When BOINC 7.4 is released to the public, you who run older Nvidia GPUs may notice that your GPU is no longer being used by BOINC for Macintosh. This happens only when you updated your drivers to 340.xx or later, or otherwise added CUDA 6.5 support on your computer. Nvidia released CUDA 6.5 earlier this year and in its documentation it states that it will drop support for all compute capability GPUs 1.0 to 1.3.

Despite NVIDIA's claims that CUDA 6.5 still supports GPUs with compute capabilities 1.0 through 1.3, we have found that it does not. Please see:
http://setiathome.berkeley.edu/forum_thread.php?id=75633&postid=1571811#1571811 and
https://devtalk.nvidia.com/default/topic/774899/cuda-programming-and-performance/340-52-driver-breaks-opencl-support-for-pre-fermi-cc-1-x-cards/ for more information.

We have found that once CUDA 6.5 is installed on Macintosh computers, OpenCL applications (e.g. Seti Astropulse v7 and Collatz Conjecture) cause frequent kernel panics (i.e, they crash the OS!) on systems with older NVIDIA GPUs.

In addition, the people working on SETI@home have found that OpenCL applications return incorrect results on Windows computers with older GPUs which have the NVIDIA 340.xx range of drivers (those which have CUDA 6.5), even though it appears to the user that it works properly. For the moment Seti will rely on the use of the validators to weed out these erroneous tasks from the correct ones.

Notice we are talking about OpenCL applications, not just CUDA apps.

The Macintosh installer for BOINC 7.4.26 includes a ReadMe, which now says:
CUDA UPGRADE WARNING: Do not upgrade to CUDA 6.5 or later if you have an older NVIDIA GPU with Compute Capability 1.3 or less. You can check your GPU’s Compute Capability at https://developer.nvidia.com/cuda-gpus

To use BOINC 7.4.26 and beyond with older NVIDIA GPUs on Macintosh computers, you must install an older version of the drivers.

You can find older CUDA drivers at http://www.nvidia.com/object/mac-driver-archive.html. Note: after mounting the downloaded disk image, you may need to control-click on the CUDA installer package to open it.

Below is an example of how BOINC will report your GPU:

NVIDIA GPU 0: Quadro FX 3800M cannot be used for CUDA or OpenCL computation with CUDA driver 6.5 or later
CUDA: NVIDIA GPU 0 (not used): Quadro FX 3800M (driver version 340.84, CUDA version 6.5, compute capability 1.1, 1024MB, 862MB available, 634 GFLOPS peak)
OpenCL: NVIDIA GPU 0 (not used): Quadro FX 3800M (driver version 340.84, device version OpenCL 1.0 CUDA, 1024MB, 862MB available, 634 GFLOPS peak)

Note that there is no cc_config.xml option to force the GPU to be used anyway, as it doesn't make sense to allow a GPU that returns only erroneous work to be used.

Explanation behind the decision not to support older Nvidia GPUs that run CUDA 6.5 drivers:

Charlie Fenton, BOINC developer for Macintosh wrote:

While one can debate the issue for Windows where the "only" issue is silently returning incorrect results, we have demonstrated that running OpenCL apps on Macs with older NVIDIA GPUs and CUDA 6.5 installed causes _frequent_ kernel panics, which are the Mac equivalent of Windows blue screens of death.

Having the OS crash and restart every 10 minutes (literally) is totally unacceptable. Users will tend to blame BOINC, which is clearly a very bad situation.

As it turns out, older versions of BOINC did not recognize the existence of CUDA at all after CUDA 6.5 was installed, because CUDA 6.5 also dropped 32-bit support. (This has been fixed in BOINC 7.4.26.) So users of older versions of BOINC on Macs are fortunately spared the kernel panics.

So it was imperative for BOINC to refuse to run situations that will crash the OS. Because of the (different) proven problems on Windows, we decided to extend that protection to Windows as well.