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.
Programs and Processes
Remote Procedure Calls
Appendix A. Using Linux Manual Pages
Appendix B. UNIX Error Messages
Appendix C. RPC Syntax Diagrams
Appendix D. Profiling Programs