6-24 Vol. 3A

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 overflow 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.