/* *  linux/boot/head.s * *  (C) 1991  Linus Torvalds *//* *  head.s contains the 32-bit startup code. * * NOTE!!! Startup happens at absolute address 0x00000000, which is also where * the page directory will exist. The startup code will be overwritten by * the page directory. */.text.globl _idt,_gdt,_pg_dir,_tmp_floppy_area_pg_dir:startup_32:	movl $0x10,%eax	mov %ax,%ds	mov %ax,%es	mov %ax,%fs	mov %ax,%gs	lss _stack_start,%esp	call setup_idt	call setup_gdt	movl $0x10,%eax		# reload all the segment registers	mov %ax,%ds		# after changing gdt. CS was already	mov %ax,%es		# reloaded in 'setup_gdt'	mov %ax,%fs	mov %ax,%gs	lss _stack_start,%esp	xorl %eax,%eax1:	incl %eax		# check that A20 really IS enabled	movl %eax,0x000000	# loop forever if it isn't	cmpl %eax,0x100000	je 1b/* * NOTE! 486 should set bit 16, to check for write-protect in supervisor * mode. Then it would be unnecessary with the "verify_area()"-calls. * 486 users probably want to set the NE (#5) bit also, so as to use * int 16 for math errors. */	movl %cr0,%eax		# check math chip	andl $0x80000011,%eax	# Save PG,PE,ET/* "orl $0x10020,%eax" here for 486 might be good */	orl $2,%eax		# set MP	movl %eax,%cr0	call check_x87	jmp after_page_tables/* * We depend on ET to be correct. This checks for 287/387. */check_x87:	fninit	fstsw %ax	cmpb $0,%al	je 1f			/* no coprocessor: have to set bits */	movl %cr0,%eax	xorl $6,%eax		/* reset MP, set EM */	movl %eax,%cr0	ret.align 21:	.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 then loads *  idt. Everything that wants to install itself *  in the idt-table may do so themselves. Interrupts *  are enabled elsewhere, when we can be relatively *  sure everything is ok. This routine will be over- *  written by the page tables. */setup_idt:	lea ignore_int,%edx	movl $0x00080000,%eax	movw %dx,%ax		/* selector = 0x0008 = cs */	movw $0x8E00,%dx	/* interrupt gate - dpl=0, present */	lea _idt,%edi	mov $256,%ecxrp_sidt:	movl %eax,(%edi)	movl %edx,4(%edi)	addl $8,%edi	dec %ecx	jne rp_sidt	lidt idt_descr	ret/* *  setup_gdt * *  This routines sets up a new gdt and loads it. *  Only two entries are currently built, the same *  ones that were built in init.s. The routine *  is VERY complicated at two whole lines, so this *  rather long comment is certainly needed :-). *  This routine will beoverwritten by the page tables. */setup_gdt:	lgdt gdt_descr	ret/* * I put the kernel page tables right after the page directory, * using 4 of them to span 16 Mb of physical memory. People with * more than 16MB will have to expand this. */.org 0x1000pg0:.org 0x2000pg1:.org 0x3000pg2:.org 0x4000pg3:.org 0x5000/* * tmp_floppy_area is used by the floppy-driver when DMA cannot * reach to a buffer-block. It needs to be aligned, so that it isn't * on a 64kB border. */_tmp_floppy_area:	.fill 1024,1,0after_page_tables:	pushl $0		# These are the parameters to main :-)	pushl $0	pushl $0	pushl $L6		# return address for main, if it decides to.	pushl $_main	jmp setup_pagingL6:	jmp L6			# main should never return here, but				# just in case, we know what happens./* This is the default interrupt "handler" :-) */int_msg:	.asciz "Unknown interrupt\n\r".align 2ignore_int:	pushl %eax	pushl %ecx	pushl %edx	push %ds	push %es	push %fs	movl $0x10,%eax	mov %ax,%ds	mov %ax,%es	mov %ax,%fs	pushl $int_msg	call _printk	popl %eax	pop %fs	pop %es	pop %ds	popl %edx	popl %ecx	popl %eax	iret/* * Setup_paging * * This routine sets up paging by setting the page bit * in cr0. The page tables are set up, identity-mapping * the first 16MB. The pager assumes that no illegal * addresses are produced (ie >4Mb on a 4Mb machine). * * NOTE! Although all physical memory should be identity * mapped by this routine, only the kernel page functions * use the >1Mb addresses directly. All "normal" functions * use just the lower 1Mb, or the local data space, which * will be mapped to some other place - mm keeps track of * that. * * For those with more memory than 16 Mb - tough luck. I've * not got it, why should you :-) The source is here. Change * it. (Seriously - it shouldn't be too difficult. Mostly * change some constants etc. I left it at 16Mb, as my machine * even cannot be extended past that (ok, but it was cheap :-) * I've tried to show which constants to change by having * some kind of marker at them (search for "16Mb"), but I * won't guarantee that's all :-( ) */.align 2setup_paging:	movl $1024*5,%ecx		/* 5 pages - pg_dir+4 page tables */	xorl %eax,%eax	xorl %edi,%edi			/* pg_dir is at 0x000 */	cld;rep;stosl	movl $pg0+7,_pg_dir		/* set present bit/user r/w */	movl $pg1+7,_pg_dir+4		/*  --------- " " --------- */	movl $pg2+7,_pg_dir+8		/*  --------- " " --------- */	movl $pg3+7,_pg_dir+12		/*  --------- " " --------- */	movl $pg3+4092,%edi	movl $0xfff007,%eax		/*  16Mb - 4096 + 7 (r/w user,p) */	std1:	stosl			/* fill pages backwards - more efficient :-) */	subl $0x1000,%eax	jge 1b	xorl %eax,%eax		/* pg_dir is at 0x0000 */	movl %eax,%cr3		/* cr3 - page directory start */	movl %cr0,%eax	orl $0x80000000,%eax	movl %eax,%cr0		/* set paging (PG) bit */	ret			/* this also flushes prefetch-queue */.align 2.word 0idt_descr:	.word 256*8-1		# idt contains 256 entries	.long _idt.align 2.word 0gdt_descr:	.word 256*8-1		# so does gdt (not that that's any	.long _gdt		# magic number, but it works for me :^)	.align 3_idt:	.fill 256,8,0		# idt is uninitialized_gdt:	.quad 0x0000000000000000	/* NULL descriptor */	.quad 0x00c09a0000000fff	/* 16Mb */	.quad 0x00c0920000000fff	/* 16Mb */	.quad 0x0000000000000000	/* TEMPORARY - don't use */	.fill 252,8,0			/* space for LDT's and TSS's etc */