DOCUMENT:Q69292  03-NOV-1999  [win16ddk]
TITLE   :PRB: Video Driver GP-Faults When Handling Large Bitmaps
PRODUCT :Microsoft Windows Device Driver Kit
PROD/VER::3.0,3.1
OPER/SYS:
KEYWORDS:

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The information in this article applies to:

 - Microsoft Windows Device Development Kit (DDK) for Windows, versions 3.0, 3.1 
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SYMPTOMS
========

Some display drivers experience a GP-fault or unrecoverable application error
(UAE) when a large bitmap is loaded onto the desktop as "wallpaper." This fault
occurs when the bitmap is aligned on a page boundary and the driver attempts to
read the next page. In standard mode, the next page is often valid and the bug
escapes detection. In enhanced mode, the next page is frequently invalid and the
GP-fault occurs.

CAUSE
=====

This GP-fault can be attributed to the way the main loop is constructed in the
video driver DIB-handling routines. The main loop is written so that a byte is
fetched from the DIB and the data pointer is advanced immediately, before
processing the byte. This is fine until the very last byte in the DIB is
processed.

MORE INFORMATION
================

This problem has manifested itself in a few different ways. For example, in one
instance, the driver code fetched the final DIB byte (which was at an offset of
FFFFh in a given segment). The driver incremented the offset, determined that it
wrapped to zero, and incremented the value in the DIB segment register. Because
this was the final byte of the DIB, the new value in the DIB segment register
was an invalid selector. Using an invalid selector causes a GP-fault.

In a second example, the driver code loaded each DIB byte at the bottom of the
main loop (loop termination logic is at the top of the loop). This caused a
final, invalid byte to be loaded at the bottom of the loop (before the loop
termination test). This worked for many bitmaps because there are often a few
extra bytes of space after the final DIB byte. One bitmap, however, fit exactly
within the segment limit. When the program tried to fetch one extra byte in this
case, a GP-fault occurred because the fetch violated the segment limit defined
in the local descriptor table.

Additional query words: 3.00 3.10

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Keywords          :  
Technology        : kbAudDeveloper kbWin3xSearch kbWinDDKSearch kbWinDDK300 kbWinDDK310
Version           : :3.0,3.1

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