System Memory

In UNIX, when an executable program is read into system memory by the kernel and executed, it becomes a process. We can consider system memory to be divided into two distinct regions or spaces. First is user space , which is where user processes run. The system manages individual user processes within this space and prevents them from interfering with one another. Processes in user space, termed user processes , are said to be in user mode . Second is a region called kernel space , which is where the kernel executes and provides its services. As noted previously, user processes can only access kernel space through system calls. When the user process runs a portion of the kernel code via a system call, the process is known temporarily as a kernel process and is said to be in kernel mode . While in kernel mode, the process will have special (root) privileges and access to key system data structures. This change in mode, from user to kernel, is called a context switch .

In UNIX environments, kernels are reentrant, and thus several processes can be in kernel mode at the same time. If the system has a single processor, then only one process will be making progress at any given time while the others are blocked. The operating system uses a bit, stored in the program status word ( PSW ), to keep track of the current mode of the process.