This paper presents a novel 2.5D multicore processor which consists of 3 distinct silicon dies: a processor die with 8 MIPS-cores, a 16kB SRAM die, and an accelerator die for multimedia and communication applications. These dies are interconnected into multi-modes, like core-core (up to 32 cores), core-memory (4x storage capacity) and core-accelerator (4.4x speedup in H.264 decoder), to establish a scalable and reconfigurable platform with less tape-out die area cost. A pair of 8Gbps SerDes is custom designed for each of the 12 inter-die communication channels, achieving a 2.5D I/O bandwidth of 24GB/s. The processor was implemented in GF 65nm process, and operates at 500MHz under 1.2V supply, with 1.08W power dissipation.