Though same physical memory may be shared but still the memory is separated for CPU and GPU. Size of GPU memory can be set with BIOS. With newer APUs having HSA support (like Kaveri), HUMA and HSA enhancements allows the CPU and GPU cores to dynamically share the same virtual memory address space. I guess, the new SVM features in OpenCL 2.0 will allow the same benefit on these APU.
Regards,