Document _tos.
 [rsc] --rw-rw-r-- M 771144 glenda sys 4191 Feb 11 09:09 sys/man/2/exec
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	/n/sourcesdump/2006/0212/plan9/sys/man/2/exec:1,6
	  .TH EXEC 2
	  .SH NAME
	- exec, execl, _clock, _privates, _nprivates \- execute a file
	+ exec, execl, _privates, _nprivates, _tps \- execute a file
	  .SH SYNOPSIS
	  .B #include <u.h>
	  .br
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	/n/sourcesdump/2006/0212/plan9/sys/man/2/exec:14,41
	  int execl(char *name, ...)
	  .PP
	  .B
	- long	*_clock;
	- .PP
	- .B
	  void	**_privates;
	  .PP
	  .B
	  int	_nprivates;
	+ .PP
	+ .B
	+ #include <tos.h>
	+ .PP
	+ .ft L
	+ typedef struct Tos Tos;
	+ struct Tos {
	+     struct { ... } prof;    /* profiling data */
	+     uvlong  cyclefreq;      /* cycle clock frequency */
	+     vlong   kcycles;        /* kernel cycles */
	+     vlong   pcycles;        /* process cycles (kernel + user) */
	+     ulong   pid;            /* process id */
	+     ulong   clock;          /* profiling clock */
	+     /* top of stack */
	+ };
	+ .PP
	+ .B
	+ extern Tos *_tos;
	  .fi
	  .SH DESCRIPTION
	  .I Exec
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	/n/sourcesdump/2006/0212/plan9/sys/man/2/exec:129,134
	  (see
	  .IR notify (2)).
	  .PP
	- When the new program begins, the global cell
	- .B _clock
	- is set to the address of a cell that keeps approximate time
	- expended by the process at user level.
	- The time is measured in milliseconds but is updated at
	- a system-dependent lower rate.
	- This clock is typically used by the profiler but is available
	- to all programs.
	- .PP
	  The global cell
	  .B _privates
	  points to an array of
	/n/sourcesdump/2006/0211/plan9/sys/man/2/exec:129,134 - 
	/n/sourcesdump/2006/0212/plan9/sys/man/2/exec:135,162
	  .B _nprivates
	  elements of per-process private data.
	  This storage is private for each process, even if the processes share data segments.
	+ .PP
	+ When the new program begins, the global pointer
	+ .B _tos
	+ is set to the address of a structure
	+ that holds information
	+ allowing accurate time keeping and clock reading in user space.
	+ If there is a user-space accessible fast clock (a processor
	+ cycle counter),
	+ .B cyclefreq
	+ will be set to its frequency in Hz.
	+ .B Kcycles
	+ .RB ( pcycles )
	+ counts the number of cycles 
	+ this process has spent in kernel mode
	+ (kernel and user mode).
	+ .B Clock
	+ is the user-profiling clock (see
	+ .IR prof (1)).
	+ Its time is measured in milliseconds but is updated at
	+ a system-dependent lower rate.
	+ This clock is typically used by the profiler but is available
	+ to all programs.
	  .PP
	  The above conventions apply to C programs; the raw system
	  interface to the new image is as follows: