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
  29  */
  30 startup_32:
  31         cld
  32         movl $(KERNEL_DS),%eax
  33         mov %ax,%ds
  34         mov %ax,%es
  35         mov %ax,%fs
  36         mov %ax,%gs
  37         lss _stack_start,%esp
  38         call setup_idt
  39         xorl %eax,%eax
  40 1:      incl %eax               # check that A20 really IS enabled
  41         movl %eax,0x000000      # loop forever if it isn't
  42         cmpl %eax,0x100000
  43         je 1b
  44 /*
  45  * Initialize eflags.  Some BIOS's leave bits like NT set.  This would
  46  * confuse the debugger if this code is traced.
  47  * XXX - best to initialize before switching to protected mode.
  48  */
  49         pushl $0
  50         popfl
  51 /*
  52  * Copy bootup parameters out of the way. First 2kB of
  53  * _empty_zero_page is for boot parameters, second 2kB
  54  * is for the command line.
  55  */
  56         movl $0x90000,%esi
  57         movl $_empty_zero_page,%edi
  58         movl $512,%ecx
  59         cld
  60         rep
  61         movsl
  62         xorl %eax,%eax
  63         movl $512,%ecx
  64         rep
  65         stosl
  66         cmpw $(CL_MAGIC),CL_MAGIC_ADDR
  67         jne 1f
  68         movl $_empty_zero_page+2048,%edi
  69         movzwl CL_OFFSET,%esi
  70         addl $(CL_BASE_ADDR),%esi
  71         movl $2048,%ecx
  72         rep
  73         movsb
  74 1:
  75 /*
  76  * Clear BSS
  77  */
  78         xorl %eax,%eax
  79         movl $__edata,%edi
  80         movl $__end,%ecx
  81         subl %edi,%ecx
  82         cld
  83         rep
  84         stosb
  85 /* check if it is 486 or 386. */
  86 /*
  87  * XXX - this does a lot of unnecessary setup.  Alignment checks don't
  88  * apply at our cpl of 0 and the stack ought to be aligned already, and
  89  * we don't need to preserve eflags.
  90  */
  91         movl %esp,%edi          # save stack pointer
  92         andl $0xfffffffc,%esp   # align stack to avoid AC fault
  93         pushfl                  # push EFLAGS
  94         popl %eax               # get EFLAGS
  95         movl %eax,%ecx          # save original EFLAGS
  96         xorl $0x40000,%eax      # flip AC bit in EFLAGS
  97         pushl %eax              # copy to EFLAGS
  98         popfl                   # set EFLAGS
  99         pushfl                  # get new EFLAGS
 100         popl %eax               # put it in eax
 101         xorl %ecx,%eax          # change in flags
 102         andl $0x40000,%eax      # check if AC bit changed
 103         jnz 1f                  # 486
 104         pushl %ecx              # restore original EFLAGS
 105         popfl
 106         movl %edi,%esp          # restore esp
 107         movl %cr0,%eax          # 386
 108         andl $0x80000011,%eax   # Save PG,PE,ET
 109         orl $2,%eax             # set MP
 110         jmp 2f  
 111 /*
 112  * NOTE! 486 should set bit 16, to check for write-protect in supervisor
 113  * mode. Then it would be unnecessary with the "verify_area()"-calls.
 114  * 486 users probably want to set the NE (#5) bit also, so as to use
 115  * int 16 for math errors.
 116  * XXX - the above is out of date.  We set all the bits, but don't take
 117  * advantage of WP (26 Dec 92).
 118  */
 119 1:      pushl %ecx              # restore original EFLAGS
 120         popfl
 121         movl %edi,%esp          # restore esp
 122         movl %cr0,%eax          # 486
 123         andl $0x80000011,%eax   # Save PG,PE,ET
 124         orl $0x50022,%eax       # set AM, WP, NE and MP
 125 2:      movl %eax,%cr0
 126         call check_x87
 127         call setup_paging
 128         lgdt gdt_descr
 129         lidt idt_descr
 130         ljmp $(KERNEL_CS),$1f
 131 1:      movl $(KERNEL_DS),%eax  # reload all the segment registers
 132         mov %ax,%ds             # after changing gdt.
 133         mov %ax,%es
 134         mov %ax,%fs
 135         mov %ax,%gs
 136         lss _stack_start,%esp
 137         pushl $0                # These are the parameters to main :-)
 138         pushl $0
 139         pushl $0
 140         cld                     # gcc2 wants the direction flag cleared at all times
 141         call _start_kernel
 142 L6:
 143         jmp L6                  # main should never return here, but
 144                                 # just in case, we know what happens.
 145 
 146 /*
 147  * We depend on ET to be correct. This checks for 287/387.
 148  */
 149 check_x87:
 150         movl $0,_hard_math
 151         fninit
 152         fstsw %ax
 153         cmpb $0,%al
 154         je 1f
 155         movl %cr0,%eax          /* no coprocessor: have to set bits */
 156         xorl $4,%eax            /* set EM */
 157         movl %eax,%cr0
 158         ret
 159 .align 2
 160 1:      movl $1,_hard_math
 161         .byte 0xDB,0xE4         /* fsetpm for 287, ignored by 387 */
 162         ret
 163 
 164 /*
 165  *  setup_idt
 166  *
 167  *  sets up a idt with 256 entries pointing to
 168  *  ignore_int, interrupt gates. It doesn't actually load
 169  *  idt - that can be done only after paging has been enabled
 170  *  and the kernel moved to 0xC0000000. Interrupts
 171  *  are enabled elsewhere, when we can be relatively
 172  *  sure everything is ok.
 173  */
 174 setup_idt:
 175         lea ignore_int,%edx
 176         movl $(KERNEL_CS << 16),%eax
 177         movw %dx,%ax            /* selector = 0x0010 = cs */
 178         movw $0x8E00,%dx        /* interrupt gate - dpl=0, present */
 179 
 180         lea _idt,%edi
 181         mov $256,%ecx
 182 rp_sidt:
 183         movl %eax,(%edi)
 184         movl %edx,4(%edi)
 185         addl $8,%edi
 186         dec %ecx
 187         jne rp_sidt
 188         ret
 189 
 190 
 191 /*
 192  * Setup_paging
 193  *
 194  * This routine sets up paging by setting the page bit
 195  * in cr0. The page tables are set up, identity-mapping
 196  * the first 4MB.  The rest are initialized later.
 197  *
 198  * (ref: added support for up to 32mb, 17Apr92)  -- Rik Faith
 199  * (ref: update, 25Sept92)  -- croutons@crunchy.uucp 
 200  * (ref: 92.10.11 - Linus Torvalds. Corrected 16M limit - no upper memory limit)
 201  */
 202 .align 2
 203 setup_paging:
 204         movl $1024*2,%ecx               /* 2 pages - swapper_pg_dir+1 page table */
 205         xorl %eax,%eax
 206         movl $_swapper_pg_dir,%edi      /* swapper_pg_dir is at 0x1000 */
 207         cld;rep;stosl
 208 /* Identity-map the kernel in low 4MB memory for ease of transition */
 209         movl $_pg0+7,_swapper_pg_dir            /* set present bit/user r/w */
 210 /* But the real place is at 0xC0000000 */
 211         movl $_pg0+7,_swapper_pg_dir+3072       /* set present bit/user r/w */
 212         movl $_pg0+4092,%edi
 213         movl $0x03ff007,%eax            /*  4Mb - 4096 + 7 (r/w user,p) */
 214         std
 215 1:      stosl                   /* fill the page backwards - more efficient :-) */
 216         subl $0x1000,%eax
 217         jge 1b
 218         cld
 219         movl $_swapper_pg_dir,%eax
 220         movl %eax,%cr3                  /* cr3 - page directory start */
 221         movl %cr0,%eax
 222         orl $0x80000000,%eax
 223         movl %eax,%cr0          /* set paging (PG) bit */
 224         ret                     /* this also flushes the prefetch-queue */
 225 
 226 /*
 227  * page 0 is made non-existent, so that kernel NULL pointer references get
 228  * caught. Thus the swapper page directory has been moved to 0x1000
 229  */
 230 .org 0x1000
 231 _swapper_pg_dir:
 232 /*
 233  * The page tables are initialized to only 4MB here - the final page
 234  * tables are set up later depending on memory size.
 235  */
 236 .org 0x2000
 237 _pg0:
 238 
 239 .org 0x3000
 240 _empty_bad_page:
 241 
 242 .org 0x4000
 243 _empty_bad_page_table:
 244 
 245 .org 0x5000
 246 _empty_zero_page:
 247 
 248 .org 0x6000
 249 /*
 250  * tmp_floppy_area is used by the floppy-driver when DMA cannot
 251  * reach to a buffer-block. It needs to be aligned, so that it isn't
 252  * on a 64kB border.
 253  */
 254 _tmp_floppy_area:
 255         .fill 1024,1,0
 256 /*
 257  * floppy_track_buffer is used to buffer one track of floppy data: it
 258  * has to be separate from the tmp_floppy area, as otherwise a single-
 259  * sector read/write can mess it up. It can contain one full track of
 260  * data (18*2*512 bytes).
 261  */
 262 _floppy_track_buffer:
 263         .fill 512*2*18,1,0
 264 
 265 /* This is the default interrupt "handler" :-) */
 266 int_msg:
 267         .asciz "Unknown interrupt\n"
 268 .align 2
 269 ignore_int:
 270         cld
 271         pushl %eax
 272         pushl %ecx
 273         pushl %edx
 274         push %ds
 275         push %es
 276         push %fs
 277         movl $(KERNEL_DS),%eax
 278         mov %ax,%ds
 279         mov %ax,%es
 280         mov %ax,%fs
 281         pushl $int_msg
 282         call _printk
 283         popl %eax
 284         pop %fs
 285         pop %es
 286         pop %ds
 287         popl %edx
 288         popl %ecx
 289         popl %eax
 290         iret
 291 
 292 /*
 293  * The interrupt descriptor table has room for 256 idt's
 294  */
 295 .align 4
 296 .word 0
 297 idt_descr:
 298         .word 256*8-1           # idt contains 256 entries
 299         .long 0xc0000000+_idt
 300 
 301 .align 4
 302 _idt:
 303         .fill 256,8,0           # idt is uninitialized
 304 
 305 .align 4
 306 .word 0
 307 gdt_descr:
 308         .word (8+2*NR_TASKS)*8-1
 309         .long 0xc0000000+_gdt
 310 
 311 /*
 312  * This gdt setup gives the kernel a 1GB address space at virtual
 313  * address 0xC0000000 - space enough for expansion, I hope.
 314  */
 315 .align 4
 316 _gdt:
 317         .quad 0x0000000000000000        /* NULL descriptor */
 318         .quad 0x0000000000000000        /* not used */
 319         .quad 0xc0c39a000000ffff        /* 0x10 kernel 1GB code at 0xC0000000 */
 320         .quad 0xc0c392000000ffff        /* 0x18 kernel 1GB data at 0xC0000000 */
 321         .quad 0x00cbfa000000ffff        /* 0x23 user   3GB code at 0x00000000 */
 322         .quad 0x00cbf2000000ffff        /* 0x2b user   3GB data at 0x00000000 */
 323         .quad 0x0000000000000000        /* not used */
 324         .quad 0x0000000000000000        /* not used */
 325         .fill 2*NR_TASKS,8,0            /* space for LDT's and TSS's etc */

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