Block Multiplexer Channels - IBM System/370 145 Manual

processor storage on a one-byte basis. A one-byte store into processor
storage requires 607.5 nanoseconds, while a one-byte fetch requires 540
nanoseconds.
The data rate of a selector channel can be increased by installation
of the optional Channel Word Buffer feature. When this feature is
present, a four-byte buffer is included for each installed selector
channel. (The Channel Word Buffer feature does not apply to the IFA or
the byte multiplexer channel.) Data is transferred between a channel
buffer and processor storage up to four bytes at a time to increase the
channel data rate and reduce the number of required processor storage
accesses, thereby causing less interference with the
cpu.
If the word
buffer is installed, an individual selector channel can sustain a data
rate of 1.85 MB, while the maximum obtainable aggregate data rate of all
operating channels is approximately 5 MB. The transfer times stated for
single-byte fetches and stores are not altered by use of the word
buffer. The Channel Word Buffer feature is a prerequisite for
attachment of the 2305 Model 2.
The order of priority implemented for handling simultaneous requests
for an I/O interruption is (high to low): byte multiplexer channel, "IFA
or channell, chann.el 2, channel 3, channel 4. Simultaneous requests
for a command- or a data-chaining operation are handled on a rotating
basis in the following high-to-low priority sequence: IFA or channell,
channel 2, channel 3, IFA or channell, etc. Selector channels and the
IFA transfer data to and from processor storage during a share-cycle.
The high-to-low priority sequence for servicing share-cycle requests is
channel 2, IFA, channel 3, channel 2, etc., when the IFA is present, and
channell, channel 2, channel 3, channel 4, channel 1, etc., when the
IFA is not installed. The use of a rotating technique ensures that no
channel has more than one request serviced when a service request is
outstanding for another channel.
Tables 10.20.1 and 10.20.2 show examples of maximum-speed I/O
configurations with currently announced I/O devices that will operate on
the Model 145 without and with the. Channel Word Buffer feature
installed. The aggregate channel data rate listed is the rate that can
be sustained at any instant without data overrun when all installed
high-speed channels are operating concurrently.
BLOCK MULTIPLEXER CHANNELS
Block multiplexing mode for the Model 145 is an optional, no-charge
feature. It permits any or all installed selector channels to operate
as block multiplexer channels. When a system is ordered, the user
specifies those channels that are to operate only as selector channels
and those that are to have the capability of operating in either
selector or block multiplexer mode. Block multiplexing will be of most
benefit to system throughput when used with direct access devices with
rotational posi.tion sensing capability, such as the 3330-series and the
2305. The block multiplexer feature must be installed if 3330-series or
2305 disk storage is attached to the Model 145, OS is used, and multiple
requesting and rotational position sensing support are desired. (DOS
does not support block multiplexing.)
The setting of a channel mode bit in a control register determines
whether a channel with block multiplexing capability operates in block
multiplexer or selector mode. Those channels designated as selector
channels only are not affected by this mode bit. The mode bit is set to
selector mode at IPL and can be altered by programming at any time.
When a
START
I/O instruction is issued to a channel with block
multiplexing capability on the Model 145, the current setting of the
channel mode bit determines the mode in which the subchannel involved
will operate.
32 A Guide to the IBM System/370 Model 145