boot/head.S

/* */
   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 #include <linux/tasks.h>
  22 
  23 /*
  24  * swapper_pg_dir is the main page directory, address 0x00001000
  25  */
  26 startup_32:
  27         cld
  28         movl $0x10,%eax
  29         mov %ax,%ds
  30         mov %ax,%es
  31         mov %ax,%fs
  32         mov %ax,%gs
  33         lss _stack_start,%esp
  34         call setup_idt
  35         xorl %eax,%eax
  36 1:      incl %eax               # check that A20 really IS enabled
  37         movl %eax,0x000000      # loop forever if it isn't
  38         cmpl %eax,0x100000
  39         je 1b
  40 /*
  41  * Initialize eflags.  Some BIOS's leave bits like NT set.  This would
  42  * confuse the debugger if this code is traced.
  43  * XXX - best to initialize before switching to protected mode.
  44  */
  45         pushl $0
  46         popfl
  47 /* check if it is 486 or 386. */
  48 /*
  49  * XXX - this does a lot of unnecessary setup.  Alignment checks don't
  50  * apply at our cpl of 0 and the stack ought to be aligned already, and
  51  * we don't need to preserve eflags.
  52  */
  53         movl %esp,%edi          # save stack pointer
  54         andl $0xfffffffc,%esp   # align stack to avoid AC fault
  55         pushfl                  # push EFLAGS
  56         popl %eax               # get EFLAGS
  57         movl %eax,%ecx          # save original EFLAGS
  58         xorl $0x40000,%eax      # flip AC bit in EFLAGS
  59         pushl %eax              # copy to EFLAGS
  60         popfl                   # set EFLAGS
  61         pushfl                  # get new EFLAGS
  62         popl %eax               # put it in eax
  63         xorl %ecx,%eax          # change in flags
  64         andl $0x40000,%eax      # check if AC bit changed
  65         jnz 1f                  # 486
  66         pushl %ecx              # restore original EFLAGS
  67         popfl
  68         movl %edi,%esp          # restore esp
  69         movl %cr0,%eax          # 386
  70         andl $0x80000011,%eax   # Save PG,PE,ET
  71         orl $2,%eax             # set MP
  72         jmp 2f  
  73 /*
  74  * NOTE! 486 should set bit 16, to check for write-protect in supervisor
  75  * mode. Then it would be unnecessary with the "verify_area()"-calls.
  76  * 486 users probably want to set the NE (#5) bit also, so as to use
  77  * int 16 for math errors.
  78  * XXX - the above is out of date.  We set all the bits, but don't take
  79  * advantage of WP (26 Dec 92).
  80  */
  81 1:      pushl %ecx              # restore original EFLAGS
  82         popfl
  83         movl %edi,%esp          # restore esp
  84         movl %cr0,%eax          # 486
  85         andl $0x80000011,%eax   # Save PG,PE,ET
  86         orl $0x50022,%eax       # set AM, WP, NE and MP
  87 2:      movl %eax,%cr0
  88         call check_x87
  89         call setup_paging
  90         lgdt gdt_descr
  91         lidt idt_descr
  92         ljmp $0x08,$1f
  93 1:      movl $0x10,%eax         # reload all the segment registers
  94         mov %ax,%ds             # after changing gdt.
  95         mov %ax,%es
  96         mov %ax,%fs
  97         mov %ax,%gs
  98         lss _stack_start,%esp
  99         pushl $0                # These are the parameters to main :-)
 100         pushl $0
 101         pushl $0
 102         cld                     # gcc2 wants the direction flag cleared at all times
 103         call _start_kernel
 104 L6:
 105         jmp L6                  # main should never return here, but
 106                                 # just in case, we know what happens.
 107 
 108 /*
 109  * We depend on ET to be correct. This checks for 287/387.
 110  */
 111 check_x87:
 112         movl $0,_hard_math
 113         fninit
 114         fstsw %ax
 115         cmpb $0,%al
 116         je 1f
 117         movl %cr0,%eax          /* no coprocessor: have to set bits */
 118         xorl $6,%eax            /* reset MP, set EM */
 119         movl %eax,%cr0
 120         ret
 121 .align 2
 122 1:      movl $1,_hard_math
 123         .byte 0xDB,0xE4         /* fsetpm for 287, ignored by 387 */
 124         ret
 125 
 126 /*
 127  *  setup_idt
 128  *
 129  *  sets up a idt with 256 entries pointing to
 130  *  ignore_int, interrupt gates. It doesn't actually load
 131  *  idt - that can be done only after paging has been enabled
 132  *  and the kernel moved to 0xC0000000. Interrupts
 133  *  are enabled elsewhere, when we can be relatively
 134  *  sure everything is ok. This routine will be over-
 135  *  written by the page tables.
 136  */
 137 setup_idt:
 138         lea ignore_int,%edx
 139         movl $0x00080000,%eax
 140         movw %dx,%ax            /* selector = 0x0008 = cs */
 141         movw $0x8E00,%dx        /* interrupt gate - dpl=0, present */
 142 
 143         lea _idt,%edi
 144         mov $256,%ecx
 145 rp_sidt:
 146         movl %eax,(%edi)
 147         movl %edx,4(%edi)
 148         addl $8,%edi
 149         dec %ecx
 150         jne rp_sidt
 151         ret
 152 
 153 
 154 /*
 155  * Setup_paging
 156  *
 157  * This routine sets up paging by setting the page bit
 158  * in cr0. The page tables are set up, identity-mapping
 159  * the first 4MB.  The rest are initialized later.
 160  *
 161  * (ref: added support for up to 32mb, 17Apr92)  -- Rik Faith
 162  * (ref: update, 25Sept92)  -- croutons@crunchy.uucp 
 163  * (ref: 92.10.11 - Linus Torvalds. Corrected 16M limit - no upper memory limit)
 164  */
 165 .align 2
 166 setup_paging:
 167         movl $1024*2,%ecx               /* 2 pages - swapper_pg_dir+1 page table */
 168         xorl %eax,%eax
 169         movl $_swapper_pg_dir,%edi      /* swapper_pg_dir is at 0x1000 */
 170         cld;rep;stosl
 171 /* Identity-map the kernel in low 4MB memory for ease of transition */
 172         movl $_pg0+7,_swapper_pg_dir            /* set present bit/user r/w */
 173 /* But the real place is at 0xC0000000 */
 174         movl $_pg0+7,_swapper_pg_dir+3072       /* set present bit/user r/w */
 175         movl $_pg0+4092,%edi
 176         movl $0x03ff007,%eax            /*  4Mb - 4096 + 7 (r/w user,p) */
 177         std
 178 1:      stosl                   /* fill the page backwards - more efficient :-) */
 179         subl $0x1000,%eax
 180         jge 1b
 181         cld
 182         movl $_swapper_pg_dir,%eax
 183         movl %eax,%cr3                  /* cr3 - page directory start */
 184         movl %cr0,%eax
 185         orl $0x80000000,%eax
 186         movl %eax,%cr0          /* set paging (PG) bit */
 187         ret                     /* this also flushes the prefetch-queue */
 188 
 189 /*
 190  * page 0 is made non-existent, so that kernel NULL pointer references get
 191  * caught. Thus the swapper page directory has been moved to 0x1000
 192  */
 193 .org 0x1000
 194 _swapper_pg_dir:
 195 /*
 196  * The page tables are initialized to only 4MB here - the final page
 197  * tables are set up later depending on memory size.
 198  */
 199 .org 0x2000
 200 _pg0:
 201 
 202 .org 0x3000
 203 _empty_bad_page:
 204 
 205 .org 0x4000
 206 _empty_bad_page_table:
 207 
 208 .org 0x5000
 209 /*
 210  * tmp_floppy_area is used by the floppy-driver when DMA cannot
 211  * reach to a buffer-block. It needs to be aligned, so that it isn't
 212  * on a 64kB border.
 213  */
 214 _tmp_floppy_area:
 215         .fill 1024,1,0
 216 /*
 217  * floppy_track_buffer is used to buffer one track of floppy data: it
 218  * has to be separate from the tmp_floppy area, as otherwise a single-
 219  * sector read/write can mess it up. It can contain one full track of
 220  * data (18*2*512 bytes).
 221  */
 222 _floppy_track_buffer:
 223         .fill 512*2*18,1,0
 224 
 225 /* This is the default interrupt "handler" :-) */
 226 int_msg:
 227         .asciz "Unknown interrupt\n\r"
 228 .align 2
 229 ignore_int:
 230         cld
 231         pushl %eax
 232         pushl %ecx
 233         pushl %edx
 234         push %ds
 235         push %es
 236         push %fs
 237         movl $0x10,%eax
 238         mov %ax,%ds
 239         mov %ax,%es
 240         mov %ax,%fs
 241         pushl $int_msg
 242         call _printk
 243         popl %eax
 244         pop %fs
 245         pop %es
 246         pop %ds
 247         popl %edx
 248         popl %ecx
 249         popl %eax
 250         iret
 251 
 252 /*
 253  * The interrupt descriptor table has room for 256 idt's
 254  */
 255 .align 4
 256 .word 0
 257 idt_descr:
 258         .word 256*8-1           # idt contains 256 entries
 259         .long 0xc0000000+_idt
 260 
 261 .align 4
 262 _idt:
 263         .fill 256,8,0           # idt is uninitialized
 264 
 265 /*
 266  * The real GDT is also 256 entries long - no real reason
 267  */
 268 .align 4
 269 .word 0
 270 gdt_descr:
 271         .word (4+2*NR_TASKS)*8-1
 272         .long 0xc0000000+_gdt
 273 
 274 /*
 275  * This gdt setup gives the kernel a 1GB address space at virtual
 276  * address 0xC0000000 - space enough for expansion, I hope.
 277  */
 278 .align 4
 279 _gdt:
 280         .quad 0x0000000000000000        /* NULL descriptor */
 281         .quad 0xc0c39a000000ffff        /* 1GB at 0xC0000000 */
 282         .quad 0xc0c392000000ffff        /* 1GB */
 283         .quad 0x0000000000000000        /* TEMPORARY - don't use */
 284         .fill 2*NR_TASKS,8,0            /* space for LDT's and TSS's etc */
/* */
root/boot/head.S
