
 *
 *  Copyright (C) 1991, 1992, 1993  Linus Torvalds
 */

/*
 *  head.S contains the 32-bit startup code.
 *
 * NOTE!!! Startup happens at absolute address 0x00001000, which is also where
 * the page directory will exist. The startup code will be overwritten by
 * the page directory.
 *
 * Page 0 is deliberately kept safe, since System Management Mode code in 
 * laptops may need to access the BIOS data stored there.  This is also
 * useful for future device drivers that either access the BIOS via VM86 
 * mode.
 */
.text

#include <linux/segment.h>

startup_32:
	cld
	cli
	movl $(KERNEL_DS),%eax
	mov %ax,%ds
	mov %ax,%es
	mov %ax,%fs
	mov %ax,%gs
	lss _stack_start,%esp
	xorl %eax,%eax
1:	incl %eax		# check that A20 really IS enabled
	movl %eax,0x000000	# loop forever if it isn't
	cmpl %eax,0x100000
	je 1b
/*
 * Initialize eflags.  Some BIOS's leave bits like NT set.  This would
 * confuse the debugger if this code is traced.
 * XXX - best to initialize before switching to protected mode.
 */
	pushl $0
	popfl
/*
 * Clear BSS
 */
	xorl %eax,%eax
	movl $__edata,%edi
	movl $__end,%ecx
	subl %edi,%ecx
	cld
	rep
	stosb
/*
 * Do the decompression, and jump to the new kernel..
 */
	call _decompress_kernel
	ljmp $(KERNEL_CS), $0x100000