6. DATA SOURCES › 6.6 I/O Measurements and Device Activity › 6.6.1 What Is an I/O?

6.6.1 What Is an I/O?

During execution, the program, task, or TSO session that
constitutes an address space usually performs input and
output operations that involve devices outside the CPU and
central storage.  Each such operation is really a call to an
access method (AM), which is a set of subroutines that manage
the actual transmittal process.  The AM has responsibility
for any required restructuring of the data for transmittal
(e.g., blocking or segmenting to match storage or transmittal
protocols), high-level error detection and correction, and
notification of the calling task when the I/O process has
completed.  The AM may be able to respond to the call with a
reference to data already present in an in-storage buffer.
If not, the AM will issue one or more calls to the I/O
Supervisor (IOS).

I/O activity usually occurs asynchronously with the tasks
occupying the CPU.  The I/O request represents the beginning
of a complex sequence that may include periods of mechanical
action or telecommunications transmission during which the
CPU is effectively disconnected from the target device.  The
other end of the asynchronous I/O sequence is the interrupt
that is presented to the main processor(s) when data
transmission has completed or else when there is an error or
other condition that the subsystem cannot handle by itself.
A single I/O request can thus lead to one or more interrupts.
The interrupt leads to reactivation of AM code and, finally,
the application code that started the process.

The sections that follow discuss:

    1 - Where Does CA MICS Obtain Its I/O Counts?
    2 - What Is and Is Not Counted as an I/O?