Intel IA-32 - Manual (Page 207)

Vol. 3A 5-31
INTERRUPT AND EXCEPTION HANDLING
Interrupt 4—Overflow Exception (#OF)
Exception Class Trap.
Description
Indicates that an overflow trap occurred when an INTO instruction was executed. The INTO
instruction checks the state of the OF flag in the EFLAGS register. If the OF flag is set, an over-
flow trap is generated.
Some arithmetic instructions (such as the ADD and SUB) perform both signed and unsigned
arithmetic. These instructions set the OF and CF flags in the EFLAGS register to indicate signed
overflow and unsigned overflow, respectively. When performing arithmetic on signed operands,
the OF flag can be tested directly or the INTO instruction can be used. The benefit of using the
INTO instruction is that if the overflow exception is detected, an exception handler can be called
automatically to handle the overflow condition.
Exception Error Code
None.
Saved Instruction Pointer
The saved contents of CS and EIP registers point to the instruction following the INTO
instruction.
Program State Change
Even though the EIP points to the instruction following the INTO instruction, the state of the
program is essentially unchanged because the INTO instruction does not affect any register or
memory locations. The program can thus resume normal execution upon returning from the
overflow exception handler.