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4–70 Alpha Architecture Handbook
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 generates 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 generates 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.