By default, Windows systems create kernel memory dump files. The kernel memory dump file is an intermediate-size dump file that records only kernel memory and can occupy several megabytes of disk space. A kernel memory dump file takes longer to create than a small dump file and thus increases the downtime associated with a system failure. On most systems, the increase in downtime is minimal.
Kernel memory dumps contain additional information that might assist troubleshooting. When a Stop error occurs, Windows saves a kernel memory dump file to a file named %SystemRoot%\Memory.dmp and creates a small memory dump file in the %SystemRoot%\ Minidump folder.
A kernel memory dump file records only kernel memory information, which expedites the dump file creation process. The kernel memory dump file does not include unallocated memory or any memory allocated to user-mode programs. It includes only memory allocated to the Executive, kernel, Hardware Abstraction Layer (HAL), and file system cache, in addition to nonpaged pool memory allocated to kernel-mode drivers and other kernel-mode routines.
The size of the kernel memory dump file will vary, but it is always less than the size of the system memory. When Windows creates the dump file, it first writes the information to the paging file. Therefore, the paging file might grow to the size of the physical memory. Later, the dump file information is extracted from the paging file to the actual memory dump file. To ensure that you have sufficient free space, verify that the system drive would have free space greater than the size of physical memory if the paging file were extended to the size of physical memory. Although you cannot exactly predict the size of a kernel memory dump file, a good rule of thumb is that roughly 50 MB to 800 MB, or one-third the size of physical memory, must be available on the boot volume for the paging file.
For most purposes, a kernel memory dump file is sufficient for troubleshooting Stop errors. It contains more information than a small memory dump file and is smaller than a complete memory dump file. It omits those portions of memory that are unlikely to have been involved in the problem. However, some problems do require a complete memory dump file for troubleshooting.
By default, a new kernel memory dump file overwrites an existing one. To change the default setting, clear the Overwrite Any Existing File check box. You can also rename or move an existing dump file prior to troubleshooting.
Source of Information : Windows 7 Resource Kit 2009 Microsoft Press