Linux 0.11/boot/head.s

   /*
    *  linux/boot/head.s
    *
    *  (C) 1991  Linus Torvalds
    */
   
   /*
    *  head.s contains the 32-bit startup code.
    *
   * NOTE!!! Startup happens at absolute address 0x00000000, which is also where
   * the page directory will exist. The startup code will be overwritten by
   * the page directory.
   */
  .text
  .globl _idt,_gdt,_pg_dir,_tmp_floppy_area
  _pg_dir:
  startup_32:
          movl $0x10,%eax
          mov %ax,%ds
          mov %ax,%es
          mov %ax,%fs
          mov %ax,%gs
          lss _stack_start,%esp
          call setup_idt
          call setup_gdt
          movl $0x10,%eax         # reload all the segment registers
          mov %ax,%ds             # after changing gdt. CS was already
          mov %ax,%es             # reloaded in 'setup_gdt'
          mov %ax,%fs
          mov %ax,%gs
          lss _stack_start,%esp
          xorl %eax,%eax
  1:      incl %eax               # check that A20 really IS enabled
          movl %eax,0x000000      # loop forever if it isn't
          cmpl %eax,0x100000
          je 1b
  /*
   * NOTE! 486 should set bit 16, to check for write-protect in supervisor
   * mode. Then it would be unnecessary with the "verify_area()"-calls.
   * 486 users probably want to set the NE (#5) bit also, so as to use
   * int 16 for math errors.
   */
          movl %cr0,%eax          # check math chip
          andl $0x80000011,%eax   # Save PG,PE,ET
  /* "orl $0x10020,%eax" here for 486 might be good */
          orl $2,%eax             # set MP
          movl %eax,%cr0
          call check_x87
          jmp after_page_tables
  
  /*
   * We depend on ET to be correct. This checks for 287/387.
   */
  check_x87:
          fninit
          fstsw %ax
          cmpb $0,%al
          je 1f                   /* no coprocessor: have to set bits */
          movl %cr0,%eax
          xorl $6,%eax            /* reset MP, set EM */
          movl %eax,%cr0
          ret
  .align 2
  1:      .byte 0xDB,0xE4         /* fsetpm for 287, ignored by 387 */
          ret
  
  /*
   *  setup_idt
   *
   *  sets up a idt with 256 entries pointing to
   *  ignore_int, interrupt gates. It then loads
   *  idt. Everything that wants to install itself
   *  in the idt-table may do so themselves. Interrupts
   *  are enabled elsewhere, when we can be relatively
   *  sure everything is ok. This routine will be over-
   *  written by the page tables.
   */
  setup_idt:
          lea ignore_int,%edx
          movl $0x00080000,%eax
          movw %dx,%ax            /* selector = 0x0008 = cs */
          movw $0x8E00,%dx        /* interrupt gate - dpl=0, present */
  
          lea _idt,%edi
          mov $256,%ecx
  rp_sidt:
          movl %eax,(%edi)
          movl %edx,4(%edi)
          addl $8,%edi
          dec %ecx
          jne rp_sidt
          lidt idt_descr
          ret
  
  /*
   *  setup_gdt
   *
   *  This routines sets up a new gdt and loads it.
   *  Only two entries are currently built, the same
  *  ones that were built in init.s. The routine
  *  is VERY complicated at two whole lines, so this
  *  rather long comment is certainly needed :-).
  *  This routine will beoverwritten by the page tables.
  */
 setup_gdt:
         lgdt gdt_descr
         ret
 
 /*
  * I put the kernel page tables right after the page directory,
  * using 4 of them to span 16 Mb of physical memory. People with
  * more than 16MB will have to expand this.
  */
 .org 0x1000
 pg0:
 
 .org 0x2000
 pg1:
 
 .org 0x3000
 pg2:
 
 .org 0x4000
 pg3:
 
 .org 0x5000
 /*
  * tmp_floppy_area is used by the floppy-driver when DMA cannot
  * reach to a buffer-block. It needs to be aligned, so that it isn't
  * on a 64kB border.
  */
 _tmp_floppy_area:
         .fill 1024,1,0
 
 after_page_tables:
         pushl $0                # These are the parameters to main :-)
         pushl $0
         pushl $0
         pushl $L6               # return address for main, if it decides to.
         pushl 
___FCKpd___0
 
main
         jmp setup_paging
 L6:
         jmp L6                  # main should never return here, but
                                 # just in case, we know what happens.
 
 /* This is the default interrupt "handler" :-) */
 int_msg:
         .asciz "Unknown interrupt/n/r"
 .align 2
 ignore_int:
         pushl %eax
         pushl %ecx
         pushl %edx
         push %ds
         push %es
         push %fs
         movl $0x10,%eax
         mov %ax,%ds
         mov %ax,%es
         mov %ax,%fs
         pushl $int_msg
         call _printk
         popl %eax
         pop %fs
         pop %es
         pop %ds
         popl %edx
         popl %ecx
         popl %eax
         iret
 
 
 /*
  * Setup_paging
  *
  * This routine sets up paging by setting the page bit
  * in cr0. The page tables are set up, identity-mapping
  * the first 16MB. The pager assumes that no illegal
  * addresses are produced (ie >4Mb on a 4Mb machine).
  *
  * NOTE! Although all physical memory should be identity
  * mapped by this routine, only the kernel page functions
  * use the >1Mb addresses directly. All "normal" functions
  * use just the lower 1Mb, or the local data space, which
  * will be mapped to some other place - mm keeps track of
  * that.
  *
  * For those with more memory than 16 Mb - tough luck. I've
  * not got it, why should you :-) The source is here. Change
  * it. (Seriously - it shouldn't be too difficult. Mostly
  * change some constants etc. I left it at 16Mb, as my machine
  * even cannot be extended past that (ok, but it was cheap :-)
  * I've tried to show which constants to change by having
  * some kind of marker at them (search for "16Mb"), but I
  * won't guarantee that's all :-( )
  */
 .align 2
 setup_paging:
         movl $1024*5,%ecx               /* 5 pages - pg_dir+4 page tables */
         xorl %eax,%eax
         xorl %edi,%edi                  /* pg_dir is at 0x000 */
         cld;rep;stosl
         movl $pg0+7,_pg_dir             /* set present bit/user r/w */
         movl $pg1+7,_pg_dir+4           /*  --------- " " --------- */
         movl $pg2+7,_pg_dir+8           /*  --------- " " --------- */
         movl $pg3+7,_pg_dir+12          /*  --------- " " --------- */
         movl $pg3+4092,%edi
         movl $0xfff007,%eax             /*  16Mb - 4096 + 7 (r/w user,p) */
         std
 1:      stosl                   /* fill pages backwards - more efficient :-) */
         subl $0x1000,%eax
         jge 1b
         xorl %eax,%eax          /* pg_dir is at 0x0000 */
         movl %eax,%cr3          /* cr3 - page directory start */
         movl %cr0,%eax
         orl $0x80000000,%eax
         movl %eax,%cr0          /* set paging (PG) bit */
         ret                     /* this also flushes prefetch-queue */
 
 .align 2
 .word 0
 idt_descr:
         .word 256*8-1           # idt contains 256 entries
         .long _idt
 .align 2
 .word 0
 gdt_descr:
         .word 256*8-1           # so does gdt (not that that's any
         .long _gdt              # magic number, but it works for me :^)
 
         .align 3
 _idt:   .fill 256,8,0           # idt is uninitialized
 
 _gdt:   .quad 0x0000000000000000        /* NULL descriptor */
         .quad 0x00c09a0000000fff        /* 16Mb */
         .quad 0x00c0920000000fff        /* 16Mb */
         .quad 0x0000000000000000        /* TEMPORARY - don't use */
         .fill 252,8,0                   /* space for LDT's and TSS's etc */