Hardware vendors developing a Microsoft SCSI miniport driver may find that their miniport will not load under Windows 95 when the SCSI adapter's BIOS is enabled. Ios.log shows the following listing:

The Windows 95 I/O Supervisor (Ios.vxd) attempts to detect unsafe INT 13h hookers during Windows 95 initialization. Part of this detection mechanism is an attempt to detect INT 13h hooks that were installed by a modified Master Boot Record (MBR). MBR INT 13h hooks are usually installed by disk utilities that perform services such as disk compression or encryption, which make it unsafe for Windows 95 to take over control of the disk using 32-bit disk drivers. MBR INT 13h hookers may also be installed by certain viruses.

Under certain circumstances, it is possible for IOS to treat the INT 13h handler installed by a SCSI BIOS as an unsafe MBR INT 13h hooker. This occurs when the BIOS allocates system RAM and moves its INT 13h entry point into this memory. The INT 13h hook is treated as an unsafe MBR INT 13h hook because it lies at an address below A000:0000h in RAM, rather than above A000:0000h in what is assumed to be ROM.

IOS tries to detect when a BIOS has installed an INT 13h handler by examining the code in the handler's entry point. It looks for code sequences that jump directly to an address above A000:0000h. Specifically, it looks for the following five code sequences to call or jump to an address above A000:0000h:

1.     push    ds
       push    cs
       pop     ds
       jmp     xxxx:yyyy
   						

2.     push    ds
       push    cs
       pop     ds
       jmp     dword ptr [MyFunc]
   						

3.     push    ds
       push    cs
       pop     ds
       call    dword ptr [MyFunc]
   						

4.     call    xxxx:yyyy
   						

5.     jmp     xxxx:yyyy
   						

SCSI miniport developers who are encountering this problem should modify their INT 13h entry point to use one of the above code sequences, and thus allow IOS to load their miniport driver.

This behavior is by design.