Hardware Reference
In-Depth Information
The DOS Boot Process
MS-DOS and similar operating systems (PC-DOS, DR-DOS, and Freedos) use the fol-
lowing boot process:
1.
The initial system file (called
IO.SYS
or
IBMBIO.COM
) is loaded, and the initialization
code copies itself into the highest region of contiguous DOS memory and transfers
control to the copy.
2.
The initialization code loads
MSDOS.SYS
(or
IBMDOS.COM
), which initializes the base
device drivers, determines equipment status, resets the disk system, resets and initial-
izes attached devices, and sets the system default parameters.
3.
CONFIG.SYS
is read multiple times. On the first pass any
DEVICE
statements are pro-
cessed in the order in which they appear; the drivers named are loaded and executed.
Then, any
INSTALL
statements are processed in the order in which they appear; the
programs named are loaded andexecuted. The
SHELL
statement isprocessed andloads
the specified command processor with the specified parameters. If the
CONFIG.SYS
file
contains no
SHELL
statement, the default
\COMMAND.COM
processor is loaded with de-
faultparameters.Loadingthecommandprocessoroverwritestheinitializationcodein
memory (because the job of the initialization code is finished).
4.
If
AUTOEXEC.BAT
is present,
COMMAND.COM
loads and runs
AUTOEXEC.BAT
. After the
commands in
AUTOEXEC.BAT
have been executed, the DOS prompt appears (unless
AUTOEXEC.BAT
calls an application program or shell of some kind, in which case the
usermightoperatethesystemwithouteverseeingaDOSprompt).Ifno
AUTOEXEC.BAT
file is present,
COMMAND.COM
executes the internal
DATE
and
TIME
commands, displays a
copyright message, and displays the DOS prompt.
The Windows 9x/Me Boot Process
The Windows 9x/Me boot process can be broken into two phases:
• The
IO.SYS
file is loaded and run. (Windows 9x's
IO.SYS
combines the functions of
DOS's
IO.SYS
and
MSDOS.SYS
.)
• The
WIN.COM
file is loaded and run.
Phase 1—Loading and Running
IO.SYS
IO.SYS
1.
The
IO.SYS
initialization code initializes the base device drivers, determines equip-
ment status, resets the disk system, resets and initializes attached devices, and sets the
systemdefaultparameters. The
Starting Windows
messageisdisplayedfor2seconds,
or until you press a Windows function key.
2.
The
IO.SYS
initialization code reads the
MSDOS.SYS
configuration file.
