1 .\" $NetBSD: ctxsw.9,v 1.2 1996/12/02 00:11:31 tls Exp $
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46 .Nd switch to another process context
48 .Fd #include <sys/param.h>
49 .Fd #include <sys/proc.h>
59 function implements the machine independent prelude to a process context
61 It is called from only a few distinguished places in the kernel
62 code as a result of the principle of non-preemtable kernel mode execution.
63 The three major uses of
65 can be enumerated as follows:
66 .Bl -enum -offset indent
72 when the current process
73 voluntarily relinquishes the CPU to wait for some resource to become
77 (e.g. a system call, device interrupt)
78 when the kernel prepares a return to user-mode execution.
80 typically handled by machine dependent trap-handling code after detection
81 of a change in the signal disposition of the current process, or when a
82 higher priority process might be available to run.
84 communicated by the machine independent scheduling routines by calling
88 in the signal handling code
91 if a signal is delivered that causes a process to stop.
95 records the amount of time the current process has been running in the
96 process structure and checks this value against the CPU time limits
97 allocated to the process
100 Exceeding the soft limit results in a
102 signal to be posted to the process, while exceeding the hard limit will
105 After these administrative tasks are done,
107 hands over control to the machine dependent routine
109 which will perform the actual process context switch.
112 first saves the context of the current process.
115 to determine which process to run next.
116 Finally, it reads in the saved context of the new process and starts to
117 execute the new process.
122 except that it does not save the context of the old process.
123 This function is useful when the kernel does not have an old process
124 context to save, such as when CPUs other than the boot CPU perform their
125 first task switch, or when the kernel does not care about the state of the
126 old process, such as in
128 when the kernel terminates the current process and switches into a new
133 all of these functions must be called with the