Although all of the functions provided by expanded virtual machine
assist can be disabled by values in control register 6, any of the
functions of expanded virtual machine assist can be enabled or disabled by the setting of bits in a field defined in the MICBLOK (displace.ent 1'14', label MICEYMA) in conjunction with values in control register 6
that enable expanded virtual machine assist. Specifically, if bits 0 and 6 of control register 6 are both on and the real machine is in
virtual supervisor state, then the finer controls provided in the
specified MICBLOK field are active. If those two bits in control
register 6 are not both on, or if the real machine is not in virtual
supervisor state, then the settings of the MICBLOK assist controls are ignored. During virtual supervisor state execution, a particular function of
expanded virtual machine assist is enabled only if: • Expanded virtual machine assist is enabled in control register 6
(bits 0 and 6 are both on), and • The bit defined for that function in the MICBLOK assist control field
is on. Virtual machine assist is unaffected by the MICBLOK assist control
field.
Some bits in the MICBLOK ass1st control field enable and disable
several functions of expanded virtual machine assist. This is because
of the similarity in the definitions of the controlled functions. The
following table contains the bit definitions of the MICBLOK assist
control field. o Load PSi 1 Purge TLB
2 Set clock comparator, set CPU timer
3 Start I/O 4 Set system store then AND system mask,
store then OR system mask
5 Store CPU timer
6 Test channel
These MICBLOK assist control bits can also be used to enable or
disable all of expanded virtual machine assist. If bits 0 and 6 of
control register 6 are both on, and the assist control bits are off,
then CP assist is enabled but expanded virtual machine assist is
disabled. VIRTUAL INTERVAL TIMER ASSIST Virtual interval timer assist is the hardware support for an interval
timer in the virtual machine. The virtual machine may be in either EC
mode or BC mode. The virtual interval timer is in the virtual machine's
page frame 0 and functions like the real interval timer.
The virtual interval timer runs without either the CP assist or the
extended virtual machine assist being active. The virtual interval tiaer assist is only active if virtual machine assist is active. Bits 0 and 7 of control register 6 control the virtual interval timer assist.
Appendix A: VM/370 Extended Control-Program Support 1-421

Eit 23 of the virtual interval timer is decremented like the real
interval timer. If a virtual machine is executing (the real PSi is in
problem state) and the real interval tiaer needs updating, both the real
interval timer and the virtual interval timer are updated. The virtual
interval timer is decremented only if the following conditions are true: • The real PSi is in the problem state (bit 15 = 1) • Virtual machine assist is enabled (control register 6 bit 0 = 1) • Virtual interval timer assist is enabled (control register 6 bit 7 =
1)
A virtual interval timer interrupt request is recognized when the
virtual interval timer is decremented from a positive value, including
zero, to a negative value. When this condition occurs, an interval timer interrupt is presented to the virtual aachine.
If the virtual interval tiaer interrupt cannot be presented to the
virtual machine, the virtual interval timer interrupt is presented to
the real machine. When presented to the real machine, the virtual
interval timer interrupt is distinguished from the real interval timer interrupt by a unique external interrupt code (bit 7 of the halfword
interrupt code set to 1 and bits 0-6 set to 0). Bits 8-15 of the
interrupt code are zero unless set to one for another condition that may be concurrently indicated. (Locations X'84' and X'85' are set to zero.) 1-422 IBM VM/370 System Logic and Problem Determination--Voluae 1