A_XMEMAllocate this state in X memory
A_YMEMAllocate this state in Y memory
AB_YMEM. Each attribute above turns one off and turns the other on (e.g.
Also for CG56 stars, portholes can assert attributes
P_YMEM, which work in exactly the same way as
A_YMEM. The default attribute for a 56001 porthole is
P_XMEM, which allocates the porthole buffer in X memory. Specifying the
P_YMEMattribute places the porthole buffer in Y memory.