In the future, advanced integrated circuit processing and packaging technology will allow for several design options for multiprocessor microprocessors. In this paper we consider three architectures: shared-primary cache, shared-secondary cache, and shared-memory. We evaluate these three architectures using a complete system simulation environment which models the CPU, memory hierarchy and I/O devices in sufficient detail to boot and run a commercial operating system. Within our simulation environment, we measure performance using representative hand and compiler generated parallel applications, and a multiprogramming workload. Our results show that when applications exhibit fine-grained sharing, both shared-primary and shared-secondary architectures perform similarly when the full costs of sharing the primary cache are included.