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The FreeBSD Documentation Project

Copyright (c) 2002 Sergey Lyubka <devnull@uptsoft.com>
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$FreeBSD$
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<chapter id="boot">
  <chapterinfo>
    <authorgroup>
      <author>
        <firstname>Sergey</firstname>
	<surname>Lyubka</surname>
	<contrib>Contributed by </contrib>
      </author> <!-- devnull@uptsoft.com  12 Jun 2002 -->
    </authorgroup>
  </chapterinfo>
  <title>Bootstrapping and kernel initialization</title>

  <sect1 id="boot-synopsis">
    <title>Synopsis</title>

    <para>This chapter is an overview of the boot and system
      initialization process, starting from the BIOS (firmware) POST,
      to the first user process creation.  Since the initial steps of
      system startup are very architecture dependent, the IA-32
      architecture is used as an example.</para>
  </sect1>

  <sect1 id="boot-overview">
    <title>Overview</title>

    <para>A computer running FreeBSD can boot by several methods,
      although the most common method, booting from a harddisk where
      the OS is installed, will be discussed here.  The boot process
      is divided into several steps:</para>

    <itemizedlist>
      <listitem><para>BIOS POST</para></listitem>
      <listitem><para><literal>boot0</literal> stage</para></listitem>
      <listitem><para><literal>boot2</literal> stage</para></listitem>
      <listitem><para>loader stage</para></listitem>
      <listitem><para>kernel initialization</para></listitem>
    </itemizedlist>

    <para>The <literal>boot0</literal> and <literal>boot2</literal>
      stages are also referred to as <emphasis>bootstrap stages 1 and
      2</emphasis> in &man.boot.8; as the first steps in FreeBSD's
      3-stage bootstrapping procedure.  Various information is printed
      on the screen at each stage, so you may visually recognize them
      using the table that follows.  Please note that the actual data
      may differ from machine to machine:</para>

    <informaltable>
      <tgroup cols="2">
        <tbody>
          <row>
            <entry><para>may vary</para></entry>

	    <entry><para>BIOS (firmware) messages</para></entry>
          </row>

          <row>
            <entry><para>
<screen>F1    FreeBSD
F2    BSD
F5    Disk 2</screen>
            </para></entry>

	    <entry><para><literal>boot0</literal></para></entry>
          </row>

          <row>
            <entry><para>
<screen>>>FreeBSD/i386 BOOT
Default: 1:ad(1,a)/boot/loader
boot:</screen>
            </para></entry>

	    <entry><para><literal>boot2</literal><footnote><para>This
	      prompt will appear if the user presses a key just after
	      selecting an OS to boot at the <literal>boot0</literal>
              stage.</para></footnote></para></entry>
          </row>

          <row>
            <entry><para>
<screen>BTX loader 1.0 BTX version is 1.01
BIOS drive A: is disk0
BIOS drive C: is disk1
BIOS 639kB/64512kB available memory
FreeBSD/i386 bootstrap loader, Revision 0.8
Console internal video/keyboard
(jkh@bento.freebsd.org, Mon Nov 20 11:41:23 GMT 2000)
/kernel text=0x1234 data=0x2345 syms=[0x4+0x3456]
Hit [Enter] to boot immediately, or any other key for command prompt
Booting [kernel] in 9 seconds..._</screen>
            </para></entry>

            <entry><para>loader</para></entry>
          </row>

          <row>
            <entry><para>
	      <screen>Copyright (c) 1992-2002 The FreeBSD Project.
Copyright (c) 1979, 1980, 1983, 1986, 1988, 1989, 1991, 1992, 1993, 1994
        The Regents of the University of California. All rights reserved.
FreeBSD 4.6-RC #0: Sat May  4 22:49:02 GMT 2002
    devnull@kukas:/usr/obj/usr/src/sys/DEVNULL
Timecounter "i8254"  frequency 1193182 Hz</screen></para></entry>

            <entry><para>kernel</para></entry>
          </row>
        </tbody>
      </tgroup>
    </informaltable>
  </sect1>

  <sect1 id="boot-bios">
    <title>BIOS POST</title>

    <para>When the PC powers on, the processor's registers are set
      to some predefined values.  One of the registers is the
      <emphasis>instruction pointer</emphasis> register, and its value
      after a power on is well defined: it is a 32-bit value of
      0xfffffff0.  The instruction pointer register points to code to
      be executed by the processor.  One of the registers is the
      <literal>cr1</literal> 32-bit control register, and its value
      just after the reboot is 0.  One of the cr1's bits, the bit PE
      (Protected Enabled) indicates whether the processor is running
      in protected or real mode.  Since at boot time this bit is
      cleared, the processor boots in real mode.  Real mode means,
      among other things, that linear and physical addresses are
      identical.</para>

    <para>The value of 0xfffffff0 is slightly less then 4Gb, so unless
      the machine has 4Gb physical memory, it cannot point to a valid
      memory address.  The computer's hardware translates this address
      so that it points to a BIOS memory block.</para>

    <para>BIOS stands for <emphasis>Basic Input Output
      System</emphasis>, and it is a chip on the motherboard that has
      a relatively small amount of read-only memory (ROM).  This
      memory contains various low-level routines that are specific to
      the hardware supplied with the motherboard.  So, the processor
      will first jump to the address 0xfffffff0, which really resides
      in the BIOS's memory.  Usually this address contains a jump
      instruction to the BIOS's POST routines.</para>

    <para>POST stands for <emphasis>Power On Self Test</emphasis>.
      This is a set of routines including the memory check, system bus
      check and other low-level stuff so that the CPU can initialize
      the computer properly.  The important step on this stage is
      determining the boot device.  All modern BIOS's allow the boot
      device to be set manually, so you can boot from a floppy,
      CD-ROM, harddisk etc.</para>

    <para>The very last thing in the POST is the <literal>INT
      0x19</literal> instruction.  That instruction reads 512 bytes
      from the first sector of boot device into the memory at address
      0x7c00.  The term <emphasis>first sector</emphasis> originates
      from harddrive architecture, where the magnetic plate is divided
      to a number of cylindrical tracks.  Tracks are numbered, and
      every track is divided by a number (usually 64) sectors.  Track
      number 0 is the outermost on the magnetic plate, and sector 1,
      the first sector (tracks, or, cylinders, are numbered starting
      from 0, but sectors - starting from 1), has a special meaning.
      It is also called Master Boot Record, or MBR.  The remaining
      sectors on the first track are never used <footnote><para>Some
      utilities such as &man.disklabel.8; may store the information in
      this area, mostly in the second
      sector.</para></footnote>.</para>
  </sect1>

  <sect1 id="boot-boot0">
    <title><literal>boot0</literal> stage</title>

    <para>Take a look at the file <filename>/boot/boot0</filename>.
      This is a small 512-byte file, and it is exactly what FreeBSD's
      installation procedure wrote to your harddisk's MBR if you chose
      the <quote>bootmanager</quote> option at installation time.</para>

    <para>As mentioned previously, the <literal>INT 0x19</literal>
      instruction loads an MBR, i.e. the <filename>boot0</filename>
      content, into the memory at address 0x7c00.  Taking a look at
      the file <filename>sys/boot/i386/boot0/boot0.s</filename> can
      give a guess at what is happening there - this is the boot
      manager, which is an awesome piece of code written by Robert
      Nordier.</para>

    <para>The MBR, or, <filename>boot0</filename>, has a special
      structure starting from offset 0x1be, called the
      <emphasis>partition table</emphasis>.  It has 4 records of 16
      bytes each, called <emphasis>partition records</emphasis>, which
      represent how the harddisk(s) are partitioned, or, in FreeBSD's
      terminology, sliced.  One byte of those 16 says whether a
      partition (slice) is bootable or not.  Exactly one record must
      have that flag set, otherwise <filename>boot0</filename>'s code
      will refuse to proceed.</para>

    <para>A partition record has the following fields:</para>

    <itemizedlist>
      <listitem>
        <para>the 1-byte filesystem type</para>
      </listitem>

      <listitem>
        <para>the 1-byte bootable flag</para>
      </listitem>

      <listitem>
        <para>the 6 byte descriptor in CHS format</para>
      </listitem>

      <listitem>
        <para>the 8 byte descriptor in LBA format</para>
      </listitem>
    </itemizedlist>

    <para>A partition record descriptor has the information about
      where exactly the partition resides on the drive.  Both
      descriptors, LBA and CHS, describe the same information, but in
      different ways: LBA (Logical Block Addressing) has the starting
      sector for the partition and the partition's length, while CHS
      (Cylinder Head Sector) has coordinates for the first and last
      sectors of the partition.</para>

    <para>The boot manager scans the partition table and prints the
      menu on the screen so the user can select what disk and what
      slice to boot.  By pressing an appropriate key,
      <filename>boot0</filename> performs the following
      actions:</para>

    <itemizedlist>
      <listitem>
	<para>modifies the bootable flag for the selected partition to
	  make it bootable, and clears the previous</para>
      </listitem>

      <listitem>
	<para>saves itself to disk to remember what partition (slice)
	  has been selected so to use it as the default on the next
	  boot</para>
      </listitem>

      <listitem>
	<para>loads the first sector of the selected partition (slice)
	  into memory and jumps there</para>
      </listitem>
    </itemizedlist>

    <para>What kind of data should reside on the very first sector of
      a bootable partition (slice), in our case, a FreeBSD slice?  As
      you may have already guessed, it is
      <filename>boot2</filename>.</para>
  </sect1>

  <sect1 id="boot-boot2">
    <title><literal>boot2</literal> stage</title>

    <para>You might wonder, why <literal>boot2</literal> comes after
      <literal>boot0</literal>, and not boot1.  Actually, there is a
      512-byte file called <filename>boot1</filename> in the directory
      <filename>/boot</filename> as well.  It is used for booting from
      a floppy.  When booting from a floppy,
      <filename>boot1</filename> plays the same role as
      <filename>boot0</filename> for a harddisk: it locates
      <filename>boot2</filename> and runs it.</para>

    <para>You may have realized that a file
      <filename>/boot/mbr</filename> exists as well.  It is a
      simplified version of <filename>boot0</filename>.  The code in
      <filename>mbr</filename> does not provide a menu for the user,
      it just blindly boots the partition marked active.</para>

    <para>The code implementing <filename>boot2</filename> resides in
      <filename>sys/boot/i386/boot2/</filename>, and the executable
      itself is in <filename>/boot</filename>.  The files
      <filename>boot0</filename> and <filename>boot2</filename> that
      are in <filename>/boot</filename> are not used by the bootstrap,
      but by utilities such as <application>boot0cfg</application>.
      The actual position for <filename>boot0</filename> is in the
      MBR.  For <filename>boot2</filename> it is the beginning of a
      bootable FreeBSD slice.  These locations are not under the
      filesystem's control, so they are invisible to commands like
      <application>ls</application>.</para>

    <para>The main task for <literal>boot2</literal> is to load the
      file <filename>/boot/loader</filename>, which is the third stage
      in the bootstrapping procedure.  The code in
      <literal>boot2</literal> cannot use any services like
      <function>open()</function> and <function>read()</function>,
      since the kernel is not yet loaded.  It must scan the harddisk,
      knowing about the filesystem structure, find the file
      <filename>/boot/loader</filename>, read it into memory using a
      BIOS service, and then pass the execution to the loader's entry
      point.</para>

    <para>Besides that, <literal>boot2</literal> prompts for user
      input so the loader can be booted from different disk, unit,
      slice and partition.</para>

    <para>The <literal>boot2</literal> binary is created in special
      way:</para>

    <programlisting><filename>sys/boot/i386/boot2/Makefile</filename>
boot2: boot2.ldr boot2.bin ${BTX}/btx/btx
	btxld -v -E ${ORG2} -f bin -b ${BTX}/btx/btx -l boot2.ldr \
		-o boot2.ld -P 1 boot2.bin</programlisting>

    <para>This Makefile snippet shows that &man.btxld.8; is used to
      link the binary.  BTX, which stands for BooT eXtender, is a
      piece of code that provides a protected mode environment for the
      program, called the client, that it is linked with.  So
      <literal>boot2</literal> is a BTX client, i.e. it uses the
      service provided by BTX.</para>

    <para>The <application>btxld</application> utility is the linker.
      It links two binaries together.  The difference between
      &man.btxld.8; and &man.ld.1; is that
      <application>ld</application> usually links object files into a
      shared object or executable, while
      <application>btxld</application> links an object file with the
      BTX, producing the binary file suitable to be put on the
      beginning of the partition for the system boot.</para>

    <para><literal>boot0</literal> passes the execution to BTX's entry
      point.  BTX then switches the processor to protected mode, and
      prepares a simple environment before calling the client.  This
      includes:</para>

    <itemizedlist>
      <listitem><para>virtual v86 mode.  That means, the BTX is a v86
        monitor.  Real mode instructions like pushf, popf, cli, sti, if
        called by the client, will work.</para></listitem>

      <listitem><para>Interrupt Descriptor Table (IDT) is set up so
        all hardware interrupts are routed to the default BIOS's
        handlers, and interrupt 0x30 is set up to be the syscall
        gate.</para></listitem>

      <listitem><para>Two system calls: <function>exec</function> and
        <function>exit</function>, are defined:</para>

    <programlisting><filename>sys/boot/i386/btx/lib/btxsys.s:</filename>
		.set INT_SYS,0x30		# Interrupt number
#
# System call: exit
#
__exit: 	xorl %eax,%eax			# BTX system
		int $INT_SYS			#  call 0x0
#
# System call: exec
#
__exec: 	movl $0x1,%eax			# BTX system
		int $INT_SYS			#  call 0x1</programlisting></listitem>
    </itemizedlist>

    <para>BTX creates a Global Descriptor Table (GDT):</para>

    <programlisting><filename>sys/boot/i386/btx/btx/btx.s:</filename>
gdt:		.word 0x0,0x0,0x0,0x0		# Null entry
		.word 0xffff,0x0,0x9a00,0xcf	# SEL_SCODE
		.word 0xffff,0x0,0x9200,0xcf	# SEL_SDATA
		.word 0xffff,0x0,0x9a00,0x0	# SEL_RCODE
		.word 0xffff,0x0,0x9200,0x0	# SEL_RDATA
		.word 0xffff,MEM_USR,0xfa00,0xcf# SEL_UCODE
		.word 0xffff,MEM_USR,0xf200,0xcf# SEL_UDATA
		.word _TSSLM,MEM_TSS,0x8900,0x0 # SEL_TSS</programlisting>

    <para>The client's code and data start from address MEM_USR
      (0xa000), and a selector (SEL_UCODE) points to the client's code
      segment.  The SEL_UCODE descriptor has Descriptor Privilege
      Level (DPL) 3, which is the lowest privilege level.  But the
      <literal>INT 0x30</literal> instruction handler resides in a
      segment pointed to by the SEL_SCODE (supervisor code) selector,
      as shown from the code that creates an IDT:</para>

  <programlisting>		mov $SEL_SCODE,%dh		# Segment selector
init.2: 	shr %bx				# Handle this int?
		jnc init.3			# No
		mov %ax,(%di)			# Set handler offset
		mov %dh,0x2(%di)		#  and selector
		mov %dl,0x5(%di)		# Set P:DPL:type
		add $0x4,%ax			# Next handler</programlisting>

    <para>So, when the client calls <function>__exec()</function>, the
      code will be executed with the highest privileges.  This allows
      the kernel to change the protected mode data structures, such as
      page tables, GDT, IDT, etc later, if needed.</para>

    <para><literal>boot2</literal> defines an important structure,
      <literal>struct bootinfo</literal>.  This structure is
      initialized by <literal>boot2</literal> and passed to the
      loader, and then further to the kernel.  Some nodes of this
      structures are set by <literal>boot2</literal>, the rest by the
      loader.  This structure, among other information, contains the
      kernel filename, BIOS harddisk geometry, BIOS drive number for
      boot device, physical memory available, <literal>envp</literal>
      pointer etc.  The definition for it is:</para>

    <programlisting><filename>/usr/include/machine/bootinfo.h</filename>
struct bootinfo {
	u_int32_t	bi_version;
	u_int32_t	bi_kernelname;		/* represents a char * */
	u_int32_t	bi_nfs_diskless;	/* struct nfs_diskless * */
				/* End of fields that are always present. */
#define	bi_endcommon	bi_n_bios_used
	u_int32_t	bi_n_bios_used;
	u_int32_t	bi_bios_geom[N_BIOS_GEOM];
	u_int32_t	bi_size;
	u_int8_t	bi_memsizes_valid;
	u_int8_t	bi_bios_dev;		/* bootdev BIOS unit number */
	u_int8_t	bi_pad[2];
	u_int32_t	bi_basemem;
	u_int32_t	bi_extmem;
	u_int32_t	bi_symtab;		/* struct symtab * */
	u_int32_t	bi_esymtab;		/* struct symtab * */
				/* Items below only from advanced bootloader */
	u_int32_t	bi_kernend;		/* end of kernel space */
	u_int32_t	bi_envp;		/* environment */
	u_int32_t	bi_modulep;		/* preloaded modules */
};</programlisting>

  <para><literal>boot2</literal> enters into an infinite loop waiting
    for user input, then calls <function>load()</function>.  If the
    user does not press anything, the loop brakes by a timeout, so
    <function>load()</function> will load the default file
    (<filename>/boot/loader</filename>).  Functions <function>ino_t
    lookup(char *filename)</function> and <function>int xfsread(ino_t
    inode, void *buf, size_t nbyte)</function> are used to read the
    content of a file into memory.  <filename>/boot/loader</filename>
    is an ELF binary, but where the ELF header is prepended with
    a.out's <literal>struct exec</literal> structure.
    <function>load()</function> scans the loader's ELF header, loading
    the content of <filename>/boot/loader</filename> into memory, and
    passing the execution to the loader's entry:</para>

  <programlisting><filename>sys/boot/i386/boot2/boot2.c:</filename>
    __exec((caddr_t)addr, RB_BOOTINFO | (opts & RBX_MASK),
	   MAKEBOOTDEV(dev_maj[dsk.type], 0, dsk.slice, dsk.unit, dsk.part),
	   0, 0, 0, VTOP(&amp;bootinfo));</programlisting>
  </sect1>

  <sect1 id="boot-loader">
    <title><application>loader</application> stage</title>

    <para><application>loader</application> is a BTX client as well.
      I will not describe it here in detail, there is a comprehensive
      manpage written by Mike Smith, &man.loader.8;.  The underlying
      mechanisms and BTX were discussed above.</para>

    <para>The main task for the loader is to boot the kernel.  When
      the kernel is loaded into memory, it is being called by the
      loader:</para>

  <programlisting><filename>sys/boot/common/boot.c:</filename>
    /* Call the exec handler from the loader matching the kernel */
    module_formats[km->m_loader]->l_exec(km);</programlisting>
  </sect1>

  <sect1 id="boot-kernel">
    <title>Kernel initialization</title>

    <para>To where exactly is the execution passed by the loader,
      i.e. what is the kernel's actual entry point.  Let us take a
      look at the command that links the kernel:</para>

    <programlisting><filename>sys/conf/Makefile.i386:</filename>
ld -elf -Bdynamic -T /usr/src/sys/conf/ldscript.i386  -export-dynamic \
-dynamic-linker /red/herring -o kernel -X locore.o \
&lt;lots of kernel .o files&gt;</programlisting>

    <para>A few interesting things can be seen in this line.  First,
      the kernel is an ELF dynamically linked binary, but the dynamic
      linker for kernel is <filename>/red/herring</filename>, which is
      definitely a bogus file.  Second, taking a look at the file
      <filename>sys/conf/ldscript.i386</filename> gives an idea about
      what <application>ld</application> options are used when
      compiling a kernel.  Reading through the first few lines, the
      string</para>

  <programlisting><filename>sys/conf/ldscript.i386:</filename>
ENTRY(btext)</programlisting>

    <para>says that a kernel's entry point is the symbol `btext'.
      This symbol is defined in <filename>locore.s</filename>:</para>

  <programlisting><filename>sys/i386/i386/locore.s:</filename>
	.text
/**********************************************************************
 *
 * This is where the bootblocks start us, set the ball rolling...
 *
 */
NON_GPROF_ENTRY(btext)</programlisting>

    <para>First what is done is the register EFLAGS is set to a
      predefined value of 0x00000002, and then all the segment
      registers are initialized:</para>

    <programlisting><filename>sys/i386/i386/locore.s</filename>
/* Don't trust what the BIOS gives for eflags. */
	pushl	$PSL_KERNEL
	popfl

/*
 * Don't trust what the BIOS gives for %fs and %gs.  Trust the bootstrap
 * to set %cs, %ds, %es and %ss.
 */
	mov	%ds, %ax
	mov	%ax, %fs
	mov	%ax, %gs</programlisting>

    <para>btext calls the routines
      <function>recover_bootinfo()</function>,
      <function>identify_cpu()</function>,
      <function>create_pagetables()</function>, which are also defined
      in <filename>locore.s</filename>.  Here is a description of what
      they do:</para>

    <informaltable>
      <tgroup cols=2 align=left>
      <tbody>
        <row>
          <entry><function>recover_bootinfo</function></entry>

          <entry>This routine parses the parameters to the kernel
            passed from the bootstrap.  The kernel may have been
            booted in 3 ways: by the loader, described above, by the
            old disk boot blocks, and by the old diskless boot
            procedure.  This function determines the booting method,
            and stores the <literal>struct bootinfo</literal>
            structure into the kernel memory.</entry>
        </row>

        <row>
          <entry><function>identify_cpu</function></entry>

	  <entry>This functions tries to find out what CPU it is
	    running on, storing the value found in a variable
	    <varname>_cpu</varname>.</entry>
        </row>

        <row>
          <entry><function>create_pagetables</function></entry>

	  <entry>This function allocates and fills out a Page Table
	    Directory at the top of the kernel memory area.</entry>
        </row>
       </tbody>
      </tgroup>
    </informaltable>

    <para>The next steps are enabling VME, if the CPU supports
      it:</para>

    <programlisting>	testl	$CPUID_VME, R(_cpu_feature)
	jz	1f
	movl	%cr4, %eax
	orl	$CR4_VME, %eax
	movl	%eax, %cr4</programlisting>

    <para>Then, enabling paging:</para>
    <programlisting>/* Now enable paging */
	movl	R(_IdlePTD), %eax
	movl	%eax,%cr3			/* load ptd addr into mmu */
	movl	%cr0,%eax			/* get control word */
	orl	$CR0_PE|CR0_PG,%eax		/* enable paging */
	movl	%eax,%cr0			/* and let's page NOW! */</programlisting>

    <para>The next three lines of code are because the paging was set,
      so the jump is needed to continue the execution in virtualized
      address space:</para>

    <programlisting>	pushl	$begin				/* jump to high virtualized address */
	ret

/* now running relocated at KERNBASE where the system is linked to run */
begin:</programlisting>

    <para>The function <function>init386()</function> is called, with
      a pointer to the first free physical page, after that
      <function>mi_startup()</function>.  <function>init386</function>
      is an architecture dependent initialization function, and
      <function>mi_startup()</function> is an architecture independent
      one (the 'mi_' prefix stands for Machine Independent).  The
      kernel never returns from <function>mi_startup()</function>, and
      by calling it, the kernel finishes booting:</para>

    <programlisting><filename>sys/i386/i386/locore.s:</filename>
	movl	physfree, %esi
	pushl	%esi				/* value of first for init386(first) */
	call	_init386			/* wire 386 chip for unix operation */
	call	_mi_startup			/* autoconfiguration, mountroot etc */
	hlt		/* never returns to here */</programlisting>

    <sect2>
      <title><function>init386()</function></title>

      <para><function>init386()</function> is defined in
        <filename>sys/i386/i386/machdep.c</filename> and performs
        low-level initialization, specific to the i386 chip.  The
        switch to protected mode was performed by the loader.  The
        loader has created the very first task, in which the kernel
        continues to operate.  Before running straight away to the
        code, I will enumerate the tasks the processor must complete
        to initialize protected mode execution:</para>

      <itemizedlist>
        <listitem>
	  <para>Initialize the kernel tunable parameters, passed from
	    the bootstrapping program.</para>
	  </listitem>

        <listitem>
	  <para>Prepare the GDT.</para>
	</listitem>

        <listitem>
	  <para>Prepare the IDT.</para>
	</listitem>

        <listitem>
	  <para>Initialize the system console.</para>
	</listitem>

        <listitem>
	  <para>Initialize the DDB, if it is compiled into
	    kernel.</para>
	</listitem>

        <listitem>
	  <para>Initialize the TSS.</para>
	</listitem>

        <listitem>
	  <para>Prepare the LDT.</para>
	</listitem>

        <listitem>
	  <para>Set up proc0's pcb.</para>
	</listitem>
      </itemizedlist>

      <para>What <function>init386()</function> first does is
        initialize the tunable parameters passed from bootstrap.  This
        is done by setting the environment pointer (envp) and calling
        <function>init_param1()</function>.  The envp pointer has been
        passed from loader in the <literal>bootinfo</literal>
        structure:</para>

      <programlisting><filename>sys/i386/i386/machdep.c:</filename>
		kern_envp = (caddr_t)bootinfo.bi_envp + KERNBASE;

	/* Init basic tunables, hz etc */
	init_param1();</programlisting>

      <para><function>init_param1()</function> is defined in
        <filename>sys/kern/subr_param.c</filename>.  That file has a
        number of sysctls, and two functions,
        <function>init_param1()</function> and
        <function>init_param2()</function>, that are called from
        <function>init386()</function>:</para>

      <programlisting><filename>sys/kern/subr_param.c</filename>
	hz = HZ;
	TUNABLE_INT_FETCH("kern.hz", &amp;hz);</programlisting>

      <para>TUNABLE_&lt;typename&gt;_FETCH is used to fetch the value
        from the environment:</para>

    <programlisting><filename>/usr/src/sys/sys/kernel.h</filename>
#define	TUNABLE_INT_FETCH(path, var)	getenv_int((path), (var))
</programlisting>

      <para>Sysctl <literal>kern.hz</literal> is the system clock tick.  Along with
        this, the following sysctls are set by
        <function>init_param1()</function>: <literal>kern.maxswzone,
        kern.maxbcache, kern.maxtsiz, kern.dfldsiz, kern.dflssiz,
        kern.maxssiz, kern.sgrowsiz</literal>.</para>

      <para>Then <function>init386()</function> prepares the Global
        Descriptors Table (GDT).  Every task on an x86 is running in
        its own virtual address space, and this space is addressed by
        a segment:offset pair.  Say, for instance, the current
        instruction to be executed by the processor lies at CS:EIP,
        then the linear virtual address for that instruction would be
        <quote>the virtual address of code segment CS</quote> + EIP.  For
        convenience, segments begin at virtual address 0 and end at a
        4Gb boundary.  Therefore, the instruction's linear virtual
        address for this example would just be the value of EIP.
        Segment registers such as CS, DS etc are the selectors,
        i.e. indexes, into GDT (to be more precise, an index is not a
        selector itself, but the INDEX field of a selector).
        FreeBSD's GDT holds descriptors for 15 selectors per
        CPU:</para>

      <programlisting><filename>sys/i386/i386/machdep.c:</filename>
union descriptor gdt[NGDT * MAXCPU];	/* global descriptor table */

<filename>sys/i386/include/segments.h:</filename>
/*
 * Entries in the Global Descriptor Table (GDT)
 */
#define	GNULL_SEL	0	/* Null Descriptor */
#define	GCODE_SEL	1	/* Kernel Code Descriptor */
#define	GDATA_SEL	2	/* Kernel Data Descriptor */
#define	GPRIV_SEL	3	/* SMP Per-Processor Private Data */
#define	GPROC0_SEL	4	/* Task state process slot zero and up */
#define	GLDT_SEL	5	/* LDT - eventually one per process */
#define	GUSERLDT_SEL	6	/* User LDT */
#define	GTGATE_SEL	7	/* Process task switch gate */
#define	GBIOSLOWMEM_SEL	8	/* BIOS low memory access (must be entry 8) */
#define	GPANIC_SEL	9	/* Task state to consider panic from */
#define GBIOSCODE32_SEL	10	/* BIOS interface (32bit Code) */
#define GBIOSCODE16_SEL	11	/* BIOS interface (16bit Code) */
#define GBIOSDATA_SEL	12	/* BIOS interface (Data) */
#define GBIOSUTIL_SEL	13	/* BIOS interface (Utility) */
#define GBIOSARGS_SEL	14	/* BIOS interface (Arguments) */</programlisting>

      <para>Note that those #defines are not selectors themselves, but
        just a field INDEX of a selector, so they are exactly the
        indices of the GDT.  for example, an actual selector for the
        kernel code (GCODE_SEL) has the value 0x08.</para>

      <para>The next step is to initialize the Interrupt Descriptor
        Table (IDT).  This table is to be referenced by the processor
        when a software or hardware interrupt occurs.  For example, to
        make a system call, user application issues the <literal>INT
        0x80</literal> instruction.  This is a software interrupt, so
        the processor's hardware looks up a record with index 0x80 in
        the IDT.  This record points to the routine that handles this
        interrupt, in this particular case, this will be the kernel's
        syscall gate.  The IDT may have a maximum of 256 (0x100)
        records.  The kernel allocates NIDT records for the IDT, where
        NIDT is the maximum (256):</para>

      <programlisting><filename>sys/i386/i386/machdep.c:</filename>
static struct gate_descriptor idt0[NIDT];
struct gate_descriptor *idt = &amp;idt0[0];	/* interrupt descriptor table */
</programlisting>

      <para>For each interrupt, an appropriate handler is set.  The
        syscall gate for <literal>INT 0x80</literal> is set as
        well:</para>

      <programlisting><filename>sys/i386/i386/machdep.c:</filename>
 	setidt(0x80, &amp;IDTVEC(int0x80_syscall),
			SDT_SYS386TGT, SEL_UPL, GSEL(GCODE_SEL, SEL_KPL));</programlisting>

      <para>So when a userland application issues the <literal>INT
        0x80</literal> instruction, control will transfer to the
        function <function>_Xint0x80_syscall</function>, which is in
        the kernel code segment and will be executed with supervisor
        privileges.</para>

      <para>Console and DDB are then initialized:</para>

      <programlisting><filename>sys/i386/i386/machdep.c:</filename>
	cninit();
/* skipped */
#ifdef DDB
	kdb_init();
	if (boothowto & RB_KDB)
		Debugger("Boot flags requested debugger");
#endif</programlisting>

      <para>The Task State Segment is another x86 protected mode
        structure, the TSS is used by the hardware to store task
        information when a task switch occurs.</para>

      <para>The Local Descriptors Table is used to reference userland
        code and data.  Several selectors are defined to point to the
        LDT, they are the system call gates and the user code and data
        selectors:</para>

      <programlisting><filename>/usr/include/machine/segments.h</filename>
#define	LSYS5CALLS_SEL	0	/* forced by intel BCS */
#define	LSYS5SIGR_SEL	1
#define	L43BSDCALLS_SEL	2	/* notyet */
#define	LUCODE_SEL	3
#define LSOL26CALLS_SEL	4	/* Solaris >= 2.6 system call gate */
#define	LUDATA_SEL	5
/* separate stack, es,fs,gs sels ? */
/* #define	LPOSIXCALLS_SEL	5*/	/* notyet */
#define LBSDICALLS_SEL	16	/* BSDI system call gate */
#define NLDT		(LBSDICALLS_SEL + 1)
</programlisting>

    <para>Next, proc0's Process Control Block (<literal>struct
      pcb</literal>) structure is initialized.  proc0 is a
      <literal>struct proc</literal> structure that describes a kernel
      process.  It is always present while the kernel is running,
      therefore it is declared as global:</para>

    <programlisting><filename>sys/kern/kern_init.c:</filename>
    struct	proc proc0;</programlisting>

    <para>The structure <literal>struct pcb</literal> is a part of a
      proc structure.  It is defined in
      <filename>/usr/include/machine/pcb.h</filename> and has a
      process's information specific to the i386 architecture, such as
      registers values.</para>
    </sect2>

    <sect2>
      <title><function>mi_startup()</function></title>

      <para>This function performs a bubble sort of all the system
        initialization objects and then calls the entry of each object
        one by one:</para>

      <programlisting><filename>sys/kern/init_main.c:</filename>
	for (sipp = sysinit; *sipp; sipp++) {

		/* ... skipped ... */

		/* Call function */
		(*((*sipp)->func))((*sipp)->udata);
		/* ... skipped ... */
	}</programlisting>

    <para>Although the sysinit framework is described in the
      Developers' Handbook, I will discuss the internals of it.</para>

    <para>Every system initialization object (sysinit object) is
      created by calling a SYSINIT() macro.  Let us take as example an
      <literal>announce</literal> sysinit object.  This object prints
      the copyright message:</para>

    <programlisting><filename>sys/kern/init_main.c:</filename>
static void
print_caddr_t(void *data __unused)
{
	printf("%s", (char *)data);
}
SYSINIT(announce, SI_SUB_COPYRIGHT, SI_ORDER_FIRST, print_caddr_t, copyright)</programlisting>

    <para>The subsystem ID for this object is SI_SUB_COPYRIGHT
      (0x0800001), which comes right after the SI_SUB_CONSOLE
      (0x0800000).  So, the copyright message will be printed out
      first, just after the console initialization.</para>

    <para>Let us take a look at what exactly the macro
      <literal>SYSINIT()</literal> does.  It expands to a
      <literal>C_SYSINIT()</literal> macro.  The
      <literal>C_SYSINIT()</literal> macro then expands to a static
      <literal>struct sysinit</literal> structure declaration with
      another <literal>DATA_SET</literal> macro call:</para>
      <programlisting><filename>/usr/include/sys/kernel.h:</filename>
      #define C_SYSINIT(uniquifier, subsystem, order, func, ident) \
      static struct sysinit uniquifier ## _sys_init = { \ subsystem, \
      order, \ func, \ ident \ }; \ DATA_SET(sysinit_set,uniquifier ##
      _sys_init);

#define	SYSINIT(uniquifier, subsystem, order, func, ident)	\
	C_SYSINIT(uniquifier, subsystem, order,			\
	(sysinit_cfunc_t)(sysinit_nfunc_t)func, (void *)ident)</programlisting>

    <para>The <literal>DATA_SET()</literal> macro expands to a
      <literal>MAKE_SET()</literal>, and that macro is the point where
      the all sysinit magic is hidden:</para>

    <programlisting><filename>/usr/include/linker_set.h</filename>
#define MAKE_SET(set, sym)						\
	static void const * const __set_##set##_sym_##sym = &amp;sym;	\
	__asm(".section .set." #set ",\"aw\"");				\
	__asm(".long " #sym);						\
	__asm(".previous")
#endif
#define TEXT_SET(set, sym) MAKE_SET(set, sym)
#define DATA_SET(set, sym) MAKE_SET(set, sym)</programlisting>

    <para>In our case, the following declaration will occur:</para>

    <programlisting>static struct sysinit announce_sys_init = {
	SI_SUB_COPYRIGHT,
	SI_ORDER_FIRST,
	(sysinit_cfunc_t)(sysinit_nfunc_t)  print_caddr_t,
	(void *) copyright
};

static void const *const __set_sysinit_set_sym_announce_sys_init =
    &amp;announce_sys_init;
__asm(".section .set.sysinit_set" ",\"aw\"");
__asm(".long " "announce_sys_init");
__asm(".previous");</programlisting>

    <para>The first <literal>__asm</literal> instruction will create
      an ELF section within the kernel's executable.  This will happen
      at kernel link time.  The section will have the name
      <literal>.set.sysinit_set</literal>.  The content of this section is one 32-bit
      value, the address of announce_sys_init structure, and that is
      what the second <literal>__asm</literal> is.  The third
      <literal>__asm</literal> instruction marks the end of a section.
      If a directive with the same section name occurred before, the
      content, i.e. the 32-bit value, will be appended to the existing
      section, so forming an array of 32-bit pointers.</para>

    <para>Running <application>objdump</application> on a kernel
      binary, you may notice the presence of such small
      sections:</para>

    <screen>&prompt.user; <userinput>objdump -h /kernel</userinput>
  7 .set.cons_set 00000014  c03164c0  c03164c0  002154c0  2**2
                  CONTENTS, ALLOC, LOAD, DATA
  8 .set.kbddriver_set 00000010  c03164d4  c03164d4  002154d4  2**2
                  CONTENTS, ALLOC, LOAD, DATA
  9 .set.scrndr_set 00000024  c03164e4  c03164e4  002154e4  2**2
                  CONTENTS, ALLOC, LOAD, DATA
 10 .set.scterm_set 0000000c  c0316508  c0316508  00215508  2**2
                  CONTENTS, ALLOC, LOAD, DATA
 11 .set.sysctl_set 0000097c  c0316514  c0316514  00215514  2**2
                  CONTENTS, ALLOC, LOAD, DATA
 12 .set.sysinit_set 00000664  c0316e90  c0316e90  00215e90  2**2
                  CONTENTS, ALLOC, LOAD, DATA</screen>

    <para>This screen dump shows that the size of .set.sysinit_set
      section is 0x664 bytes, so <literal>0x664/sizeof(void
      *)</literal> sysinit objects are compiled into the kernel.  The
      other sections such as <literal>.set.sysctl_set</literal>
      represent other linker sets.</para>

    <para>By defining a variable of type <literal>struct
      linker_set</literal> the content of
      <literal>.set.sysinit_set</literal> section will be <quote>collected</quote>
      into that variable:</para>
      <programlisting><filename>sys/kern/init_main.c:</filename>
      extern struct linker_set sysinit_set; /* XXX */</programlisting>

    <para>The <literal>struct linker_set</literal> is defined as
      follows:</para>

    <programlisting><filename>/usr/include/linker_set.h:</filename>
  struct linker_set {
	int	ls_length;
	void	*ls_items[1];		/* really ls_length of them, trailing NULL */
};</programlisting>

    <para>The first node will be equal to the number of a sysinit
      objects, and the second node will be a NULL-terminated array of
      pointers to them.</para>

    <para>Returning to the <function>mi_startup()</function>
      discussion, it is must be clear now, how the sysinit objects are
      being organized.  The <function>mi_startup()</function> function
      sorts them and calls each.  The very last object is the system
      scheduler:</para>

    <programlisting><filename>/usr/include/sys/kernel.h:</filename>
enum sysinit_sub_id {
	SI_SUB_DUMMY		= 0x0000000,	/* not executed; for linker*/
	SI_SUB_DONE		= 0x0000001,	/* processed*/
	SI_SUB_CONSOLE		= 0x0800000,	/* console*/
	SI_SUB_COPYRIGHT	= 0x0800001,	/* first use of console*/
...
	SI_SUB_RUN_SCHEDULER	= 0xfffffff	/* scheduler: no return*/
};</programlisting>

    <para>The system scheduler sysinit object is defined in the file
      <filename>sys/vm/vm_glue.c</filename>, and the entry point for
      that object is <function>scheduler()</function>.  That function
      is actually an infinite loop, and it represents a process with
      PID 0, the swapper process.  The proc0 structure, mentioned
      before, is used to describe it.</para>

    <para>The first user process, called <emphasis>init</emphasis>, is
      created by the sysinit object <literal>init</literal>:</para>

    <programlisting><filename>sys/kern/init_main.c:</filename>
static void
create_init(const void *udata __unused)
{
	int error;
	int s;

	s = splhigh();
	error = fork1(&amp;proc0, RFFDG | RFPROC, &amp;initproc);
	if (error)
		panic("cannot fork init: %d\n", error);
	initproc->p_flag |= P_INMEM | P_SYSTEM;
	cpu_set_fork_handler(initproc, start_init, NULL);
	remrunqueue(initproc);
	splx(s);
}
SYSINIT(init,SI_SUB_CREATE_INIT, SI_ORDER_FIRST, create_init, NULL)</programlisting>

  <para>The <function>create_init()</function> allocates a new process
    by calling <function>fork1()</function>, but does not mark it
    runnable.  When this new process is scheduled for execution by the
    scheduler, the <function>start_init()</function> will be called.
    That function is defined in <filename>init_main.c</filename>.  It
    tries to load and exec the <filename>init</filename> binary,
    probing <filename>/sbin/init</filename> first, then
    <filename>/sbin/oinit</filename>,
    <filename>/sbin/init.bak</filename>, and finally
    <filename>/stand/sysinstall</filename>:</para>

  <programlisting><filename>sys/kern/init_main.c:</filename>
static char init_path[MAXPATHLEN] =
#ifdef	INIT_PATH
    __XSTRING(INIT_PATH);
#else
    "/sbin/init:/sbin/oinit:/sbin/init.bak:/stand/sysinstall";
#endif</programlisting>

  </sect2>
</sect1>

</chapter>

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