PRB: Video Driver GP-Faults When Handling Large Bitmaps

• Microsoft Windows Device Development Kit (DDK) for Windows versions 3.0 and 3.1

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.