There are several methods that you can use to start WinDbg as the kernel debugger.
Method 1:
Start WinDbg from the command prompt with the kernel debugging
command-line options:
start windbg -k platform port speed
where platform is the target machine type (i386, MIPS, Alpha, PPC), port is the COM port (COM1 ... COMn) to which the serial cable on the host is attached, and speed is the com port speed (9600, 19200, 57600, ...). See Section 4.1.11 for a description of the command-line options.
Method 2:
Start WinDbg from the command prompt with ntoskrnl or ntoskrnl.exe as a
command-line option:
start windbg ntoskrnl[.exe]
Use the Kernel Debugger dialog from the Options menu to specify the target platform, com port, and speed.
Method3:
Start WinDbg by double-clicking the WinDbg icon in the
Program Manager. Use the Kernel Debugger command from the Options menu to
enable kernel debugging and specify the target platform, com port, and speed.
Once you have set up a kernel debugging session with the appropriate parameters, save it as a named workspace with the Save As command from the Program menu. You can then start a kernel debugging session with those parameters by specifying the workspace name on the command line:
start windbg -w workspacename ntoskrnl