INT 2F U - Microsoft Profiler (PROF.COM/VPROD.386) - START PROFILING
AX = 4503h
Notes:  Profiling is also turned on by the key combinations
LeftShift + RightShift + Alt
LeftShift + RightShift + Ctrl
for PROF.COM, this call programs the CMOS clock by reading register
0Ch, and setting bit 6 of register 0Bh.  It then makes sure that IRQ8
is unmasked

See also: 2F4504

2F4503 - Microsoft Profiler (PROF.COM/VPROD.386) - START PROFILING