Kernel
The protected core of the operating system, with the deepest access to memory and hardware.
Glossary
User Space
The more protected area where ordinary programs run, away from the system's core.
In one line
The more protected area where ordinary programs run, away from the system's core.
Also calleduserland
User space is the everyday neighbourhood of software, kept deliberately apart from the protected core. Programs here have restricted access and must ask the system for what they cannot reach directly. This separation is a basic safety measure, ensuring ordinary software cannot disturb the machine's foundations.
Running a driver in user space trades a little speed for a great deal of safety, since a failure there can be contained rather than allowed to disturb the core. A driver that stumbles in this outer area can often be restarted quietly while everything else continues.
The boundary between user space and the core is firmly guarded. When a program needs something only the core can provide, it makes a formal request and waits for an answer, rather than reaching across the line itself. This keeps the outer world from interfering with the inner one.
Choosing where a driver should live, in the outer neighbourhood or close to the core, is a thoughtful trade-off. Most parts are happiest in user space, where their occasional troubles stay safely contained and easy to recover from.
In practice
In practice, user space is like the public area of a building, while the core is the restricted staff-only section. Visitors do their business in the open areas and make requests at a desk when they need something from behind the locked doors. Ordinary programs live in just such a public space.
