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 
  11 .text
  12 .globl _idt,_gdt,
  13 .globl _swapper_pg_dir,_pg0
  14 .globl _empty_bad_page
  15 .globl _empty_bad_page_table
  16 .globl _empty_zero_page
  17 .globl _tmp_floppy_area,_floppy_track_buffer
  18 
  19 #include <linux/tasks.h>
  20 #include <linux/segment.h>
  21 
  22 #define CL_MAGIC_ADDR   0x90020
  23 #define CL_MAGIC        0xA33F
  24 #define CL_BASE_ADDR    0x90000
  25 #define CL_OFFSET       0x90022
  26 
  27 /*
  28  * swapper_pg_dir is the main page directory, address 0x00001000 (or at
  29  * address 0x00101000 for a compressed boot).
  30  */
  31 startup_32:
  32         cld
  33         movl $(KERNEL_DS),%eax
  34         mov %ax,%ds
  35         mov %ax,%es
  36         mov %ax,%fs
  37         mov %ax,%gs
  38         lss _stack_start,%esp
  39 /*
  40  * Clear BSS first so that there are no surprises...
  41  */
  42         xorl %eax,%eax
  43         movl $__edata,%edi
  44         movl $__end,%ecx
  45         subl %edi,%ecx
  46         cld
  47         rep
  48         stosb
  49 /*
  50  * start system 32-bit setup. We need to re-do some of the things done
  51  * in 16-bit mode for the "real" operations.
  52  */
  53         call setup_idt
  54         xorl %eax,%eax
  55 1:      incl %eax               # check that A20 really IS enabled
  56         movl %eax,0x000000      # loop forever if it isn't
  57         cmpl %eax,0x100000
  58         je 1b
  59 /*
  60  * Initialize eflags.  Some BIOS's leave bits like NT set.  This would
  61  * confuse the debugger if this code is traced.
  62  * XXX - best to initialize before switching to protected mode.
  63  */
  64         pushl $0
  65         popfl
  66 /*
  67  * Copy bootup parameters out of the way. First 2kB of
  68  * _empty_zero_page is for boot parameters, second 2kB
  69  * is for the command line.
  70  */
  71         movl $0x90000,%esi
  72         movl $_empty_zero_page,%edi
  73         movl $512,%ecx
  74         cld
  75         rep
  76         movsl
  77         xorl %eax,%eax
  78         movl $512,%ecx
  79         rep
  80         stosl
  81         cmpw $(CL_MAGIC),CL_MAGIC_ADDR
  82         jne 1f
  83         movl $_empty_zero_page+2048,%edi
  84         movzwl CL_OFFSET,%esi
  85         addl $(CL_BASE_ADDR),%esi
  86         movl $2048,%ecx
  87         rep
  88         movsb
  89 1:
  90 /* check if it is 486 or 386. */
  91 /*
  92  * XXX - this does a lot of unnecessary setup.  Alignment checks don't
  93  * apply at our cpl of 0 and the stack ought to be aligned already, and
  94  * we don't need to preserve eflags.
  95  */
  96         movl %esp,%edi          # save stack pointer
  97         andl $0xfffffffc,%esp   # align stack to avoid AC fault
  98         movl $3,_x86
  99         pushfl                  # push EFLAGS
 100         popl %eax               # get EFLAGS
 101         movl %eax,%ecx          # save original EFLAGS
 102         xorl $0x40000,%eax      # flip AC bit in EFLAGS
 103         pushl %eax              # copy to EFLAGS
 104         popfl                   # set EFLAGS
 105         pushfl                  # get new EFLAGS
 106         popl %eax               # put it in eax
 107         xorl %ecx,%eax          # change in flags
 108         andl $0x40000,%eax      # check if AC bit changed
 109         je is386
 110         movl $4,_x86
 111         movl %ecx,%eax
 112         xorl $0x200000,%eax     # check ID flag
 113         pushl %eax
 114         popfl                   # if we are on a 486,
 115         pushfl                  # we can't change it
 116         popl %eax
 117         xorl %ecx,%eax
 118         andl $0x200000,%eax
 119         je is486
 120         movl $5,_x86            # 586 setup same as 486 at least for now
 121 is486:  pushl %ecx              # restore original EFLAGS
 122         popfl
 123         movl %edi,%esp          # restore esp
 124         movl %cr0,%eax          # 486
 125         andl $0x80000011,%eax   # Save PG,PE,ET
 126         orl $0x50022,%eax       # set AM, WP, NE and MP
 127         jmp 2f
 128 is386:  pushl %ecx              # restore original EFLAGS
 129         popfl
 130         movl %edi,%esp          # restore esp
 131         movl %cr0,%eax          # 386
 132         andl $0x80000011,%eax   # Save PG,PE,ET
 133         orl $2,%eax             # set MP
 134 2:      movl %eax,%cr0
 135         call check_x87
 136         call setup_paging
 137         lgdt gdt_descr
 138         lidt idt_descr
 139         ljmp $(KERNEL_CS),$1f
 140 1:      movl $(KERNEL_DS),%eax  # reload all the segment registers
 141         mov %ax,%ds             # after changing gdt.
 142         mov %ax,%es
 143         mov %ax,%fs
 144         mov %ax,%gs
 145         lss _stack_start,%esp
 146         xorl %eax,%eax
 147         lldt %ax
 148         pushl %eax              # These are the parameters to main :-)
 149         pushl %eax
 150         pushl %eax
 151         cld                     # gcc2 wants the direction flag cleared at all times
 152         call _start_kernel
 153 L6:
 154         jmp L6                  # main should never return here, but
 155                                 # just in case, we know what happens.
 156 
 157 /*
 158  * We depend on ET to be correct. This checks for 287/387.
 159  */
 160 check_x87:
 161         movl $0,_hard_math
 162         clts
 163         fninit
 164         fstsw %ax
 165         cmpb $0,%al
 166         je 1f
 167         movl %cr0,%eax          /* no coprocessor: have to set bits */
 168         xorl $4,%eax            /* set EM */
 169         movl %eax,%cr0
 170         ret
 171 .align 2
 172 1:      movl $1,_hard_math
 173         .byte 0xDB,0xE4         /* fsetpm for 287, ignored by 387 */
 174         ret
 175 
 176 /*
 177  *  setup_idt
 178  *
 179  *  sets up a idt with 256 entries pointing to
 180  *  ignore_int, interrupt gates. It doesn't actually load
 181  *  idt - that can be done only after paging has been enabled
 182  *  and the kernel moved to 0xC0000000. Interrupts
 183  *  are enabled elsewhere, when we can be relatively
 184  *  sure everything is ok.
 185  */
 186 setup_idt:
 187         lea ignore_int,%edx
 188         movl $(KERNEL_CS << 16),%eax
 189         movw %dx,%ax            /* selector = 0x0010 = cs */
 190         movw $0x8E00,%dx        /* interrupt gate - dpl=0, present */
 191 
 192         lea _idt,%edi
 193         mov $256,%ecx
 194 rp_sidt:
 195         movl %eax,(%edi)
 196         movl %edx,4(%edi)
 197         addl $8,%edi
 198         dec %ecx
 199         jne rp_sidt
 200         ret
 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  * (ref: added support for up to 32mb, 17Apr92)  -- Rik Faith
 211  * (ref: update, 25Sept92)  -- croutons@crunchy.uucp 
 212  * (ref: 92.10.11 - Linus Torvalds. Corrected 16M limit - no upper memory limit)
 213  */
 214 .align 2
 215 setup_paging:
 216         movl $1024*2,%ecx               /* 2 pages - swapper_pg_dir+1 page table */
 217         xorl %eax,%eax
 218         movl $_swapper_pg_dir,%edi      /* swapper_pg_dir is at 0x1000 */
 219         cld;rep;stosl
 220 /* Identity-map the kernel in low 4MB memory for ease of transition */
 221         movl $_pg0+7,_swapper_pg_dir            /* set present bit/user r/w */
 222 /* But the real place is at 0xC0000000 */
 223         movl $_pg0+7,_swapper_pg_dir+3072       /* set present bit/user r/w */
 224         movl $_pg0+4092,%edi
 225         movl $0x03ff007,%eax            /*  4Mb - 4096 + 7 (r/w user,p) */
 226         std
 227 1:      stosl                   /* fill the page backwards - more efficient :-) */
 228         subl $0x1000,%eax
 229         jge 1b
 230         cld
 231         movl $_swapper_pg_dir,%eax
 232         movl %eax,%cr3                  /* cr3 - page directory start */
 233         movl %cr0,%eax
 234         orl $0x80000000,%eax
 235         movl %eax,%cr0          /* set paging (PG) bit */
 236         ret                     /* this also flushes the prefetch-queue */
 237 
 238 /*
 239  * page 0 is made non-existent, so that kernel NULL pointer references get
 240  * caught. Thus the swapper page directory has been moved to 0x1000
 241  *
 242  * XXX Actually, the swapper page directory is at 0x1000 plus 1 megabyte,
 243  * with the introduction of the compressed boot code.  Theoretically,
 244  * the original design of overlaying the startup code with the swapper
 245  * page directory is still possible --- it would reduce the size of the kernel
 246  * by 2-3k.  This would be a good thing to do at some point.....
 247  */
 248 .org 0x1000
 249 _swapper_pg_dir:
 250 /*
 251  * The page tables are initialized to only 4MB here - the final page
 252  * tables are set up later depending on memory size.
 253  */
 254 .org 0x2000
 255 _pg0:
 256 
 257 .org 0x3000
 258 _empty_bad_page:
 259 
 260 .org 0x4000
 261 _empty_bad_page_table:
 262 
 263 .org 0x5000
 264 _empty_zero_page:
 265 
 266 .org 0x6000
 267 /*
 268  * tmp_floppy_area is used by the floppy-driver when DMA cannot
 269  * reach to a buffer-block. It needs to be aligned, so that it isn't
 270  * on a 64kB border.
 271  */
 272 _tmp_floppy_area:
 273         .fill 1024,1,0
 274 /*
 275  * floppy_track_buffer is used to buffer one track of floppy data: it
 276  * has to be separate from the tmp_floppy area, as otherwise a single-
 277  * sector read/write can mess it up. It can contain one full track of
 278  * data (18*2*512 bytes).
 279  */
 280 _floppy_track_buffer:
 281         .fill 512*2*18,1,0
 282 
 283 /* This is the default interrupt "handler" :-) */
 284 int_msg:
 285         .asciz "Unknown interrupt\n"
 286 .align 2
 287 ignore_int:
 288         cld
 289         pushl %eax
 290         pushl %ecx
 291         pushl %edx
 292         push %ds
 293         push %es
 294         push %fs
 295         movl $(KERNEL_DS),%eax
 296         mov %ax,%ds
 297         mov %ax,%es
 298         mov %ax,%fs
 299         pushl $int_msg
 300         call _printk
 301         popl %eax
 302         pop %fs
 303         pop %es
 304         pop %ds
 305         popl %edx
 306         popl %ecx
 307         popl %eax
 308         iret
 309 
 310 /*
 311  * The interrupt descriptor table has room for 256 idt's
 312  */
 313 .align 4
 314 .word 0
 315 idt_descr:
 316         .word 256*8-1           # idt contains 256 entries
 317         .long 0xc0000000+_idt
 318 
 319 .align 4
 320 _idt:
 321         .fill 256,8,0           # idt is uninitialized
 322 
 323 .align 4
 324 .word 0
 325 gdt_descr:
 326         .word (8+2*NR_TASKS)*8-1
 327         .long 0xc0000000+_gdt
 328 
 329 /*
 330  * This gdt setup gives the kernel a 1GB address space at virtual
 331  * address 0xC0000000 - space enough for expansion, I hope.
 332  */
 333 .align 4
 334 _gdt:
 335         .quad 0x0000000000000000        /* NULL descriptor */
 336         .quad 0x0000000000000000        /* not used */
 337         .quad 0xc0c39a000000ffff        /* 0x10 kernel 1GB code at 0xC0000000 */
 338         .quad 0xc0c392000000ffff        /* 0x18 kernel 1GB data at 0xC0000000 */
 339         .quad 0x00cbfa000000ffff        /* 0x23 user   3GB code at 0x00000000 */
 340         .quad 0x00cbf2000000ffff        /* 0x2b user   3GB data at 0x00000000 */
 341         .quad 0x0000000000000000        /* not used */
 342         .quad 0x0000000000000000        /* not used */
 343         .fill 2*NR_TASKS,8,0            /* space for LDT's and TSS's etc */

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