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,_floppy_track_buffer
  16 _pg_dir:
  17 startup_32:
  18         cld
  19         movl $0x10,%eax
  20         mov %ax,%ds
  21         mov %ax,%es
  22         mov %ax,%fs
  23         mov %ax,%gs
  24         lss _stack_start,%esp
  25         call setup_idt
  26         call setup_gdt
  27         movl $0x10,%eax         # reload all the segment registers
  28         mov %ax,%ds             # after changing gdt. CS was already
  29         mov %ax,%es             # reloaded in 'setup_gdt'
  30         mov %ax,%fs
  31         mov %ax,%gs
  32         lss _stack_start,%esp
  33         xorl %eax,%eax
  34 1:      incl %eax               # check that A20 really IS enabled
  35         movl %eax,0x000000      # loop forever if it isn't
  36         cmpl %eax,0x100000
  37         je 1b
  38 /* check if it is 486 or 386. */
  39         movl %esp,%edi          # save stack pointer
  40         andl $0xfffffffc,%esp   # align stack to avoid AC fault
  41         pushfl                  # push EFLAGS
  42         popl %eax               # get EFLAGS
  43         movl %eax,%ecx          # save original EFLAGS
  44         xorl $0x40000,%eax      # flip AC bit in EFLAGS
  45         pushl %eax              # copy to EFLAGS
  46         popfl                   # set EFLAGS
  47         pushfl                  # get new EFLAGS
  48         popl %eax               # put it in eax
  49         xorl %ecx,%eax          # check if AC bit is changed. zero is 486.
  50         jz 1f                   # 486
  51         pushl %ecx              # restore original EFLAGS
  52         popfl
  53         movl %edi,%esp          # restore esp
  54         movl %cr0,%eax          # 386
  55         andl $0x80000011,%eax   # Save PG,PE,ET
  56         orl $2,%eax             # set MP
  57         jmp 2f  
  58 /*
  59  * NOTE! 486 should set bit 16, to check for write-protect in supervisor
  60  * mode. Then it would be unnecessary with the "verify_area()"-calls.
  61  * 486 users probably want to set the NE (#5) bit also, so as to use
  62  * int 16 for math errors.
  63  */
  64 1:      pushl %ecx              # restore original EFLAGS
  65         popfl
  66         movl %edi,%esp          # restore esp
  67         movl %cr0,%eax          # 486
  68         andl $0x80000011,%eax   # Save PG,PE,ET
  69         orl $0x10022,%eax       # set NE and MP
  70 2:      movl %eax,%cr0
  71         call check_x87
  72         jmp after_page_tables
  73 
  74 /*
  75  * We depend on ET to be correct. This checks for 287/387.
  76  */
  77 check_x87:
  78         fninit
  79         fstsw %ax
  80         cmpb $0,%al
  81         je 1f
  82         movl %cr0,%eax          /* no coprocessor: have to set bits */
  83         xorl $6,%eax            /* reset MP, set EM */
  84         movl %eax,%cr0
  85         ret
  86 .align 2
  87 1:      .byte 0xDB,0xE4         /* fsetpm for 287, ignored by 387 */
  88         ret
  89 
  90 /*
  91  *  setup_idt
  92  *
  93  *  sets up a idt with 256 entries pointing to
  94  *  ignore_int, interrupt gates. It then loads
  95  *  idt. Everything that wants to install itself
  96  *  in the idt-table may do so themselves. Interrupts
  97  *  are enabled elsewhere, when we can be relatively
  98  *  sure everything is ok. This routine will be over-
  99  *  written by the page tables.
 100  */
 101 setup_idt:
 102         lea ignore_int,%edx
 103         movl $0x00080000,%eax
 104         movw %dx,%ax            /* selector = 0x0008 = cs */
 105         movw $0x8E00,%dx        /* interrupt gate - dpl=0, present */
 106 
 107         lea _idt,%edi
 108         mov $256,%ecx
 109 rp_sidt:
 110         movl %eax,(%edi)
 111         movl %edx,4(%edi)
 112         addl $8,%edi
 113         dec %ecx
 114         jne rp_sidt
 115         lidt idt_descr
 116         ret
 117 
 118 /*
 119  *  setup_gdt
 120  *
 121  *  This routines sets up a new gdt and loads it.
 122  *  Only two entries are currently built, the same
 123  *  ones that were built in init.s. The routine
 124  *  is VERY complicated at two whole lines, so this
 125  *  rather long comment is certainly needed :-).
 126  *  This routine will beoverwritten by the page tables.
 127  */
 128 setup_gdt:
 129         lgdt gdt_descr
 130         ret
 131 
 132 /*
 133  * I put the kernel page tables right after the page directory,
 134  * using 4 of them to span 16 Mb of physical memory. People with
 135  * more than 16MB will have to expand this.
 136  */
 137 .org 0x1000
 138 pg0:
 139 
 140 .org 0x2000
 141 pg1:
 142 
 143 .org 0x3000
 144 pg2:
 145 
 146 .org 0x4000
 147 pg3:
 148 
 149 .org 0x5000
 150 /*
 151  * tmp_floppy_area is used by the floppy-driver when DMA cannot
 152  * reach to a buffer-block. It needs to be aligned, so that it isn't
 153  * on a 64kB border.
 154  */
 155 _tmp_floppy_area:
 156         .fill 1024,1,0
 157 /*
 158  * floppy_track_buffer is used to buffer one track of floppy data: it
 159  * has to be separate from the tmp_floppy area, as otherwise a single-
 160  * sector read/write can mess it up. It can contain one full track of
 161  * data (18*2*512 bytes).
 162  */
 163 _floppy_track_buffer:
 164         .fill 512*2*18,1,0
 165 
 166 after_page_tables:
 167         call setup_paging
 168         pushl $0                # These are the parameters to main :-)
 169         pushl $0
 170         pushl $0
 171         cld                     # gcc2 wants the direction flag cleared at all times
 172         call _start_kernel
 173 L6:
 174         jmp L6                  # main should never return here, but
 175                                 # just in case, we know what happens.
 176 
 177 /* This is the default interrupt "handler" :-) */
 178 int_msg:
 179         .asciz "Unknown interrupt\n\r"
 180 .align 2
 181 ignore_int:
 182         cld
 183         pushl %eax
 184         pushl %ecx
 185         pushl %edx
 186         push %ds
 187         push %es
 188         push %fs
 189         movl $0x10,%eax
 190         mov %ax,%ds
 191         mov %ax,%es
 192         mov %ax,%fs
 193         pushl $int_msg
 194         call _printk
 195         popl %eax
 196         pop %fs
 197         pop %es
 198         pop %ds
 199         popl %edx
 200         popl %ecx
 201         popl %eax
 202         iret
 203 
 204 
 205 /*
 206  * Setup_paging
 207  *
 208  * This routine sets up paging by setting the page bit
 209  * in cr0. The page tables are set up, identity-mapping
 210  * the first 16MB. The pager assumes that no illegal
 211  * addresses are produced (ie >4Mb on a 4Mb machine).
 212  *
 213  * NOTE! Although all physical memory should be identity
 214  * mapped by this routine, only the kernel page functions
 215  * use the >1Mb addresses directly. All "normal" functions
 216  * use just the lower 1Mb, or the local data space, which
 217  * will be mapped to some other place - mm keeps track of
 218  * that.
 219  *
 220  * For those with more memory than 16 Mb - tough luck. I've
 221  * not got it, why should you :-) The source is here. Change
 222  * it. (Seriously - it shouldn't be too difficult. Mostly
 223  * change some constants etc. I left it at 16Mb, as my machine
 224  * even cannot be extended past that (ok, but it was cheap :-)
 225  * I've tried to show which constants to change by having
 226  * some kind of marker at them (search for "16Mb"), but I
 227  * won't guarantee that's all :-( )
 228  */
 229 .align 2
 230 setup_paging:
 231         movl $1024*5,%ecx               /* 5 pages - pg_dir+4 page tables */
 232         xorl %eax,%eax
 233         xorl %edi,%edi                  /* pg_dir is at 0x000 */
 234         cld;rep;stosl
 235         movl $pg0+7,_pg_dir             /* set present bit/user r/w */
 236         movl $pg1+7,_pg_dir+4           /*  --------- " " --------- */
 237         movl $pg2+7,_pg_dir+8           /*  --------- " " --------- */
 238         movl $pg3+7,_pg_dir+12          /*  --------- " " --------- */
 239         movl $pg3+4092,%edi
 240         movl $0xfff007,%eax             /*  16Mb - 4096 + 7 (r/w user,p) */
 241         std
 242 1:      stosl                   /* fill pages backwards - more efficient :-) */
 243         subl $0x1000,%eax
 244         jge 1b
 245         cld
 246         xorl %eax,%eax          /* pg_dir is at 0x0000 */
 247         movl %eax,%cr3          /* cr3 - page directory start */
 248         movl %cr0,%eax
 249         orl $0x80000000,%eax
 250         movl %eax,%cr0          /* set paging (PG) bit */
 251         ret                     /* this also flushes prefetch-queue */
 252 
 253 .align 2
 254 .word 0
 255 idt_descr:
 256         .word 256*8-1           # idt contains 256 entries
 257         .long _idt
 258 .align 2
 259 .word 0
 260 gdt_descr:
 261         .word 256*8-1           # so does gdt (not that that's any
 262         .long _gdt              # magic number, but it works for me :^)
 263 
 264         .align 3
 265 _idt:   .fill 256,8,0           # idt is uninitialized
 266 
 267 _gdt:   .quad 0x0000000000000000        /* NULL descriptor */
 268         .quad 0x00c09a0000000fff        /* 16Mb */
 269         .quad 0x00c0920000000fff        /* 16Mb */
 270         .quad 0x0000000000000000        /* TEMPORARY - don't use */
 271         .fill 252,8,0                   /* space for LDT's and TSS's etc */
/* */
root/boot/head.s
