PRB: Windows 95 Doesn't Load SCSI Miniport Driver

• Microsoft Win32 Device Development Kit (DDK) for Windows 95

SYPTOMS

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:

    UnKnown int 13 hooker MBRINT13

CAUSE

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 entrypoint into this memory. The INT 13h hook is treated as an unsafe MBR INT 13h hook because it lies at an adress below A000:0000h in RAM, rather than above A000:0000h in what is assumed to be ROM.

RESOLUTION

IOS tries to detect when a BIOS has installed an INT 13h handler by examining the code in the handler's entrypoint. 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 [foo]

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

4.
    call    xxxx:yyyy

5.
    jmp     xxxx:yyyy

STATUS

This behavior is by design.