XUFLOW

Derivation: eXponent Under FLOW

XUFLOW specifies if an exponent underflow causes a program interrupt. An exponent underflow occurs when a floating point number becomes too small to be represented. The underflow setting is determined at enclave initialization and is updated when new languages are introduced into the application (via fetch or dynamic call, for example). Otherwise, it does not vary while the application is running.

Language Environment preserves the language semantics for C/C++ and COBOL regardless of the XUFLOW setting. Language Environment preserves the language semantics for PL/I only when XUFLOW is set to AUTO or ON. Language Environment does not preserve the language semantics for PL/I when XUFLOW is set to OFF.

An exponent underflow caused by a C/C++ or COBOL program does not cause a condition to be raised.

The default value for non-CICS applications is XUFLOW(AUTO).

The default value for CICS® applications is XUFLOW(AUTO).


Syntax

              .-AUTO-.      
>>-Xuflow--(--+------+--)--------------------------------------><
              +-ON---+      
              '-OFF--'

AUTO: An exponent underflow causes or does not cause a program interrupt dynamically, based on the HLLs that make up the application. Enablement is determined without user intervention. XUFLOW(AUTO) causes condition management to process underflows only in those applications where the semantics of the application languages require it. Normally, XUFLOW(AUTO) provides the best efficiency while meeting language semantics.
ON: An exponent underflow causes a program interrupt. XUFLOW(ON) causes condition management to process underflows regardless of the mix of languages; therefore, this setting might be less efficient in applications that consist of languages not requiring underflows to be processed by condition management.
OFF: An exponent underflow does not cause a program interrupt; the hardware takes care of the underflow. When you set XUFLOW to OFF, the hardware processes exponent underflows. This is more efficient than condition handling to process the underflow.

Usage notes

PL/I consideration—When setting XUFLOW to OFF, be aware that the semantics of PL/I require the underflow to be signaled.
z/OS UNIX consideration—The XUFLOW option applies to the entire enclave and all threads within.