System Startup Sequence

 When you turn your computer on, it runs through some built-in diagnostic
 tests (called the POST or Power-On Self-Test).  If it detects a problem
 during these tests, you may see an error code (see POST Error Codes) or
 perhaps a full-screen interactive configuration utility will appear (this
 varies from machine to machine, depending on the brand of computer you
 have and the maker of its ROM-BIOS).

 After the POST, but before booting the disk, the ROM-BIOS performs
 ROM-Scan to see if any hardware adaptors have installed ROM into the
 areas between c000:0 and e000:0.

 Once the system gets past the POST and ROM-Scan, the sequence is basically
 the same for all IBM-compatible computers.  The following steps occur
 after the POST or whenever you press Ctrl+Alt+Del to reboot:

   1) If a diskette is in drive A, the boot sector of that drive is read
      and the system attempts to boot using system files from the diskette.
      If it is not a system disk, a message such as...

           Non-System disk or disk error
           Replace and press any key when ready

      ...appears.  You can do as it requests, or just press any key to boot
      from your hard disk.

      If no diskette is in drive A, the system attempts to boot from your
      first hard disk (always drive C).  The Master Boot Record (MBR)
      of the hard disk is read into memory and executed.

      The program code in MBR looks at the disk partition table (also in
      the MBR) and determines which operating system to start.  For most of
      us, the hard disk contains a single operating system -- MS-DOS 6 (you
      may use Fdisk to activate a different operating system).  The first
      sector of the active partition is used as the boot sector for that
      operating system.  From here on, the boot sequence is the same for
      hard disks and diskettes.

   2) The boot sector is read into memory and executed.  It contains
      instructions to look for two files in the root of the startup disk.

      IO.SYS contains the standard device drivers along with code that
      processes the rest of the boot sequence (IBMBIO.SYS in PC-DOS).

      MSDOS.SYS contains the code to handle DOS low-level function calls,
      including INT 21h services (IBMDOS.SYS in PC-DOS).

      The files are loaded into memory and control passes to IO.SYS.

   3) IO.SYS looks for DBLSPACE.BIN in the root of the boot disk.  If
      found, it is loaded into the upper part of conventional▲ memory.

      DBLSPACE.BIN intercepts all disk I/O services.  When it initializes,
      it looks for DBLSPACE.INI (also in the root) and using information it
      contains, it may swap one or more drive IDs.

      If either DBLSPACE.BIN or DBLSPACE.INI is not found, then DoubleSpace
      support will not be present.

      Note:    With DOS 6.2+, you can bypass this early load of
      DBLSPACE.BIN by pressing Ctrl+F5 when you boot.

   4) IO.SYS displays:

           Starting MS-DOS...

      It then looks for a text file▲ named CONFIG.SYS in the root of the
      boot disk.  If found, it is read and processed as a series of
      configuration commands.  If SWITCHES=/F is not found, there is a
      2-second delay.  In this interval, you can press F5 (see Clean Boot)
      or F8 (see Interactive Boot) to control the rest of the boot
      sequence.

      Note that when DoubleSpace is active, DBLSPACE.BIN has already
      intercepted disk I/O calls (and swapped drive C), so IO.SYS will find
      CONFIG.SYS on the compressed volume.

   5) If CONFIG.SYS contains a MultiConfig menu, then the menu is
      displayed:
          MS-DOS 6 Startup Menu
          ═════════════════════
          1. option
          2. option
          ... etc. ...

          Enter a choice: _

      It prompts for user input.  The MENUDEFAULT= command can specify a
      time-out value to continue the boot without waiting for a keystroke.

      The CONFIG e-var is set to match the option chosen.

   6) IO.SYS executes all commands in [COMMON] sections (including the
      "implied [COMMON]" area before the first section or the [MENU] line).
      It then proceeds to execute the particular commands specified in the
      selected (or default) section.

      Note that IO.SYS does not necessarily run commands in the order they
      appear in CONFIG.SYS.  It internally reorders commands into groups
      and then executes the groups in this order:
    
      ■ DOS=, SWITCHES=, SET etc. general configuration commands.
    
      ■ DEVICE= and DEVICEHIGH= device driver installation commands.
        Note that drive letters are assigned according to the order in
        which the device drivers are loaded in CONFIG.SYS.
    
      ■ INSTALL= and INSTALLHIGH= TSR installation commands.

      MultiConfig menu sections cannot override this general order; for
      instance, you can't run an INSTALL= line before a DEVICE= line.

   7) If a SHELL= command appears in CONFIG.SYS, then the program it
      identifies will be used as the DOS command processor.  If not, then
      the file COMMAND.COM will be used.  IO.SYS will look for COMMAND.COM
      in the root of the boot disk, and if not found, it will search \DOS
      and \MSDOS.

      IO.SYS sets the COMSPEC e-var to identify the command processor.

   8) COMMAND.COM initially sets up a minimal DOS environment containing a
      simple path (PATH=C:\DOS) and a standard prompt (PROMPT=$p$g).

   9) By default, COMMAND.COM looks for a text file▲ named AUTOEXEC.BAT in
      the root of the boot disk and begins executing the commands it
      contains.

    Note: with DOS 6.0+, You can force COMMAND.COM to execute a different
          startup batch by using SHELL=d:\path\command.com /K=batfile.bat
          in CONFIG.SYS.

 AUTOEXEC.BAT can be tailored to execute a different set of commands for
 each of the MultiConfig menu options by testing the CONFIG e-var.

 Finally, the command processor displays the command prompt and you may
 begin interactive computing.

See Also: CONFIG.SYS Commands
          Clean Boot
          Interactive Boot
          MultiConfig Menus
          DoubleSpace
          Predefined Environment Variables
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