A memory subsystem incorporating a die-stacked DRAM (DSDRAM) is disclosed. In one embodiment, a system include a processor implemented on a silicon interposer of an integrated circuit (IC) package, a DSDRAM coupled to the processor, the DSDRAM implemented on the silicon interposer of the IC package, and a DRAM implemented separately from the IC package. The DSDRAM and the DRAM form a main memory having a contiguous address space comprising a range of physical addresses. The physical addresses of the DSDRAM occupy a first contiguous portion of the address space, while the DRAM occupies a second contiguous portion of the address space. Each physical address of the contiguous address space is augmented with a first bit that, when set, indicates that a page is stored in the DRAM and the DSDRAM.