Microsoft KB Archive/142657

= Data Corruption on Windows NT 4.0 =

Article ID: 142657

Article Last Modified on 10/31/2006

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APPLIES TO


 * Microsoft Windows NT Workstation 4.0 Developer Edition
 * Microsoft Windows NT Server 4.0 Standard Edition

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This article was previously published under Q142657



SYMPTOMS
Data may become corrupted on systems running Windows NT 4.0.



CAUSE
This problem is caused by an incorrect comparison between a 32-bit value and a page table entry's (PTE's) page frame number, which, on 8086-based processors is 20-bits. Every time Windows NT flushes the entire translation buffer (TLB), it increments a 32-bit counter. Every time Windows NT frees a set of system pages (for example, removing the mapping for a memory descriptor list [MDL]) the current value of the counter is stored into the PTE's page frame number field. When the PTE is reused, Windows NT compares the page frame number field to the current value in the counter, if they are equal, the TLB has not been flushed since this PTE was last used. If they are unequal, the PTE's virtual address can be reused without flushing it out of the TLB.

The algorithm is designed to allow the counter to overflow which is why there is a compare for equality rather than for greater than. However, after the counter exceeds 20-bits (1,048,575), the compare always fails and TLB flushes are not done when necessary. The fix is to make the 32-bit counter also 20 bits.

The reason this is sporadic is the fact that system PTEs are reused in a first in last out order (FILO). Hence, when there are ample system address ranges for mapping MDLs and there are various other operations that cause TLB flushes, by the time the PTEs are reused, a flush was already done. A Pentium Pro processor is more likely to encounter the problem because on a Pentium Pro, a context switch does not invalidate the TLB as it does on other 8086-based processors.



RESOLUTION
To work around this problem, obtain the fix mentioned below.



STATUS
Microsoft has confirmed this to be a problem Windows NT version 4.0. This problem was corrected in the latest Windows NT 4.0 U.S. Service Pack. For information on obtaining the Service Pack, query on the following word in the Microsoft Knowledge Base (without the spaces):

S E R V P A C K

Keywords: kbbug kbusage KB142657

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