----  Extracted from PMR 6X601 in FE1 MYQMARK on JASPER's 01D disk.  ----
 
  There are 3 independent clock facilities in a CPU,
    the TOD Clock & Clock Comparator,
    the CPU Timer,
and the Interval Timer.
 
TOD Clock - 64-bit Unsigned number
            Bit 51 = 1 microsecond
            Bit 31 = 1.048576 seconds
            Counts Up
            Set   = SCK  = B204
            Store = STCK = B205
 
Clock Comparator - Same Format as TOD Clock
                   Interrupt x'1004' when TOD Clock > CC
                   and CR0, bit 20 (0000 0800) is on
                   Set   = SCKC  = B206
                   Store = STCKC = B207
 
CPU Timer - 64-bit Signed number
            Bit 51 = 1 microsecond (same as TOD Clock)
            Bit 31 = 1.048576 seconds
            Counts Down
            Set   = SPT  = B208
            Store = STPT = B209
            Interrupt x'1005' when bit 0 is on (negative)
            and CR0, bit 21 (0000 0400) is on
 
Interval Timer - 370 Mode only.  Does not exist in XA mode.
                 32-bit Signed number
                 Bit 23 = 1/300 of a second
                        = 3333.333... microseconds
                 Bit 31 = 13.0208333... microseconds
                 Counts Down
                 Set   = Store into Location x'50'
                 Store = Query location x'50'
                 Interrupt x'0080' when it goes negative
                 and CR0, bit 24 (0000 0080) is on
 
  Don't confuse (like I did, wasting a few days) the
Interval Timer units (13+ microseconds) with the O.S. term,
Timer Units, which is double (26+ microseconds).  Using the
O.S. simulation macros, STIMER(M) and TTIMER(M), you can
specify and get time in what they call "Timer Units".  They
are not the same units as the Interval Timer.  In the CMS
SP/4 DMSSVT source, where they handle TTIMER, they simply
double the interval timer to return their O.S. timer units.
In CMS SP/4, they use the interval timer.  In CMS SP/5.5,
they don't, even if you're in 370-mode, they still use the
Clock Comparator.