IBM System/370 Guide

a data transfer channel program. A channel available interrupt
may occur, however. during channel program execution •
•
The channel and disk control unit are free during arm motion and
rotational positioning, allowing seek and set sector operations
to be overlapped with other I/O operations on that control unit
and channel. Implementation of multiple requesting permits a disk
control unit to control concurrent execution of multiple RPS channel
programs in order to overlap seek and set sector operations for
its drives.
Performance improvement gains achieved on block multiplexer channels
are not due entirely to the fact that direct access device rotational
delays are overlapped. Also important is the ability to initiate seek
commands a number of milliseconds earlier because a block multiplexer
channel is free. The initiation of stand-alone seeks on a selector
channel is delayed during search and data transfer operations. On
a block multiplexer channel, seeks can be initiated during rotational
positioning, since the channel and disk control unit are not busy.
The concepts of rotational position sensing as described for the
3330 facility apply also to the 2305 facility. Since an arm poSitioning
seek is not required for the 2305, the channel program issued by an
RPS access method or a user can be the same as shown for the 3330
facility except that the first command will not be a seek requiring
mechanical motion.
SUMMARY OF BLOCK MULTIPLEXING OPERATIONS WITH I/O DEVICES
The following summarizes how direct access devices without and with
RPS and other I/O devices operate on a 2880 Block Multiplexer Channel
when a command-chained channel program is executed.
1. Direct access devices without RPS (2311, 2321. 2314, 2303, 2301)
assigned to a shared subchannel of a 2880 channel operate in
the same way whether the channel is in selector or block
multiplexer mode. That is, the 2880 and the disk control unit
are busy during the entire time a command-chained disk channel
program is in operation. (There is no disconnection after a
chained seek.)
2. The 3330 facility executing a command-chained channel program
disconnects after the control unit accepts an arm positioning
seek that causes arm movement. Reconnection is attempted when
the arm reaches its destination and signals device end. The
3330 facility also disconnects when its control unit accepts
a SET SECTOR command. When the sector specified arrives under
the read/write heads, the control unit attempts to reconnect
and resume the CCW chain. If the channel is busy, the control
unit repeats the reconnection procedure each time the specified
sector position is reached.
3. The 2305 facility executing a command-chained channel program
disconnects when the control unit accepts a SET SECTOR command.
Reconnection occurs as described for the 3330 facility.
4. All currently announced tape drives operate exactly the same
on a 2880 as on a selector channel. That is, the 2880 is busy
during the entire time a command-chained channel program is
in operation.
5. Buffered card and print devices (or devices operating with
buffered control units) disconnect during the mechanical motion
of the device. Reconnection occurs later to fill or empty the
33

IBM System/370 Specifications

General

Brand	IBM
Model	System/370
Category	Server
Language	English