boot/head.s

/* */
   1 /*
   2  *  linux/boot/head.s
   3  *
   4  *  (C) 1991  Linus Torvalds
   5  */
   6 
   7 /*
   8  *  head.s contains the 32-bit startup code.
   9  *
  10  * NOTE!!! Startup happens at absolute address 0x00000000, which is also where
  11  * the page directory will exist. The startup code will be overwritten by
  12  * the page directory.
  13  */
  14 .text
  15 .globl _idt,_gdt,_pg_dir,_tmp_floppy_area
  16 _pg_dir:
  17 startup_32:
  18         movl $0x10,%eax
  19         mov %ax,%ds
  20         mov %ax,%es
  21         mov %ax,%fs
  22         mov %ax,%gs
  23         lss _stack_start,%esp
  24         call setup_idt
  25         call setup_gdt
  26         movl $0x10,%eax         # reload all the segment registers
  27         mov %ax,%ds             # after changing gdt. CS was already
  28         mov %ax,%es             # reloaded in 'setup_gdt'
  29         mov %ax,%fs
  30         mov %ax,%gs
  31         lss _stack_start,%esp
  32         xorl %eax,%eax
  33 1:      incl %eax               # check that A20 really IS enabled
  34         movl %eax,0x000000      # loop forever if it isn't
  35         cmpl %eax,0x100000
  36         je 1b
  37 /*
  38  * NOTE! 486 should set bit 16, to check for write-protect in supervisor
  39  * mode. Then it would be unnecessary with the "verify_area()"-calls.
  40  * 486 users probably want to set the NE (#5) bit also, so as to use
  41  * int 16 for math errors.
  42  */
  43         movl %cr0,%eax          # check math chip
  44         andl $0x80000011,%eax   # Save PG,PE,ET
  45 /* "orl $0x10020,%eax" here for 486 might be good */
  46         orl $2,%eax             # set MP
  47         movl %eax,%cr0
  48         call check_x87
  49         jmp after_page_tables
  50 
  51 /*
  52  * We depend on ET to be correct. This checks for 287/387.
  53  */
  54 check_x87:
  55         fninit
  56         fstsw %ax
  57         cmpb $0,%al
  58         je 1f                   /* no coprocessor: have to set bits */
  59         movl %cr0,%eax
  60         xorl $6,%eax            /* reset MP, set EM */
  61         movl %eax,%cr0
  62         ret
  63 .align 2
  64 1:      .byte 0xDB,0xE4         /* fsetpm for 287, ignored by 387 */
  65         ret
  66 
  67 /*
  68  *  setup_idt
  69  *
  70  *  sets up a idt with 256 entries pointing to
  71  *  ignore_int, interrupt gates. It then loads
  72  *  idt. Everything that wants to install itself
  73  *  in the idt-table may do so themselves. Interrupts
  74  *  are enabled elsewhere, when we can be relatively
  75  *  sure everything is ok. This routine will be over-
  76  *  written by the page tables.
  77  */
  78 setup_idt:
  79         lea ignore_int,%edx
  80         movl $0x00080000,%eax
  81         movw %dx,%ax            /* selector = 0x0008 = cs */
  82         movw $0x8E00,%dx        /* interrupt gate - dpl=0, present */
  83 
  84         lea _idt,%edi
  85         mov $256,%ecx
  86 rp_sidt:
  87         movl %eax,(%edi)
  88         movl %edx,4(%edi)
  89         addl $8,%edi
  90         dec %ecx
  91         jne rp_sidt
  92         lidt idt_descr
  93         ret
  94 
  95 /*
  96  *  setup_gdt
  97  *
  98  *  This routines sets up a new gdt and loads it.
  99  *  Only two entries are currently built, the same
 100  *  ones that were built in init.s. The routine
 101  *  is VERY complicated at two whole lines, so this
 102  *  rather long comment is certainly needed :-).
 103  *  This routine will beoverwritten by the page tables.
 104  */
 105 setup_gdt:
 106         lgdt gdt_descr
 107         ret
 108 
 109 /*
 110  * I put the kernel page tables right after the page directory,
 111  * using 4 of them to span 16 Mb of physical memory. People with
 112  * more than 16MB will have to expand this.
 113  */
 114 .org 0x1000
 115 pg0:
 116 
 117 .org 0x2000
 118 pg1:
 119 
 120 .org 0x3000
 121 pg2:
 122 
 123 .org 0x4000
 124 pg3:
 125 
 126 .org 0x5000
 127 /*
 128  * tmp_floppy_area is used by the floppy-driver when DMA cannot
 129  * reach to a buffer-block. It needs to be aligned, so that it isn't
 130  * on a 64kB border.
 131  */
 132 _tmp_floppy_area:
 133         .fill 1024,1,0
 134 
 135 after_page_tables:
 136         pushl $0                # These are the parameters to main :-)
 137         pushl $0
 138         pushl $0
 139         pushl $L6               # return address for main, if it decides to.
 140         pushl $_main
 141         jmp setup_paging
 142 L6:
 143         jmp L6                  # main should never return here, but
 144                                 # just in case, we know what happens.
 145 
 146 /* This is the default interrupt "handler" :-) */
 147 int_msg:
 148         .asciz "Unknown interrupt\n\r"
 149 .align 2
 150 ignore_int:
 151         pushl %eax
 152         pushl %ecx
 153         pushl %edx
 154         push %ds
 155         push %es
 156         push %fs
 157         movl $0x10,%eax
 158         mov %ax,%ds
 159         mov %ax,%es
 160         mov %ax,%fs
 161         pushl $int_msg
 162         call _printk
 163         popl %eax
 164         pop %fs
 165         pop %es
 166         pop %ds
 167         popl %edx
 168         popl %ecx
 169         popl %eax
 170         iret
 171 
 172 
 173 /*
 174  * Setup_paging
 175  *
 176  * This routine sets up paging by setting the page bit
 177  * in cr0. The page tables are set up, identity-mapping
 178  * the first 16MB. The pager assumes that no illegal
 179  * addresses are produced (ie >4Mb on a 4Mb machine).
 180  *
 181  * NOTE! Although all physical memory should be identity
 182  * mapped by this routine, only the kernel page functions
 183  * use the >1Mb addresses directly. All "normal" functions
 184  * use just the lower 1Mb, or the local data space, which
 185  * will be mapped to some other place - mm keeps track of
 186  * that.
 187  *
 188  * For those with more memory than 16 Mb - tough luck. I've
 189  * not got it, why should you :-) The source is here. Change
 190  * it. (Seriously - it shouldn't be too difficult. Mostly
 191  * change some constants etc. I left it at 16Mb, as my machine
 192  * even cannot be extended past that (ok, but it was cheap :-)
 193  * I've tried to show which constants to change by having
 194  * some kind of marker at them (search for "16Mb"), but I
 195  * won't guarantee that's all :-( )
 196  */
 197 .align 2
 198 setup_paging:
 199         movl $1024*5,%ecx               /* 5 pages - pg_dir+4 page tables */
 200         xorl %eax,%eax
 201         xorl %edi,%edi                  /* pg_dir is at 0x000 */
 202         cld;rep;stosl
 203         movl $pg0+7,_pg_dir             /* set present bit/user r/w */
 204         movl $pg1+7,_pg_dir+4           /*  --------- " " --------- */
 205         movl $pg2+7,_pg_dir+8           /*  --------- " " --------- */
 206         movl $pg3+7,_pg_dir+12          /*  --------- " " --------- */
 207         movl $pg3+4092,%edi
 208         movl $0xfff007,%eax             /*  16Mb - 4096 + 7 (r/w user,p) */
 209         std
 210 1:      stosl                   /* fill pages backwards - more efficient :-) */
 211         subl $0x1000,%eax
 212         jge 1b
 213         xorl %eax,%eax          /* pg_dir is at 0x0000 */
 214         movl %eax,%cr3          /* cr3 - page directory start */
 215         movl %cr0,%eax
 216         orl $0x80000000,%eax
 217         movl %eax,%cr0          /* set paging (PG) bit */
 218         ret                     /* this also flushes prefetch-queue */
 219 
 220 .align 2
 221 .word 0
 222 idt_descr:
 223         .word 256*8-1           # idt contains 256 entries
 224         .long _idt
 225 .align 2
 226 .word 0
 227 gdt_descr:
 228         .word 256*8-1           # so does gdt (not that that's any
 229         .long _gdt              # magic number, but it works for me :^)
 230 
 231         .align 3
 232 _idt:   .fill 256,8,0           # idt is uninitialized
 233 
 234 _gdt:   .quad 0x0000000000000000        /* NULL descriptor */
 235         .quad 0x00c09a0000000fff        /* 16Mb */
 236         .quad 0x00c0920000000fff        /* 16Mb */
 237         .quad 0x0000000000000000        /* TEMPORARY - don't use */
 238         .fill 252,8,0                   /* space for LDT's and TSS's etc */
/* */
root/boot/head.s
