In this work, a scalable and modular architecture for massive MIMO base stations with distributed processing is proposed. New antennas can readily be added by adding a new node as each node handles all the additional involved processing. The architecture supports conjugate beamforming, zero-forcing, and MMSE, where for the two latter cases a central matrix inversion is required. The impact of the time required for this matrix inversion is carefully analyzed along with a generic frame format. As part of the contribution, careful computational, memory, and communication analyses are presented. It is shown that all computations can be mapped to a single computational structure and that a processing node consisting of a single such processing element can handle a broad range of bandwidths and number of terminals.