"uClinux (Microcontroller Linux) is a variant of Linux system
without a Memory Management Unit (MMU). It was first created
starting from Linux 2.0 kernel. Today uClinux is available for 2.0,
2.4 and 2.6 Linux kernel releases. When no MMU support is present,
the user and kernel space is the same thing; also no virtual memory
subsystem is required because all codes run at Privilege Level 0. A
basic operating system has different privilege levels: kernel space
runs on Supervisor Mode and user applications on User Mode. Only
kernel space has direct access to resources, memory and I/O."
