Which Cisco software OS uses monolithic architecture?

Cisco NX-OS is a modular operating system built on a Linux kernel, offering process separation and high availability, which is not monolithic.

Cisco IOS XE is a modular, Linux-based operating system that separates the IOS daemon from the Linux kernel, providing a more robust and fault-tolerant architecture than a monolithic design.

Cisco IOS XR is a highly modular, distributed operating system built on a microkernel, specifically designed for continuous system operation and process isolation, making it the opposite of monolithic.

Cisco IOS (Internetwork Operating System) traditionally utilizes a monolithic kernel architecture, meaning all operating system components and device drivers run together in the same kernel space, sharing the same memory and resources.