Which OS uses a distributed subsystem architecture?

IOS XE is Linux-based but runs IOS as a process; it is not primarily characterized as a distributed subsystem architecture in the same way as IOS XR.

IOS is a monolithic operating system, where all processes run in the same memory space, not a distributed subsystem architecture.

Cisco IOS XR is designed with a distributed subsystem architecture, featuring a modular operating system built on a microkernel. This design enables process isolation, allowing different system components (e.g., routing protocols, forwarding managers, interface drivers) to run as independent processes that can be restarted or upgraded without impacting the entire system's operation, thus providing enhanced stability and scalability.

CatOS is an older, monolithic operating system primarily used on Catalyst switches, not featuring a distributed subsystem architecture.