IBM System/370 145 Manual page 47

storage than does fixed assignment of an equal number of UCW's to each
block multiplexer channel. Only the requested number of UCW's, based on
the specific I/O configuration of a system, occupies control storage.
10:25 BLOCK MULTIPLEXING OPERATIONS WITH ROTATIONAL POSITION SENSING
DEVICES
Rotational position sensing (RPS) and multiple requesting are
standard features for 3330-series and 2305 direct access storage
devices. These two functions, together with block multiplexing, are
designed to increase system throughput by increasing channel throughput.
The presence of RPS in the control unit of a direct access device
enables i t to operate in block multiplexing mode. The use of rotational
position sensing reduces the number of channel programs that have to be
initiated for direct access devices that require an arm-positioning seek
(such as the 3330-series), frees channels more often during direct
access device operations--specifically, during most of the time required
to position a
trac~
to a desiredrecord--and permits disk channel
programs to be initiated sooner on block multiplexer channels than is
possible with selector channels.
Multiple requesting is implemented in a direct access device control
unit to enable i t to handle concu.rrent execution of multiple RPS channel
programs. Model 1 of 3830 Storag,e Control for 3330 modules, for
example, can simultaneously control eight RPS channel programs, one on
each of its drives.
In order to overlap seek operaitions for direct access devices without
RPS, channel scheduling routines l~ust initiate two channel programs for
each record read or write. The first is a stand-alone seek, which frees
the channel as soon as the control unit accepts the seek address. (The
control unit is also free during arm movement.) At the completion of
the seek, a device-end interruption is presented, and the data transfer
channel program is subsequently initiated to search for the desired
record and transfer the data. A selector channel is busy during the
entire search operation (execution of the SEARCH command by the control
unit) that locates the desired disk record on the track. Search time
can be significantly greater than data transfer time for disk records
smaller than half a track in size. Search time averages one-half of a
rotation for a read or write (8.3 ms for a 3330-series drive) and
requires a full rotation, less record write time, for a write
verification chained from a write,.
Use of RPS reduces the time th~e channel is busy during the search for
a disk record~ It permits the SEARCH command to be initiated just
before the desired record is to come under the read/write heads, that
is, when the desired rotational
~~sition
is reached. To accomplish
this, a "sector" concept is employed. The tracks in each cylinder of a
direct access device are consider.~d to consist of equally spaced sectors
(the number of sectors varies by d.evice). Track formatting is unchanged
but each record has a sector locait.ion as well as a record address. A
sector is not physically indicated on disk tracks, but is the length of
the track arc that passes under the read/write heads in one sector time.
For 3330-series drives, for example, sector time is defined to be
approximately 130 microseconds. ~rhus, there are 128 sectors per track
on 3330-series drives.
A disk control unit with RPS and multiple requesting can determine
the sector currently under the heads of each of its drives. A sector
counter is contained in each
driv~~.
The counter is incremented once
every sector time period and set
it.O
zero each time the index marker
passes under the heads. The sector in which a record falls is a
function of the length of all records that precede i t and of its
A Guide to the IBM System/310 Model 145
31