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The program may also have to take into account
that serialization of CPU operations, which is
performed by all interruptions and by the execution
of certain instructions, affects the sequence of
events as observed by other CPUs in a
multiprocessing configuration as well as by
channels. (See the section "Serialization H in
Chapter 5, Hprogram Execution. H)
Programming Note
This publication assigns meanings to various
operation codes, to bit positions in instructions,
channel-command words, registers, and table
entries, and to fixed locations in the low 512 bytes
of storage (addresses 0-511). Other operation
codes, bit positions, and low-storage locations are
specifically noted as being available for
programming use. The remaining ones are
unassigned and reserved for future assignment to
new facilities and other extensions of the
architecture.
To ensure that existing programs run if and
when such new facilities are installed, programs
should not depend on an indication of an exception
as a result of invalid values that are currently
defined as being checked.
If
a value must be
placed in unassigned positions that are not checked,
the program should enter zeros. When the machine
provides a code or field, the program should take
into account that new codes and bits may be
assigned in the future. The program should not use
unassigned low-storage locations for keeping
information since these locations may be assigned
in the future in such a way that the machine causes
this location to be changed.
Chapter 1. Introduction
1-3
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