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The current release of Windows® 95 4.00 does not allow floating-point or
MMX instructions within VxD's, which run in ring 0. Floating point and
MMX instructions in applications and DLL's are not restricted The reason
for the restriction is because Windows® 95 does not allow floating-point
exceptions when they are originated from ring 0.
Systems using Intel processors with MMX technology, including
OverDrive®processors with MMX technology will display a
blue screen exception error message when attempting to run software which
executes MMX instructions at the ring 0 level, under early versions of
Windows® 95 (pre-OSR2 releases)
The solution is incorporated in the version 4.02 of VMCPD VxD. The revised
VxD is included in the new OEM version of Windows® 95 called OSR2 (see MS
Distribution of the solution:
Within Windows® 95 OSR2, support for FP/MMX instructions in ring 0
is implemented in VMCPD.VxD version 4.02.
Ring 0 FPU/MMX instruction dependent software should detect the existence of
VMCPD.VxD version 4.02, at installation time and at run time by checking the
version number of the VMCPD. If an early VMCPD VxD version is deteced, then
Direct X applications should intall the new version of Direct X (Beta 3 and
above) to update the VMCPD VxD.
The following API will report the version of the VMCPD VxD:
Version 4.02 or above will allow FP/MMX instruction usage within ring
There are two major cases where the VxD is called: