Integrated circuits (ICs) employ subsystem shared cache memory for facilitating extension of low-power island (LPI) memory. An LPI subsystem and primary subsystems access a memory subsystem on a first access interface in a first power mode and the LPI subsystem accesses the memory subsystem by a second access interface in the low power mode. In the first power mode, the primary subsystems and the LPI subsystem may send a subsystem memory access request including a virtual memory address to a subsystem memory interface of the memory subsystem to access either data stored in an external memory or a version of the data stored in a shared memory circuit. In the low-power mode, the LPI subsystem sends an LPI memory access request including a direct memory address to an LPI memory interface of the memory subsystem to access the shared memory circuit to extend the LPI memory.