IBM System/370 145 Manual page 46

A nonshared block mul·tiplexer UCW requires eight bytes of control
storage. The nonshared block multiplexer UCW's in a Model 145 are not
hard-wired to specific channels and are assigned in groups of eight,
representing eight device addresses. System reset causes all nonshared
UCW's to become unassigned and available for dynamic assignment to
nonshared devices on any installed block multiplexer channel. When
system operation begins, a nonshared UCW is assigned to an I/O device
when the first START I/O instruction to the device is given, as
described below.
(A
UCW is not assigned unless the device is actually
present. )
When the first START I/O is executed for a nonshared device, the
channel determines whether a block of eight UCW·s has been assigned to
the range of eight addresses in which the device falls, 190-197, 230-
237, etc. If none are allocated, a block of eight available Ucw's is
assigned to the channel for the address range of that device. When the
first START I/O is initiated to another I/O device on the same channel
and in that block of addresses, the channel determines that a block of
UCW's has already been assigned.
This process continues for all nonshared I/O devices until each
device has a UCW assigned. If the pool of nonshared UCW's is empty when
a request is made, a channel not operational condition code setting is
given (condition code 3) and the START I/O is not executed. When a
larger UCW pool is required, a new disk cartridge must be obtained from
IBM in order to allocate more OCW·s to the pool in control storage.
Shared block multiplexer UCW's are not dynamically assigned to
devices as are the nonshared. The user must ~etermine the number of
shared UCW's required and the device addresses they are to be aSSigned
so that this information can be placed in the plug cards.. Each shared
ucw
requires one UCW area (8 bytes) plus one additional byte. Thus,
a single UCW group of eight UCW's (64 bytes) provides seven shared UCW's.
The total number of UCW groups required in a system is equal to the
number of shared control units in the I/O configuration divided. by seven
(rounded high), plus the number of UCW groups required by nonshared
control units. The number of groups assigned to the pool must be a
multiple of two.
For example, assume an I/O configuration includes the following:
• One 3830 control unit with two 3330-series modules (four drives)
on block multiplexer channel 1
• One shared control unit with six devices on block multiplexer channel 2
• One shared control unit with six devices on block multiplexer channel 3
The 3330-series string requires assignment of one nonshared UCW group
(eight UCW's), of which four UCW·s will actually be used. Four more
drives can be added to the 3330-series string. The devices on channel 2
require a single shared UCW, as do the devices on channel 3. Thus, one
group of shared UCW's, of which only two UCW's will be used, suffices.
Five more shared control units can be added to the system before the
shared UCW group is exhausted. This I/O device confi~ation requires
two UCW groups, one shared (Type
1)
and one nonshared (Type 2), or 128
bytes of control storage. If an additional string of 3330-series drives
is added to this configuration, a second nonshared UCW group is needed
and two more UCW groups must be specified for inclusion in the system
microcode (for a total control storage requirement of 256 bytes for
block multiplexer UCW's).
Dynamic assignment of nonshared block mu1tiplexer UCW's provides more
flexibility in I/O configurations and requires allocation of less control
36
A
Guide to the IBM System/370 Model 145