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Source: Microsoft Support
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Sometimes a system needs to be debugged, but the debugging information hasn't been created permanently. This article describes how to provide debugging information in the boot of Vista and later Windows system.
When the Windows system is just started to be loaded by the machine's BIOS, hit F10. (It may sometimes help to hit F10 more than once.) Within a few seconds to a minute or so, depending on where the OS is stored, an "Edit Boot Options" screen with the current boot parameters will be displayed. For 1394 debugging via channel 1, add: /debug /debugport=1394 /channel=1. For serial debugging via COM1 at 115200 baud, add: /debug /debugport=com1 /baudrate=115200. Hit enter.
Note: If desired, one may erase or alter the current boot parameters by backspacing over them.
If a kernel debugger is already attached, the debugger command window should start to show output in a few seconds.
These steps apply to any Windows system being booted, as long as the OS is Vista or later. The OS may be booted from a local or externally attached (e.g., iSCSI or Fibre Channel) disk or from a DVD.
Note: The debugging information applies only as long as the OS is running. Once the OS stops (or bugchecks), the information is lost and must be entered at the next boot where debugging is desired.
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