Multicore processors need to communicate when working on shared tasks. In classical systems, this is performed via shared objects protected by locks, which are implemented with atomic operations on the main memory. However, access to shared main memory is already a bottleneck for multicore processors. Furthermore, the access time to a shared memory is often hard to predict and therefore problematic for real-time systems. This paper presents a shared on-chip memory that is used for communication and supports atomic operations to implement locks. Access to the shared memory is arbitrated with time division multiplexing, providing time-predictable access. The shared memory supports extended time slots so that a processor can execute more than one memory operation atomically. This allows for the implementation of locking and other synchronization primitives. We evaluate this shared scratchpad memory with synchronization support on a 9-core version of the T-CREST multicore platform. Worst-case access latency to the shared scratchpad is 13 clock cycles. Access to the atomic section under full contention, when every processor core wants access to acquire a lock, is 135 clock cycles.