root/boot/head.S

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   1 /*
   2  *  linux/boot/head.s
   3  *
   4  *  Copyright (C) 1991, 1992  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,
  16 .globl _swapper_pg_dir,_pg0
  17 .globl _empty_bad_page
  18 .globl _empty_bad_page_table
  19 .globl _tmp_floppy_area,_floppy_track_buffer
  20 
  21 /*
  22  * swapper_pg_dir is the main page directory, address 0x00001000
  23  */
  24 startup_32:
  25         cld
  26         movl $0x10,%eax
  27         mov %ax,%ds
  28         mov %ax,%es
  29         mov %ax,%fs
  30         mov %ax,%gs
  31         lss _stack_start,%esp
  32         call setup_idt
  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         movl $0,_hard_math
  79         fninit
  80         fstsw %ax
  81         cmpb $0,%al
  82         je 1f
  83         movl %cr0,%eax          /* no coprocessor: have to set bits */
  84         xorl $6,%eax            /* reset MP, set EM */
  85         movl %eax,%cr0
  86         ret
  87 .align 2
  88 1:      movl $1,_hard_math
  89         .byte 0xDB,0xE4         /* fsetpm for 287, ignored by 387 */
  90         ret
  91 
  92 /*
  93  *  setup_idt
  94  *
  95  *  sets up a idt with 256 entries pointing to
  96  *  ignore_int, interrupt gates. It doesn't actually load
  97  *  idt - that can be done only after paging has been enabled
  98  *  and the kernel moved to 0xC0000000. Interrupts
  99  *  are enabled elsewhere, when we can be relatively
 100  *  sure everything is ok. This routine will be over-
 101  *  written by the page tables.
 102  */
 103 setup_idt:
 104         lea ignore_int,%edx
 105         movl $0x00080000,%eax
 106         movw %dx,%ax            /* selector = 0x0008 = cs */
 107         movw $0x8E00,%dx        /* interrupt gate - dpl=0, present */
 108 
 109         lea _idt,%edi
 110         mov $256,%ecx
 111 rp_sidt:
 112         movl %eax,(%edi)
 113         movl %edx,4(%edi)
 114         addl $8,%edi
 115         dec %ecx
 116         jne rp_sidt
 117         ret
 118 
 119 /*
 120  * page 0 is made non-existent, so that kernel NULL pointer references get
 121  * caught. Thus the swapper page directory has been moved to 0x1000
 122  */
 123 .org 0x1000
 124 _swapper_pg_dir:
 125 /*
 126  * The page tables are initialized to only 4MB here - the final page
 127  * tables are set up later depending on memory size.
 128  */
 129 .org 0x2000
 130 _pg0:
 131 
 132 .org 0x3000
 133 _empty_bad_page:
 134 
 135 .org 0x4000
 136 _empty_bad_page_table:
 137 
 138 .org 0x5000
 139 /*
 140  * tmp_floppy_area is used by the floppy-driver when DMA cannot
 141  * reach to a buffer-block. It needs to be aligned, so that it isn't
 142  * on a 64kB border.
 143  */
 144 _tmp_floppy_area:
 145         .fill 1024,1,0
 146 /*
 147  * floppy_track_buffer is used to buffer one track of floppy data: it
 148  * has to be separate from the tmp_floppy area, as otherwise a single-
 149  * sector read/write can mess it up. It can contain one full track of
 150  * data (18*2*512 bytes).
 151  */
 152 _floppy_track_buffer:
 153         .fill 512*2*18,1,0
 154 
 155 after_page_tables:
 156         call setup_paging
 157         lgdt gdt_descr
 158         lidt idt_descr
 159         ljmp $0x08,$1f
 160 1:      movl $0x10,%eax         # reload all the segment registers
 161         mov %ax,%ds             # after changing gdt.
 162         mov %ax,%es
 163         mov %ax,%fs
 164         mov %ax,%gs
 165         lss _stack_start,%esp
 166         pushl $0                # These are the parameters to main :-)
 167         pushl $0
 168         pushl $0
 169         cld                     # gcc2 wants the direction flag cleared at all times
 170         call _start_kernel
 171 L6:
 172         jmp L6                  # main should never return here, but
 173                                 # just in case, we know what happens.
 174 
 175 /* This is the default interrupt "handler" :-) */
 176 int_msg:
 177         .asciz "Unknown interrupt\n\r"
 178 .align 2
 179 ignore_int:
 180         cld
 181         pushl %eax
 182         pushl %ecx
 183         pushl %edx
 184         push %ds
 185         push %es
 186         push %fs
 187         movl $0x10,%eax
 188         mov %ax,%ds
 189         mov %ax,%es
 190         mov %ax,%fs
 191         pushl $int_msg
 192         call _printk
 193         popl %eax
 194         pop %fs
 195         pop %es
 196         pop %ds
 197         popl %edx
 198         popl %ecx
 199         popl %eax
 200         iret
 201 
 202 
 203 /*
 204  * Setup_paging
 205  *
 206  * This routine sets up paging by setting the page bit
 207  * in cr0. The page tables are set up, identity-mapping
 208  * the first 4MB.  The rest are initialized later.
 209  *
 210  * NOTE! Although all physical memory should be identity
 211  * mapped by this routine, only the kernel page functions
 212  * use the >1Mb addresses directly. All "normal" functions
 213  * use just the lower 1Mb, or the local data space, which
 214  * will be mapped to some other place - mm keeps track of
 215  * that.
 216  *
 217  * For those with more memory than 16 Mb - tough luck. I've
 218  * not got it, why should you :-) The source is here. Change
 219  * it. (Seriously - it shouldn't be too difficult. Mostly
 220  * change some constants etc. I left it at 16Mb, as my machine
 221  * even cannot be extended past that (ok, but it was cheap :-)
 222  * I've tried to show which constants to change by having
 223  * some kind of marker at them (search for "16Mb"), but I
 224  * won't guarantee that's all :-( )
 225  *
 226  * (ref: added support for up to 32mb, 17Apr92)  -- Rik Faith
 227  * (ref: update, 25Sept92)  -- croutons@crunchy.uucp 
 228  * (ref: 92.10.11 - Linus Torvalds. Corrected 16M limit - no upper memory limit)
 229  */
 230 .align 2
 231 setup_paging:
 232         movl $1024*2,%ecx               /* 2 pages - swapper_pg_dir+1 page table */
 233         xorl %eax,%eax
 234         movl $_swapper_pg_dir,%edi      /* swapper_pg_dir is at 0x1000 */
 235         cld;rep;stosl
 236 /* Identity-map the kernel in low 4MB memory for ease of transition */
 237         movl $_pg0+7,_swapper_pg_dir            /* set present bit/user r/w */
 238 /* But the real place is at 0xC0000000 */
 239         movl $_pg0+7,_swapper_pg_dir+3072       /* set present bit/user r/w */
 240         movl $_pg0+4092,%edi
 241         movl $0x03ff007,%eax            /*  4Mb - 4096 + 7 (r/w user,p) */
 242         std
 243 1:      stosl                   /* fill the page backwards - more efficient :-) */
 244         subl $0x1000,%eax
 245         jge 1b
 246         cld
 247         movl $_swapper_pg_dir,%eax
 248         movl %eax,%cr3                  /* cr3 - page directory start */
 249         movl %cr0,%eax
 250         orl $0x80000000,%eax
 251         movl %eax,%cr0          /* set paging (PG) bit */
 252         ret                     /* this also flushes the prefetch-queue */
 253 
 254 /*
 255  * The interrupt descriptor table has room for 256 idt's
 256  */
 257 .align 4
 258 .word 0
 259 idt_descr:
 260         .word 256*8-1           # idt contains 256 entries
 261         .long 0xc0000000+_idt
 262 
 263 .align 4
 264 _idt:
 265         .fill 256,8,0           # idt is uninitialized
 266 
 267 /*
 268  * The real GDT is also 256 entries long - no real reason
 269  */
 270 .align 4
 271 .word 0
 272 gdt_descr:
 273         .word 256*8-1
 274         .long 0xc0000000+_gdt
 275 
 276 /*
 277  * This gdt setup gives the kernel a 1GB address space at virtual
 278  * address 0xC0000000 - space enough for expansion, I hope.
 279  */
 280 .align 4
 281 _gdt:
 282         .quad 0x0000000000000000        /* NULL descriptor */
 283         .quad 0xc0c39a000000ffff        /* 1GB at 0xC0000000 */
 284         .quad 0xc0c392000000ffff        /* 1GB */
 285         .quad 0x0000000000000000        /* TEMPORARY - don't use */
 286         .fill 252,8,0                   /* space for LDT's and TSS's etc */

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