An operation exception is recognized when the
code. The operation code may not be assigned, or
the instruction with that operation code may not be
available on the
ing an operation exception, the first eight bits of an
instruction, or, when the first eight bits have the
hexadecimal value B2, the first 16 bits form the oper
ation code.
The operation is suppressed.
The instruction-length code is 1,2, or 3.
Programming Note
In the case of
9D, and
used to distinguish between two instructions. Bits
8-14, however, are not checked for zeros, and these
operation codes never cause an operation exception
to be recognized.
To ensure that presently written programs run if
and when the operation codes 9C, 9D, and 9E are
extended further to provide for new functions, only
zeros should be placed in bit positions 8-14. Similar
ly, zeros should be placed in bit positions 8-15 in the
instruction with the operation code 9F. In accord
ance with these recommendations, the operation
codes for the seven
this manual, such as those provided for emulat.ion or
as part of special or custom features. Consequently,
all unlisted operation codes do not necessarily cause
an operation exception to be recognized. Further
more, as part of the specified operation, these in
structions may cause modes of operation to be set up
or otherwise alter the system so as to affect the ex
ecution of subsequent instructions. In order to avoid
the possibility of accidentally causing such operation,
instructions with an unlisted operation code should
be issued only when the specific function associated
with the operation code is desired.
The operation code
tion
codes
format,
tion of invalid operation is required. It is improbable
that these operation codes will ever be assigned to
an instruction implemented in the
A privileged-operation exception is recognized when
the
problem state.
The operation is suppressed.
76 System/370
Execute Exception
The execute exception is recognized when the sub
ject instruction of EXECUTE is another
The instruction-length code is 2.
Protection Exception
A protection exception is recognized when the
protected against the type of reference, and the key
in storage associated with the location does not
match the protection key in the
when the location of the instruction, including the
location of the subject instruction of EXECUTE, is
protected against fetching. Except for some specific
instructions whose execution is suppressed, the oper
ation is terminated when a protection exception is
encountered during a reference to an operand loca
tion.
action taken on a protection exception.
loaded into an addressable register or moved to an
other storage location. When part of an operand
location is protected against storing and part is not,
storing may be performed in the unprotected part.
The contents of a protected location remain un
changed.
For a protected operand location, the instruction
length code is 1, 2, or 3, designating the length of
the instruction that caused the reference. However,
for a store-protected operand location, the
instruction-length code on some models may be
protected against fetching, the instruction-length
code is 1, 2, or 3, indicating the number of halfwords
by which the instruction address has been incre
mented. It is unpredictable whether the code is 1, 2,
or 3.
Addressing Exception
An addressing exception is recognized when the
that is not available to the
location is not available to the
tion is not provided, when the storage unit is not
configured to the
storage unit. An address designating an unavailable
storage location is referred to as invalid.
The execution of the instruction is suppressed
when the address of the instruction, including the
location of the subject instruction of EXECUTE, is