Multiple Program-Interruption Conditions - IBM 4300 Manual

MUltiple Program-Interruption Conditions
Except for PER events, only one program-
interruption condition is indicated with a program
interruption. The existence of one condition,
however, does not preclude the existence of other
conditions. When more than one program-
interruption condition exists, only the condition
having the highest priority is identified in the
interruption code.
With two conditions of the same priority, it is
unpredictable which is indicated. In particular, the
priority of access exceptions associated with the
two parts of an operand that crosses a page
boundary is unpredictable and is not necessarily
related to the sequence specified for the access of
bytes within the operand.
The type of ending which occurs (nullification,
suppression, or termination) is that which is defined
for the type of exception that is indicated in the
interruption code. However, if a condition is
indicated which permits termination, and another
condition also exists which would cause either
nullification or suppression, then the unit of
operation is suppressed.
6-16 IB M 4300 Processors Principles of Operation
The figure "Priority of Program-Interruption
Conditions" lists the priorities of all
program-interruption conditions other than PER
events. All exceptions associated with references
to storage for a particular instruction halfword or a
particular operand byte are grouped as a single
entry called "access." The priorities of access
exceptions for a single access are, in descending
order of priorities:
1. Addressing exception
2. Page-access exception
3. Protection exception
The relative priorities of any two conditions can
be found by comparing the priority numbers within
a table from left to right until a mismatch is found.
If the first inequality is between numeric
characters, either the two conditions are mutually
exclusive or, if both can occur, the condition with
the smaller number is indicated. If the first
inequality is between alphabetic characters, then
the two conditions are not exclusive, and it is
unpredictable which is indicated when both occur.