IBM System/370 Guide

channel only when the channel is not busy. Thus, only cyclic
devices (such as direct access devices with rotational position
sensing) or buffered devices (such as the 2540 Card Read Punch
and the 1403 Printer) can disconnect during the execution of
a command-chained channel program on a block multiplexer channel
and resume operation later.
2. Data transfer operations for concurrently operating devices
on a block multiplexer channel are interleaved on a first-come,
first-served basis as the desired records become available.
Thus, devices are serviced in the order in which their records
become available, not necessarily in the order in which their
channel programs are initiated.
10:25 BLOCK MULTIPLEXING OPERATIONS WITH ROTATIONAL POSITION SENSING
DEVICES
Rotational position sensing and multiple requesting are standard
on 3330 and 2305 facilities. 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 the device 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 facility), frees channels more
often during direct access device operations - specifically, during
most of the time required to position a track to a desired record -
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 it to handle concurrent execution of multiple RPS channel
programs. The control unit of the 3330 facility, for example, can
simultaneously control eight RPS channel programs, one on each of its
drives.
In order to overlap seek operations for current direct access devices
without RPS, channel scheduling routines must 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 interrupt 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 that locates the desired
disk record on the track. Search time can be significantly greater
than data transfer time for disk records s~ller than half a track
in size. Search time averages one-half of a rotation for a read or
wr~te (8.3 rus for a 3330) and requires a full rot4tion. less record
write time, for a write verification chained from a write.
Use of RPS reduces the time the channel is busy searching 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 position is reached. To accomplish this,
a Wsector
W
concept is employed. The tracks in each cylinder of a
direct access device are considered to consist of equally spaced sectors
(the number of sectors varies by device). Track formatting is unchanged
but each record has a sector location 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
30

IBM System/370 Specifications

General

Brand	IBM
Model	System/370
Category	Server
Language	English