Instruction Descriptions 4–71
A summary of the VAX trapping modes, instruction notation, and their meaning follows in
Table 4–8:
4.7.7.2 IEEE Trapping Modes
This section describes the characteristics of the four IEEE trapping modes, which are summa-
rized in Table 4–9.
When no trap mode is specified (the default):
• Arithmetic is performed on IEEE finite numbers.
• Operations give imprecise traps whenever the following occur:
– an operand is a non-finite number
– a floating overflow
– a divide-by-zero
– an invalid operation
• 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 produces a zero result without trapping.
• A conversion to integer that overflows uses the low-order bits of the integer as the
result without trapping.
• When an operation traps, the result of the operation is UNPREDICTABLE.
When /U or /V mode is specified :
• Arithmetic is performed on IEEE 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
– an invalid operation
Table 4–8: VAX Trapping Modes Summary
Trap Mode Notation Meaning
Underflow disabled No qualifier
/S
Imprecise
Precise exception completion
Underflow enabled /U
/SU
Imprecise
Precise exception completion
Integer overflow disabled No qualifier
/S
Imprecise
Precise exception completion
Integer overflow enabled /V
/SV
Imprecise
Precise exception completion