When /U or /V mode is specified:

Arithmetic is performed on VAX finite numbers.

Operations give imprecise traps whenever the following occur:

an operand is a non-finite number

an underflow

an integer overflow

a floating overflow

a divide-by-zero

Traps are imprecise and it is not always possible to determine which instruction trig- gered a trap or the operands of that instruction.

An underflow trap produces a zero result.

A conversion to integer trapping with an integer overflow produces the low-order bits of the integer value.

The result of any other operation that traps is UNPREDICTABLE.

When /S mode is specified:

Arithmetic is performed on all VAX values, both finite and non-finite.

A VAX dirty zero is treated as zero.

Exceptions are signaled for:

a VAX reserved operand, which gen erates an invalid operation exception

a floating overflow

a divide-by-zero

Exceptions are precise and an application can locate the instruction that caused the exception, along with its operand values. See Section 4.7.7.3.

An operation that underflows produces a zero result without taking an exception.

A conversion to integer that overflows uses the low-order bits of the integer as the result, without taking an exception.

When an operation takes an exception, the result of the operation is UNPREDICT- ABLE.

When /SU or /SV mode is specified:

Arithmetic is performed on all VAX values, both finite and non-finite.

A VAX dirty zero is treated as zero.

Exceptions are signaled for:

a VAX reserved operand, which gen erates an invalid operation exception

an underflow

an integer overflow

a floating overflow

a divide-by-zero

Exceptions are precise and an application can locate the instruction that caused the exception, along with its operand values. See Section 4.7.7.3.

An underflow exception produces a zero.

A conversion to integer exception with integer overflow produces the low-order bits of the integer value.

The result of any other operation that takes an exception is UNPREDICTABLE.

4–70Alpha Architecture Handbook

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Compaq ECQD2KCTE manual When /U or /V mode is specified