System memory in Linux can be divided into two distinct regions: kernel space and user space. Kernel space is where the kernel (i.e., the core of the operating system) executes (i.e., runs) and provides its services.
Memory consists of RAM (random access memory) cells, whose contents can be accessed (i.e., read and written to) at extremely high speeds but are retained only temporarily (i.e., while in use or, at most, while the power supply remains on). Its purpose is to hold programs and data that are currently in use and thereby serve as a high speed intermediary between the CPU (central processing unit) and the much slower storage, which most commonly consists of one or more hard disk drives (HDDs).
User space is that set of memory locations in which user processes (i.e., everything other than the kernel) run. A process is an executing instance of a program. One of the roles of the kernel is to manage individual user processes within this space and to prevent them from interfering with each other.
Kernel space can be accessed by user processes only through the use of system calls. System calls are requests in a Unix-like operating system by an active process for a service performed by the kernel, such as input/output (I/O) or process creation. An active process is a process that is currently progressing in the CPU, as contrasted with a process that is waiting for its next turn in the CPU. I/O is any program, operation or device that transfers data to or from a CPU and to or from a peripheral device (such as disk drives, keyboards, mice and printers).
Created February 8, 2005.