512MB limit on Mac OS X OpenCL limited to 512MB on OS X?!?

I just do a couple of OpenCL test yesterday, on some Mac, each with 1GB GDDR5 (iMac w/Radeon HD5850M dubbed “HD 5750” by Apple, and MacBook Pro w/HD6750M). On OpenCL, on MAC OS X 10.6.8 and Mac OS X 10.7.3 they all reported 512MB available Global Memory for the OpenCL device.

I would like to know if you encountered the same limitation, or what is the actual available Global Memory reported on Mac with 512MB or 256MB of GPU memory. Precise the Mac model, OS X version, it’s GPU and the amount of video memory as well as how much Global Memory OpenCL report. Thanks for your help!