The 970 is a cut-down 980. Where the 980 has 4GB of memory connected to the core through a 256-bit bus to the 8 32-bit GDDR5 memory controllers the 970 only has a 224-bit (aggregate) bus between the memory controllers and the L2 caches. The last eighth of the memory can only be accessed in a bank-switched fashion that blocks access to the primary 3.5GB bank. The primary bank can be accessed almost as fast as the 980's memory (striping across 7 chips instead of 8), but accesses to the last 0.5GB bank can't be striped across multiple chips. Those two effects together make it impossible to treat the 970 as having a full 4GB of general-purpose RAM.