pc/clock.c		1991/0704		Created.
		1991/0705
		1991/0706
		1991/0709
		1991/0711
		1991/0716
		1991/0719
		1991/0730
		1991/0801
		1991/0802
		1991/0806
		1991/0807
		1991/0808
		1991/0822
		1991/0823
		1991/1112
		1991/1113
		1991/1115
		1991/1116
		1992/0321
		1992/0820		Uncomment profiling clock.
		1992/0922		Measure cpu speed to make delay system-independent.
		1992/0923		rename constants; commenting; rewrite delay to use delayloop, set during clockinit.
		1993/0226		call mouseclock during clock interrupt
		1993/0915		convert to Brazil.
		1993/1113		remove uartclock.
		1993/1124		add arg to setvec functions; move clock to avoid prototype
		1994/0128		add hardclock during clock interrupt
		1994/0219		add uartclock during clock interrupt
		1994/0302		determine CPU frequency for 386, 486 by timing cycles
		1994/0507		call mouseclock at end of clock interrupt because it might spllo (BUG fix?)
		1994/0510		call mouseclock explicitly spllo if software cursor.
		1994/0512		change from square wave to retriggered 1/HZ interrupts. (maybe hw problems? XXX).
		1994/0525		always call mouseclock spllo.
		1994/0603		change back to square wave.
		1994/0610		add frequency guess for pentiums
		1994/0715		move uartclock to after checkalarms and hardclock (why? XXX)
		1994/0716		switch from delay to aamloop (XXX why?)
		1994/0717		change cycles for 486; avoid overflow in computation (BUG fix?)
		1994/0719		store MHz instead of Hz; change cycles for 386.
		1994/0806		change default cycles
		1994/0809		use cpufreq and cpumhz.
		1994/0810		no more setting spllo during clock interrupt? XXX why?
		1994/1029		add cpuidax, cpuiddx.
		1994/1031		add x86type table to determine proper cycles multiplier for AAM loop. determine loops value dynamically to get long sample.
		1994/1101		change cpuid print; get rid of loops check — just handle 386 different from others.
		1994/1102		change cycles for 486 class from 22 to 24.
		1994/1116		add Intel486DX2
		1994/1120		change cycles back to 22
		1994/1201		back to finding big AAM loop. change to 0.2% rounding error
		1994/1206		shut up compiler by initializing x. NOT a bug fix.
		1995/0104		move time accounting into accounttime. (XXX check for changes where this went?)
		1995/0109		do not reschedule while holding locks.
		1995/0110		do reschedule while holding locks.
		1995/0215		BUG fix: do not pass aamloop negative numbers. (XXX check aamloop to see if this is indeed a bug).
		1995/0527		remove x86 function; set conf.copymode using cpu family.
		1995/0812		let mouseclock rejoin the other clock handlers
		1995/0910		add randomclock to clock interrupt
		1996/0202		call exit during clock interrupt if kernel is exiting
		1996/0303		add call to segclock for clock interrupts during user space execution. (XXX why?)
		1996/0928		add PentiumPro to x86type.
		1997/0103		spelling
		1997/0327		formatting edits; big SMP rewrite.
		1998/0331		Use ilock instead of lock for clock0lock.
		1998/0710		Add blank line.
		1998/0717		Call fastticks(nil) in clock interrupt.
		1998/0725		Flush mmu on clock interrupt if necessary.
		1999/0129		Add fastticks, syncfastticks.
		1999/0130		Remove fast clock.
		1999/0205		add TK2MS.
		1999/0217		remove call to fastticks.
		1999/0218		put call to fastticks back. add fixtod.
		1999/0219		remove call to fixtod.
		1999/0228		remove TK2MS. (XXX where did it go?)
		1999/0301		add TK2MS back unchanged.
		1999/0310		comment why fastticks(nil) is being called.
		2000/1106		move most of the clock interrupt handler into portclock. (XXX check for changes!)
		2001/0710		comment edit.
		2001/0711		remove TK2MS (XXX where did it go?)
		2001/0905		add TK2MS back unchanged (again).
		2002/0405		add different rationale for calling fastticks(nil).
		2002/0406		remove nclockintr.
		2002/0710		remove TK2MS (again).
		2002/0822		add perfticks.
		2002/0928		Deleted.