arch/i386/kernel/head.S - maze/linux - Git at Google

 /*
  *  linux/arch/i386/kernel/head.S -- the 32-bit startup code.
  *
  *  Copyright (C) 1991, 1992  Linus Torvalds
  *
  *  Enhanced CPU detection and feature setting code by Mike Jagdis
  *  and Martin Mares, November 1997.
  */

 .text
 #include <linux/config.h>
 #include <linux/threads.h>
 #include <linux/linkage.h>
 #include <asm/segment.h>
 #include <asm/page.h>
 #include <asm/pgtable.h>
 #include <asm/desc.h>
 #include <asm/cache.h>
 #include <asm/thread_info.h>
 #include <asm/asm-offsets.h>
 #include <asm/setup.h>

 /*
  * References to members of the new_cpu_data structure.
  */

 #define X86		new_cpu_data+CPUINFO_x86
 #define X86_VENDOR	new_cpu_data+CPUINFO_x86_vendor
 #define X86_MODEL	new_cpu_data+CPUINFO_x86_model
 #define X86_MASK	new_cpu_data+CPUINFO_x86_mask
 #define X86_HARD_MATH	new_cpu_data+CPUINFO_hard_math
 #define X86_CPUID	new_cpu_data+CPUINFO_cpuid_level
 #define X86_CAPABILITY	new_cpu_data+CPUINFO_x86_capability
 #define X86_VENDOR_ID	new_cpu_data+CPUINFO_x86_vendor_id

 /*
  * This is how much memory *in addition to the memory covered up to
  * and including _end* we need mapped initially.  We need one bit for
  * each possible page, but only in low memory, which means
  * 2^32/4096/8 = 128K worst case (4G/4G split.)
  *
  * Modulo rounding, each megabyte assigned here requires a kilobyte of
  * memory, which is currently unreclaimed.
  *
  * This should be a multiple of a page.
  */
 #define INIT_MAP_BEYOND_END	(128*1024)


 /*
  * 32-bit kernel entrypoint; only used by the boot CPU.  On entry,
  * %esi points to the real-mode code as a 32-bit pointer.
  * CS and DS must be 4 GB flat segments, but we don't depend on
  * any particular GDT layout, because we load our own as soon as we
  * can.
  */
 ENTRY(startup_32)

 /*
  * Set segments to known values.
  */
 	cld
 	lgdt boot_gdt_descr - __PAGE_OFFSET
 	movl $(__BOOT_DS),%eax
 	movl %eax,%ds
 	movl %eax,%es
 	movl %eax,%fs
 	movl %eax,%gs

 /*
  * Clear BSS first so that there are no surprises...
  * No need to cld as DF is already clear from cld above...
  */
 	xorl %eax,%eax
 	movl $__bss_start - __PAGE_OFFSET,%edi
 	movl $__bss_stop - __PAGE_OFFSET,%ecx
 	subl %edi,%ecx
 	shrl $2,%ecx
 	rep ; stosl
 /*
  * Copy bootup parameters out of the way.
  * Note: %esi still has the pointer to the real-mode data.
  * With the kexec as boot loader, parameter segment might be loaded beyond
  * kernel image and might not even be addressable by early boot page tables.
  * (kexec on panic case). Hence copy out the parameters before initializing
  * page tables.
  */
 	movl $(boot_params - __PAGE_OFFSET),%edi
 	movl $(PARAM_SIZE/4),%ecx
 	cld
 	rep
 	movsl
 	movl boot_params - __PAGE_OFFSET + NEW_CL_POINTER,%esi
 	andl %esi,%esi
 	jnz 2f			# New command line protocol
 	cmpw $(OLD_CL_MAGIC),OLD_CL_MAGIC_ADDR
 	jne 1f
 	movzwl OLD_CL_OFFSET,%esi
 	addl $(OLD_CL_BASE_ADDR),%esi
 2:
 	movl $(saved_command_line - __PAGE_OFFSET),%edi
 	movl $(COMMAND_LINE_SIZE/4),%ecx
 	rep
 	movsl
 1:

 /*
  * Initialize page tables.  This creates a PDE and a set of page
  * tables, which are located immediately beyond _end.  The variable
  * init_pg_tables_end is set up to point to the first "safe" location.
  * Mappings are created both at virtual address 0 (identity mapping)
  * and PAGE_OFFSET for up to _end+sizeof(page tables)+INIT_MAP_BEYOND_END.
  *
  * Warning: don't use %esi or the stack in this code.  However, %esp
  * can be used as a GPR if you really need it...
  */
 page_pde_offset = (__PAGE_OFFSET >> 20);

 	movl $(pg0 - __PAGE_OFFSET), %edi
 	movl $(swapper_pg_dir - __PAGE_OFFSET), %edx
 	movl $0x007, %eax			/* 0x007 = PRESENT+RW+USER */
 10:
 	leal 0x007(%edi),%ecx			/* Create PDE entry */
 	movl %ecx,(%edx)			/* Store identity PDE entry */
 	movl %ecx,page_pde_offset(%edx)		/* Store kernel PDE entry */
 	addl $4,%edx
 	movl $1024, %ecx
 11:
 	stosl
 	addl $0x1000,%eax
 	loop 11b
 	/* End condition: we must map up to and including INIT_MAP_BEYOND_END */
 	/* bytes beyond the end of our own page tables; the +0x007 is the attribute bits */
 	leal (INIT_MAP_BEYOND_END+0x007)(%edi),%ebp
 	cmpl %ebp,%eax
 	jb 10b
 	movl %edi,(init_pg_tables_end - __PAGE_OFFSET)

 #ifdef CONFIG_SMP
 	xorl %ebx,%ebx				/* This is the boot CPU (BSP) */
 	jmp 3f

 /*
  * Non-boot CPU entry point; entered from trampoline.S
  * We can't lgdt here, because lgdt itself uses a data segment, but
  * we know the trampoline has already loaded the boot_gdt_table GDT
  * for us.
  */
 ENTRY(startup_32_smp)
 	cld
 	movl $(__BOOT_DS),%eax
 	movl %eax,%ds
 	movl %eax,%es
 	movl %eax,%fs
 	movl %eax,%gs

 /*
  *	New page tables may be in 4Mbyte page mode and may
  *	be using the global pages.
  *
  *	NOTE! If we are on a 486 we may have no cr4 at all!
  *	So we do not try to touch it unless we really have
  *	some bits in it to set.  This won't work if the BSP
  *	implements cr4 but this AP does not -- very unlikely
  *	but be warned!  The same applies to the pse feature
  *	if not equally supported. --macro
  *
  *	NOTE! We have to correct for the fact that we're
  *	not yet offset PAGE_OFFSET..
  */
 #define cr4_bits mmu_cr4_features-__PAGE_OFFSET
 	movl cr4_bits,%edx
 	andl %edx,%edx
 	jz 6f
 	movl %cr4,%eax		# Turn on paging options (PSE,PAE,..)
 	orl %edx,%eax
 	movl %eax,%cr4

 	btl $5, %eax		# check if PAE is enabled
 	jnc 6f

 	/* Check if extended functions are implemented */
 	movl $0x80000000, %eax
 	cpuid
 	cmpl $0x80000000, %eax
 	jbe 6f
 	mov $0x80000001, %eax
 	cpuid
 	/* Execute Disable bit supported? */
 	btl $20, %edx
 	jnc 6f

 	/* Setup EFER (Extended Feature Enable Register) */
 	movl $0xc0000080, %ecx
 	rdmsr

 	btsl $11, %eax
 	/* Make changes effective */
 	wrmsr

 6:
 	/* This is a secondary processor (AP) */
 	xorl %ebx,%ebx
 	incl %ebx

 3:
 #endif /* CONFIG_SMP */

 /*
  * Enable paging
  */
 	movl $swapper_pg_dir-__PAGE_OFFSET,%eax
 	movl %eax,%cr3		/* set the page table pointer.. */
 	movl %cr0,%eax
 	orl $0x80000000,%eax
 	movl %eax,%cr0		/* ..and set paging (PG) bit */
 	ljmp $__BOOT_CS,$1f	/* Clear prefetch and normalize %eip */
 1:
 	/* Set up the stack pointer */
 	lss stack_start,%esp

 /*
  * 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

 #ifdef CONFIG_SMP
 	andl %ebx,%ebx
 	jz  1f				/* Initial CPU cleans BSS */
 	jmp checkCPUtype
 1:
 #endif /* CONFIG_SMP */

 /*
  * start system 32-bit setup. We need to re-do some of the things done
  * in 16-bit mode for the "real" operations.
  */
 	call setup_idt

 checkCPUtype:

 	movl $-1,X86_CPUID		#  -1 for no CPUID initially

 /* check if it is 486 or 386. */
 /*
  * XXX - this does a lot of unnecessary setup.  Alignment checks don't
  * apply at our cpl of 0 and the stack ought to be aligned already, and
  * we don't need to preserve eflags.
  */

 	movb $3,X86		# at least 386
 	pushfl			# push EFLAGS
 	popl %eax		# get EFLAGS
 	movl %eax,%ecx		# save original EFLAGS
 	xorl $0x240000,%eax	# flip AC and ID bits in EFLAGS
 	pushl %eax		# copy to EFLAGS
 	popfl			# set EFLAGS
 	pushfl			# get new EFLAGS
 	popl %eax		# put it in eax
 	xorl %ecx,%eax		# change in flags
 	pushl %ecx		# restore original EFLAGS
 	popfl
 	testl $0x40000,%eax	# check if AC bit changed
 	je is386

 	movb $4,X86		# at least 486
 	testl $0x200000,%eax	# check if ID bit changed
 	je is486

 	/* get vendor info */
 	xorl %eax,%eax			# call CPUID with 0 -> return vendor ID
 	cpuid
 	movl %eax,X86_CPUID		# save CPUID level
 	movl %ebx,X86_VENDOR_ID		# lo 4 chars
 	movl %edx,X86_VENDOR_ID+4	# next 4 chars
 	movl %ecx,X86_VENDOR_ID+8	# last 4 chars

 	orl %eax,%eax			# do we have processor info as well?
 	je is486

 	movl $1,%eax		# Use the CPUID instruction to get CPU type
 	cpuid
 	movb %al,%cl		# save reg for future use
 	andb $0x0f,%ah		# mask processor family
 	movb %ah,X86
 	andb $0xf0,%al		# mask model
 	shrb $4,%al
 	movb %al,X86_MODEL
 	andb $0x0f,%cl		# mask mask revision
 	movb %cl,X86_MASK
 	movl %edx,X86_CAPABILITY

 is486:	movl $0x50022,%ecx	# set AM, WP, NE and MP
 	jmp 2f

 is386:	movl $2,%ecx		# set MP
 2:	movl %cr0,%eax
 	andl $0x80000011,%eax	# Save PG,PE,ET
 	orl %ecx,%eax
 	movl %eax,%cr0

 	call check_x87
 	lgdt cpu_gdt_descr
 	lidt idt_descr
 	ljmp $(__KERNEL_CS),$1f
 1:	movl $(__KERNEL_DS),%eax	# reload all the segment registers
 	movl %eax,%ss			# after changing gdt.

 	movl $(__USER_DS),%eax		# DS/ES contains default USER segment
 	movl %eax,%ds
 	movl %eax,%es

 	xorl %eax,%eax			# Clear FS/GS and LDT
 	movl %eax,%fs
 	movl %eax,%gs
 	lldt %ax
 	cld			# gcc2 wants the direction flag cleared at all times
 #ifdef CONFIG_SMP
 	movb ready, %cl
 	movb $1, ready
 	cmpb $0,%cl
 	je 1f			# the first CPU calls start_kernel
 				# all other CPUs call initialize_secondary
 	call initialize_secondary
 	jmp L6
 1:
 #endif /* CONFIG_SMP */
 	call start_kernel
 L6:
 	jmp L6			# main should never return here, but
 				# just in case, we know what happens.

 /*
  * We depend on ET to be correct. This checks for 287/387.
  */
 check_x87:
 	movb $0,X86_HARD_MATH
 	clts
 	fninit
 	fstsw %ax
 	cmpb $0,%al
 	je 1f
 	movl %cr0,%eax		/* no coprocessor: have to set bits */
 	xorl $4,%eax		/* set EM */
 	movl %eax,%cr0
 	ret
 	ALIGN
 1:	movb $1,X86_HARD_MATH
 	.byte 0xDB,0xE4		/* fsetpm for 287, ignored by 387 */
 	ret

 /*
  *  setup_idt
  *
  *  sets up a idt with 256 entries pointing to
  *  ignore_int, interrupt gates. It doesn't actually load
  *  idt - that can be done only after paging has been enabled
  *  and the kernel moved to PAGE_OFFSET. Interrupts
  *  are enabled elsewhere, when we can be relatively
  *  sure everything is ok.
  *
  *  Warning: %esi is live across this function.
  */
 setup_idt:
 	lea ignore_int,%edx
 	movl $(__KERNEL_CS << 16),%eax
 	movw %dx,%ax		/* selector = 0x0010 = cs */
 	movw $0x8E00,%dx	/* interrupt gate - dpl=0, present */

 	lea idt_table,%edi
 	mov $256,%ecx
 rp_sidt:
 	movl %eax,(%edi)
 	movl %edx,4(%edi)
 	addl $8,%edi
 	dec %ecx
 	jne rp_sidt
 	ret

 /* This is the default interrupt "handler" :-) */
 	ALIGN
 ignore_int:
 	cld
 #ifdef CONFIG_PRINTK
 	pushl %eax
 	pushl %ecx
 	pushl %edx
 	pushl %es
 	pushl %ds
 	movl $(__KERNEL_DS),%eax
 	movl %eax,%ds
 	movl %eax,%es
 	pushl 16(%esp)
 	pushl 24(%esp)
 	pushl 32(%esp)
 	pushl 40(%esp)
 	pushl $int_msg
 	call printk
 	addl $(5*4),%esp
 	popl %ds
 	popl %es
 	popl %edx
 	popl %ecx
 	popl %eax
 #endif
 	iret

 /*
  * Real beginning of normal "text" segment
  */
 ENTRY(stext)
 ENTRY(_stext)

 /*
  * BSS section
  */
 .section ".bss.page_aligned","w"
 ENTRY(swapper_pg_dir)
 	.fill 1024,4,0
 ENTRY(empty_zero_page)
 	.fill 4096,1,0

 /*
  * This starts the data section.
  */
 .data

 ENTRY(stack_start)
 	.long init_thread_union+THREAD_SIZE
 	.long __BOOT_DS

 ready:	.byte 0

 int_msg:
 	.asciz "Unknown interrupt or fault at EIP %p %p %p\n"

 /*
  * The IDT and GDT 'descriptors' are a strange 48-bit object
  * only used by the lidt and lgdt instructions. They are not
  * like usual segment descriptors - they consist of a 16-bit
  * segment size, and 32-bit linear address value:
  */

 .globl boot_gdt_descr
 .globl idt_descr
 .globl cpu_gdt_descr

 	ALIGN
 # early boot GDT descriptor (must use 1:1 address mapping)
 	.word 0				# 32 bit align gdt_desc.address
 boot_gdt_descr:
 	.word __BOOT_DS+7
 	.long boot_gdt_table - __PAGE_OFFSET

 	.word 0				# 32-bit align idt_desc.address
 idt_descr:
 	.word IDT_ENTRIES*8-1		# idt contains 256 entries
 	.long idt_table

 # boot GDT descriptor (later on used by CPU#0):
 	.word 0				# 32 bit align gdt_desc.address
 cpu_gdt_descr:
 	.word GDT_ENTRIES*8-1
 	.long cpu_gdt_table

 	.fill NR_CPUS-1,8,0		# space for the other GDT descriptors

 /*
  * The boot_gdt_table must mirror the equivalent in setup.S and is
  * used only for booting.
  */
 	.align L1_CACHE_BYTES
 ENTRY(boot_gdt_table)
 	.fill GDT_ENTRY_BOOT_CS,8,0
 	.quad 0x00cf9a000000ffff	/* kernel 4GB code at 0x00000000 */
 	.quad 0x00cf92000000ffff	/* kernel 4GB data at 0x00000000 */

 /*
  * The Global Descriptor Table contains 28 quadwords, per-CPU.
  */
 	.align PAGE_SIZE_asm
 ENTRY(cpu_gdt_table)
 	.quad 0x0000000000000000	/* NULL descriptor */
 	.quad 0x0000000000000000	/* 0x0b reserved */
 	.quad 0x0000000000000000	/* 0x13 reserved */
 	.quad 0x0000000000000000	/* 0x1b reserved */
 	.quad 0x0000000000000000	/* 0x20 unused */
 	.quad 0x0000000000000000	/* 0x28 unused */
 	.quad 0x0000000000000000	/* 0x33 TLS entry 1 */
 	.quad 0x0000000000000000	/* 0x3b TLS entry 2 */
 	.quad 0x0000000000000000	/* 0x43 TLS entry 3 */
 	.quad 0x0000000000000000	/* 0x4b reserved */
 	.quad 0x0000000000000000	/* 0x53 reserved */
 	.quad 0x0000000000000000	/* 0x5b reserved */

 	.quad 0x00cf9a000000ffff	/* 0x60 kernel 4GB code at 0x00000000 */
 	.quad 0x00cf92000000ffff	/* 0x68 kernel 4GB data at 0x00000000 */
 	.quad 0x00cffa000000ffff	/* 0x73 user 4GB code at 0x00000000 */
 	.quad 0x00cff2000000ffff	/* 0x7b user 4GB data at 0x00000000 */

 	.quad 0x0000000000000000	/* 0x80 TSS descriptor */
 	.quad 0x0000000000000000	/* 0x88 LDT descriptor */

 	/* Segments used for calling PnP BIOS */
 	.quad 0x00c09a0000000000	/* 0x90 32-bit code */
 	.quad 0x00809a0000000000	/* 0x98 16-bit code */
 	.quad 0x0080920000000000	/* 0xa0 16-bit data */
 	.quad 0x0080920000000000	/* 0xa8 16-bit data */
 	.quad 0x0080920000000000	/* 0xb0 16-bit data */
 	/*
 	 * The APM segments have byte granularity and their bases
 	 * and limits are set at run time.
 	 */
 	.quad 0x00409a0000000000	/* 0xb8 APM CS    code */
 	.quad 0x00009a0000000000	/* 0xc0 APM CS 16 code (16 bit) */
 	.quad 0x0040920000000000	/* 0xc8 APM DS    data */

 	.quad 0x0000920000000000	/* 0xd0 - ESPFIX 16-bit SS */
 	.quad 0x0000000000000000	/* 0xd8 - unused */
 	.quad 0x0000000000000000	/* 0xe0 - unused */
 	.quad 0x0000000000000000	/* 0xe8 - unused */
 	.quad 0x0000000000000000	/* 0xf0 - unused */
 	.quad 0x0000000000000000	/* 0xf8 - GDT entry 31: double-fault TSS */
	/*
	* linux/arch/i386/kernel/head.S -- the 32-bit startup code.
	*
	* Copyright (C) 1991, 1992 Linus Torvalds
	*
	* Enhanced CPU detection and feature setting code by Mike Jagdis
	* and Martin Mares, November 1997.
	*/

	.text
	#include <linux/config.h>
	#include <linux/threads.h>
	#include <linux/linkage.h>
	#include <asm/segment.h>
	#include <asm/page.h>
	#include <asm/pgtable.h>
	#include <asm/desc.h>
	#include <asm/cache.h>
	#include <asm/thread_info.h>
	#include <asm/asm-offsets.h>
	#include <asm/setup.h>

	/*
	* References to members of the new_cpu_data structure.
	*/

	#define X86 new_cpu_data+CPUINFO_x86
	#define X86_VENDOR new_cpu_data+CPUINFO_x86_vendor
	#define X86_MODEL new_cpu_data+CPUINFO_x86_model
	#define X86_MASK new_cpu_data+CPUINFO_x86_mask
	#define X86_HARD_MATH new_cpu_data+CPUINFO_hard_math
	#define X86_CPUID new_cpu_data+CPUINFO_cpuid_level
	#define X86_CAPABILITY new_cpu_data+CPUINFO_x86_capability
	#define X86_VENDOR_ID new_cpu_data+CPUINFO_x86_vendor_id

	/*
	* This is how much memory *in addition to the memory covered up to
	* and including _end* we need mapped initially. We need one bit for
	* each possible page, but only in low memory, which means
	* 2^32/4096/8 = 128K worst case (4G/4G split.)
	*
	* Modulo rounding, each megabyte assigned here requires a kilobyte of
	* memory, which is currently unreclaimed.
	*
	* This should be a multiple of a page.
	*/
	#define INIT_MAP_BEYOND_END (128*1024)


	/*
	* 32-bit kernel entrypoint; only used by the boot CPU. On entry,
	* %esi points to the real-mode code as a 32-bit pointer.
	* CS and DS must be 4 GB flat segments, but we don't depend on
	* any particular GDT layout, because we load our own as soon as we
	* can.
	*/
	ENTRY(startup_32)

	/*
	* Set segments to known values.
	*/
	cld
	lgdt boot_gdt_descr - __PAGE_OFFSET
	movl $(__BOOT_DS),%eax
	movl %eax,%ds
	movl %eax,%es
	movl %eax,%fs
	movl %eax,%gs

	/*
	* Clear BSS first so that there are no surprises...
	* No need to cld as DF is already clear from cld above...
	*/
	xorl %eax,%eax
	movl $__bss_start - __PAGE_OFFSET,%edi
	movl $__bss_stop - __PAGE_OFFSET,%ecx
	subl %edi,%ecx
	shrl $2,%ecx
	rep ; stosl
	/*
	* Copy bootup parameters out of the way.
	* Note: %esi still has the pointer to the real-mode data.
	* With the kexec as boot loader, parameter segment might be loaded beyond
	* kernel image and might not even be addressable by early boot page tables.
	* (kexec on panic case). Hence copy out the parameters before initializing
	* page tables.
	*/
	movl $(boot_params - __PAGE_OFFSET),%edi
	movl $(PARAM_SIZE/4),%ecx
	cld
	rep
	movsl
	movl boot_params - __PAGE_OFFSET + NEW_CL_POINTER,%esi
	andl %esi,%esi
	jnz 2f # New command line protocol
	cmpw $(OLD_CL_MAGIC),OLD_CL_MAGIC_ADDR
	jne 1f
	movzwl OLD_CL_OFFSET,%esi
	addl $(OLD_CL_BASE_ADDR),%esi
	2:
	movl $(saved_command_line - __PAGE_OFFSET),%edi
	movl $(COMMAND_LINE_SIZE/4),%ecx
	rep
	movsl
	1:

	/*
	* Initialize page tables. This creates a PDE and a set of page
	* tables, which are located immediately beyond _end. The variable
	* init_pg_tables_end is set up to point to the first "safe" location.
	* Mappings are created both at virtual address 0 (identity mapping)
	* and PAGE_OFFSET for up to _end+sizeof(page tables)+INIT_MAP_BEYOND_END.
	*
	* Warning: don't use %esi or the stack in this code. However, %esp
	* can be used as a GPR if you really need it...
	*/
	page_pde_offset = (__PAGE_OFFSET >> 20);

	movl $(pg0 - __PAGE_OFFSET), %edi
	movl $(swapper_pg_dir - __PAGE_OFFSET), %edx
	movl $0x007, %eax /* 0x007 = PRESENT+RW+USER */
	10:
	leal 0x007(%edi),%ecx /* Create PDE entry */
	movl %ecx,(%edx) /* Store identity PDE entry */
	movl %ecx,page_pde_offset(%edx) /* Store kernel PDE entry */
	addl $4,%edx
	movl $1024, %ecx
	11:
	stosl
	addl $0x1000,%eax
	loop 11b
	/* End condition: we must map up to and including INIT_MAP_BEYOND_END */
	/* bytes beyond the end of our own page tables; the +0x007 is the attribute bits */
	leal (INIT_MAP_BEYOND_END+0x007)(%edi),%ebp
	cmpl %ebp,%eax
	jb 10b
	movl %edi,(init_pg_tables_end - __PAGE_OFFSET)

	#ifdef CONFIG_SMP
	xorl %ebx,%ebx /* This is the boot CPU (BSP) */
	jmp 3f

	/*
	* Non-boot CPU entry point; entered from trampoline.S
	* We can't lgdt here, because lgdt itself uses a data segment, but
	* we know the trampoline has already loaded the boot_gdt_table GDT
	* for us.
	*/
	ENTRY(startup_32_smp)
	cld
	movl $(__BOOT_DS),%eax
	movl %eax,%ds
	movl %eax,%es
	movl %eax,%fs
	movl %eax,%gs

	/*
	* New page tables may be in 4Mbyte page mode and may
	* be using the global pages.
	*
	* NOTE! If we are on a 486 we may have no cr4 at all!
	* So we do not try to touch it unless we really have
	* some bits in it to set. This won't work if the BSP
	* implements cr4 but this AP does not -- very unlikely
	* but be warned! The same applies to the pse feature
	* if not equally supported. --macro
	*
	* NOTE! We have to correct for the fact that we're
	* not yet offset PAGE_OFFSET..
	*/
	#define cr4_bits mmu_cr4_features-__PAGE_OFFSET
	movl cr4_bits,%edx
	andl %edx,%edx
	jz 6f
	movl %cr4,%eax # Turn on paging options (PSE,PAE,..)
	orl %edx,%eax
	movl %eax,%cr4

	btl $5, %eax # check if PAE is enabled
	jnc 6f

	/* Check if extended functions are implemented */
	movl $0x80000000, %eax
	cpuid
	cmpl $0x80000000, %eax
	jbe 6f
	mov $0x80000001, %eax
	cpuid
	/* Execute Disable bit supported? */
	btl $20, %edx
	jnc 6f

	/* Setup EFER (Extended Feature Enable Register) */
	movl $0xc0000080, %ecx
	rdmsr

	btsl $11, %eax
	/* Make changes effective */
	wrmsr

	6:
	/* This is a secondary processor (AP) */
	xorl %ebx,%ebx
	incl %ebx

	3:
	#endif /* CONFIG_SMP */

	/*
	* Enable paging
	*/
	movl $swapper_pg_dir-__PAGE_OFFSET,%eax
	movl %eax,%cr3 /* set the page table pointer.. */
	movl %cr0,%eax
	orl $0x80000000,%eax
	movl %eax,%cr0 /* ..and set paging (PG) bit */
	ljmp $__BOOT_CS,$1f /* Clear prefetch and normalize %eip */
	1:
	/* Set up the stack pointer */
	lss stack_start,%esp

	/*
	* 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

	#ifdef CONFIG_SMP
	andl %ebx,%ebx
	jz 1f /* Initial CPU cleans BSS */
	jmp checkCPUtype
	1:
	#endif /* CONFIG_SMP */

	/*
	* start system 32-bit setup. We need to re-do some of the things done
	* in 16-bit mode for the "real" operations.
	*/
	call setup_idt

	checkCPUtype:

	movl $-1,X86_CPUID # -1 for no CPUID initially

	/* check if it is 486 or 386. */
	/*
	* XXX - this does a lot of unnecessary setup. Alignment checks don't
	* apply at our cpl of 0 and the stack ought to be aligned already, and
	* we don't need to preserve eflags.
	*/

	movb $3,X86 # at least 386
	pushfl # push EFLAGS
	popl %eax # get EFLAGS
	movl %eax,%ecx # save original EFLAGS
	xorl $0x240000,%eax # flip AC and ID bits in EFLAGS
	pushl %eax # copy to EFLAGS
	popfl # set EFLAGS
	pushfl # get new EFLAGS
	popl %eax # put it in eax
	xorl %ecx,%eax # change in flags
	pushl %ecx # restore original EFLAGS
	popfl
	testl $0x40000,%eax # check if AC bit changed
	je is386

	movb $4,X86 # at least 486
	testl $0x200000,%eax # check if ID bit changed
	je is486

	/* get vendor info */
	xorl %eax,%eax # call CPUID with 0 -> return vendor ID
	cpuid
	movl %eax,X86_CPUID # save CPUID level
	movl %ebx,X86_VENDOR_ID # lo 4 chars
	movl %edx,X86_VENDOR_ID+4 # next 4 chars
	movl %ecx,X86_VENDOR_ID+8 # last 4 chars

	orl %eax,%eax # do we have processor info as well?
	je is486

	movl $1,%eax # Use the CPUID instruction to get CPU type
	cpuid
	movb %al,%cl # save reg for future use
	andb $0x0f,%ah # mask processor family
	movb %ah,X86
	andb $0xf0,%al # mask model
	shrb $4,%al
	movb %al,X86_MODEL
	andb $0x0f,%cl # mask mask revision
	movb %cl,X86_MASK
	movl %edx,X86_CAPABILITY

	is486: movl $0x50022,%ecx # set AM, WP, NE and MP
	jmp 2f

	is386: movl $2,%ecx # set MP
	2: movl %cr0,%eax
	andl $0x80000011,%eax # Save PG,PE,ET
	orl %ecx,%eax
	movl %eax,%cr0

	call check_x87
	lgdt cpu_gdt_descr
	lidt idt_descr
	ljmp $(__KERNEL_CS),$1f
	1: movl $(__KERNEL_DS),%eax # reload all the segment registers
	movl %eax,%ss # after changing gdt.

	movl $(__USER_DS),%eax # DS/ES contains default USER segment
	movl %eax,%ds
	movl %eax,%es

	xorl %eax,%eax # Clear FS/GS and LDT
	movl %eax,%fs
	movl %eax,%gs
	lldt %ax
	cld # gcc2 wants the direction flag cleared at all times
	#ifdef CONFIG_SMP
	movb ready, %cl
	movb $1, ready
	cmpb $0,%cl
	je 1f # the first CPU calls start_kernel
	# all other CPUs call initialize_secondary
	call initialize_secondary
	jmp L6
	1:
	#endif /* CONFIG_SMP */
	call start_kernel
	L6:
	jmp L6 # main should never return here, but
	# just in case, we know what happens.

	/*
	* We depend on ET to be correct. This checks for 287/387.
	*/
	check_x87:
	movb $0,X86_HARD_MATH
	clts
	fninit
	fstsw %ax
	cmpb $0,%al
	je 1f
	movl %cr0,%eax /* no coprocessor: have to set bits */
	xorl $4,%eax /* set EM */
	movl %eax,%cr0
	ret
	ALIGN
	1: movb $1,X86_HARD_MATH
	.byte 0xDB,0xE4 /* fsetpm for 287, ignored by 387 */
	ret

	/*
	* setup_idt
	*
	* sets up a idt with 256 entries pointing to
	* ignore_int, interrupt gates. It doesn't actually load
	* idt - that can be done only after paging has been enabled
	* and the kernel moved to PAGE_OFFSET. Interrupts
	* are enabled elsewhere, when we can be relatively
	* sure everything is ok.
	*
	* Warning: %esi is live across this function.
	*/
	setup_idt:
	lea ignore_int,%edx
	movl $(__KERNEL_CS << 16),%eax
	movw %dx,%ax /* selector = 0x0010 = cs */
	movw $0x8E00,%dx /* interrupt gate - dpl=0, present */

	lea idt_table,%edi
	mov $256,%ecx
	rp_sidt:
	movl %eax,(%edi)
	movl %edx,4(%edi)
	addl $8,%edi
	dec %ecx
	jne rp_sidt
	ret

	/* This is the default interrupt "handler" :-) */
	ALIGN
	ignore_int:
	cld
	#ifdef CONFIG_PRINTK
	pushl %eax
	pushl %ecx
	pushl %edx
	pushl %es
	pushl %ds
	movl $(__KERNEL_DS),%eax
	movl %eax,%ds
	movl %eax,%es
	pushl 16(%esp)
	pushl 24(%esp)
	pushl 32(%esp)
	pushl 40(%esp)
	pushl $int_msg
	call printk
	addl $(5*4),%esp
	popl %ds
	popl %es
	popl %edx
	popl %ecx
	popl %eax
	#endif
	iret

	/*
	* Real beginning of normal "text" segment
	*/
	ENTRY(stext)
	ENTRY(_stext)

	/*
	* BSS section
	*/
	.section ".bss.page_aligned","w"
	ENTRY(swapper_pg_dir)
	.fill 1024,4,0
	ENTRY(empty_zero_page)
	.fill 4096,1,0

	/*
	* This starts the data section.
	*/
	.data

	ENTRY(stack_start)
	.long init_thread_union+THREAD_SIZE
	.long __BOOT_DS

	ready: .byte 0

	int_msg:
	.asciz "Unknown interrupt or fault at EIP %p %p %p\n"

	/*
	* The IDT and GDT 'descriptors' are a strange 48-bit object
	* only used by the lidt and lgdt instructions. They are not
	* like usual segment descriptors - they consist of a 16-bit
	* segment size, and 32-bit linear address value:
	*/

	.globl boot_gdt_descr
	.globl idt_descr
	.globl cpu_gdt_descr

	ALIGN
	# early boot GDT descriptor (must use 1:1 address mapping)
	.word 0 # 32 bit align gdt_desc.address
	boot_gdt_descr:
	.word __BOOT_DS+7
	.long boot_gdt_table - __PAGE_OFFSET

	.word 0 # 32-bit align idt_desc.address
	idt_descr:
	.word IDT_ENTRIES*8-1 # idt contains 256 entries
	.long idt_table

	# boot GDT descriptor (later on used by CPU#0):
	.word 0 # 32 bit align gdt_desc.address
	cpu_gdt_descr:
	.word GDT_ENTRIES*8-1
	.long cpu_gdt_table

	.fill NR_CPUS-1,8,0 # space for the other GDT descriptors

	/*
	* The boot_gdt_table must mirror the equivalent in setup.S and is
	* used only for booting.
	*/
	.align L1_CACHE_BYTES
	ENTRY(boot_gdt_table)
	.fill GDT_ENTRY_BOOT_CS,8,0
	.quad 0x00cf9a000000ffff /* kernel 4GB code at 0x00000000 */
	.quad 0x00cf92000000ffff /* kernel 4GB data at 0x00000000 */

	/*
	* The Global Descriptor Table contains 28 quadwords, per-CPU.
	*/
	.align PAGE_SIZE_asm
	ENTRY(cpu_gdt_table)
	.quad 0x0000000000000000 /* NULL descriptor */
	.quad 0x0000000000000000 /* 0x0b reserved */
	.quad 0x0000000000000000 /* 0x13 reserved */
	.quad 0x0000000000000000 /* 0x1b reserved */
	.quad 0x0000000000000000 /* 0x20 unused */
	.quad 0x0000000000000000 /* 0x28 unused */
	.quad 0x0000000000000000 /* 0x33 TLS entry 1 */
	.quad 0x0000000000000000 /* 0x3b TLS entry 2 */
	.quad 0x0000000000000000 /* 0x43 TLS entry 3 */
	.quad 0x0000000000000000 /* 0x4b reserved */
	.quad 0x0000000000000000 /* 0x53 reserved */
	.quad 0x0000000000000000 /* 0x5b reserved */

	.quad 0x00cf9a000000ffff /* 0x60 kernel 4GB code at 0x00000000 */
	.quad 0x00cf92000000ffff /* 0x68 kernel 4GB data at 0x00000000 */
	.quad 0x00cffa000000ffff /* 0x73 user 4GB code at 0x00000000 */
	.quad 0x00cff2000000ffff /* 0x7b user 4GB data at 0x00000000 */

	.quad 0x0000000000000000 /* 0x80 TSS descriptor */
	.quad 0x0000000000000000 /* 0x88 LDT descriptor */

	/* Segments used for calling PnP BIOS */
	.quad 0x00c09a0000000000 /* 0x90 32-bit code */
	.quad 0x00809a0000000000 /* 0x98 16-bit code */
	.quad 0x0080920000000000 /* 0xa0 16-bit data */
	.quad 0x0080920000000000 /* 0xa8 16-bit data */
	.quad 0x0080920000000000 /* 0xb0 16-bit data */
	/*
	* The APM segments have byte granularity and their bases
	* and limits are set at run time.
	*/
	.quad 0x00409a0000000000 /* 0xb8 APM CS code */
	.quad 0x00009a0000000000 /* 0xc0 APM CS 16 code (16 bit) */
	.quad 0x0040920000000000 /* 0xc8 APM DS data */

	.quad 0x0000920000000000 /* 0xd0 - ESPFIX 16-bit SS */
	.quad 0x0000000000000000 /* 0xd8 - unused */
	.quad 0x0000000000000000 /* 0xe0 - unused */
	.quad 0x0000000000000000 /* 0xe8 - unused */
	.quad 0x0000000000000000 /* 0xf0 - unused */
	.quad 0x0000000000000000 /* 0xf8 - GDT entry 31: double-fault TSS */