Siemens U3585-J-Z125-8-76 1 Glossary Channels, Counting of I/O operations and their duration

Glossary

channels

Channels are used for data interchange between the CPU, main memory and the peripheral devices.

Multiplexer channel (generic term):

can handle several I/O operations simultaneously.

Byte multiplexer channel (BYMUX):

data is transferred in byte-interleaved fashion between main memory and several simul- taneously active I/O devices. This type of channel is used to connect slow devices.

Block multiplexer channel (BLMUX):

data is transferred in block-interleaved fashion between main memory and several simultaneously active I/O devices. This type of channel is used to connect fast devices such as disk controllers.

counting of I/O operations and their duration

For SM2, an I/O operation is

–any version of EXCP (EXCP, EXCPW, $EXCP, $EXCPW) or

–an I/O request from the memory management system for paging (no SVC call)

addressed to the I/O control module. The I/O control module is the central system component for handling physical I/O operations.

The I/O control module normally processes the I/O request with one privileged instruction (Start Device or Start Subchannel). For each instruction a termination message is issued, upon which the I/O operation is counted.

In rare cases (e.g. disk connected to a channel operated in selector mode), two SDV instructions are issued. This is called "offline seek". The first SDV prepares the I/O device and the second initiates the I/O operation proper. Only one I/O operation is counted in this case too.

The following particularities should be noted:

I/O requests which could not be started properly (SDV with condition code differing from 0) are ignored.

So-called SENSE SDVs, which are generally issued after an I/O request that resulted in an error and serve to fetch additional diagnostic information, are only included in report 100.

In SDV fast release mode supported in some systems, a properly initiated I/O operation can be rejected at a later time by a channel. The rejected SDVs are not counted.

In determining the duration of the I/O operation, the time between the SDV and the termination message is always counted.

An SDV rejected because of an error is ignored; this results in an increase in waiting time for the device.

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