IBM System/370 145 Manual page 49

When the sector address is known or can be calculated, the channel
program below illustrates retriva.l of a record from the same BDAM data
set on a 3330-series drive attached to a block multiplexer channel. The
records are fixed-length standard format and sector numbers are
ca-lculated from record ID (by da1ta management).
Channel program 1. Initiate the seek and data transfer operation.
Command Chailling
Flag Command
Block Multiplexer
Channel and Disk
Control Unit
Status
CC
SEEK
CC
SET SECTon
CC
(Seek address)
(Sector number of
sector preceding
desired record)
Free during
arm motion
Free until
sector found
Busy SEARCH ID
EQ
(ID - sequential
position on track) (260 microseconds
average for a 3330)
CC TIC
READ DATA
(Back to search if
ID is not equal.
With the logic
shown, the first
ID inspected
normally is that of
the desired record
and the TIC command
is not executed.>
Busy
(Processor storage Busy
address of input
area)
The preceding example
indicatE~
the advantages of rotational position
sensing and block multiplexing:
• Only one channel program is :required to locate a disk record and
transfer the data, thereby el.iminating a stand-alone seek I/O
interruption and the I/O supe!rvisor processing required to schedule
a data transfer channel program. A Channel-available interruption
may occur, however, during
cb~nnel
program execution.
• The channel and disk control unit are free during arm motion and
rotational positioning, allowring 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 cQncu~rent 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_
A Guide to the IBM System/310 Model 145
39