Processor flag indicating whether signed arithmetic overflow has occurred
This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. Find sources: "Overflow flag" – news · newspapers · books · scholar · JSTOR(January 2008) (Learn how and when to remove this message)
In computer processors, the overflow flag (sometimes called the V flag) is usually a single bit in a system status register used to indicate when an arithmetic overflow has occurred in an operation, indicating that the signed two's-complement result would not fit in the number of bits used for the result. Some architectures may be configured to automatically generate an exception on an operation resulting in overflow.
An example, suppose we add 127 and 127 using 8-bit registers. 127+127 is 254, but using 8-bit arithmetic the result would be 1111 1110 binary, which is the two's complement encoding of −2, a negative number. A negative sum of positive operands (or vice versa) is an overflow. The overflow flag would then be set so the program can be aware of the problem and mitigate this or signal an error. The overflow flag is thus set when the most significant bit (here considered the sign bit) is changed by adding two numbers with the same sign (or subtracting two numbers with opposite signs). Overflow cannot occur when the sign of two addition operands are different (or the sign of two subtraction operands are the same).[1]
When binary values are interpreted as unsigned numbers, the overflow flag is meaningless and normally ignored. One of the advantages of two's complement arithmetic is that the addition and subtraction operations do not need to distinguish between signed and unsigned operands. For this reason, most computer instruction sets do not distinguish between signed and unsigned operands, generating both (signed) overflow and (unsigned) carry flags on every operation, and leaving it to following instructions to pay attention to whichever one is of interest.[2]
Internally, the overflow flag is usually generated by an exclusive or of the internal carry into and out of the sign bit.
Bitwise operations (and, or, xor, not, rotate) do not have a notion of signed overflow, so the defined value varies on different processor architectures. Some processors clear the bit unconditionally (which is useful because bitwise operations set the sign flag, and the clear overflow flag then indicates that the sign flag is valid), others leave it unchanged, and some set it to an undefined value. Shifts and multiplies do permit a well-defined value, but it is not consistently implemented. For example, the x86 instruction set only defines the overflow flag for multiplies and 1-bit shifts; multi-bit shifts leave it undefined.
^Kholodov, Igor (1 November 2008). "Overflow Detection: Signed Numbers Addition". CIS77 Introduction to Computer Systems (course notes). Bristol Community College. Retrieved 2020-12-30.
^Allen, Ian D. (25 February 2011). "The CARRY flag and OVERFLOW flag in binary arithmetic". DAT 2343 Computer Systems Architecture (course notes). Algonquin College.
the overflowflag (sometimes called the V flag) is usually a single bit in a system status register used to indicate when an arithmetic overflow has occurred...
contents of this flag to modify a register or a memory location that contains the higher part of a multi-word value. The overflowflag is set when the...
instructions. Additional status flags may bypass memory mapping and define what action the CPU should take on arithmetic overflow. The carry, parity, auxiliary...
addition cannot be represented in the destination an arithmetic overflow is set. The flags can be used to decide subsequent operations, such as conditional...
documentation and assembly languages. Along with a carry flag, a sign flag and an overflowflag, the zero flag is used to check the result of an arithmetic operation...
other without risk of overflow, so the situation stabilizes at five instructions per word added. Control register CPU flag (x86) Flag field "ARM Information...
Buffer overflow protection is any of various techniques used during software development to enhance the security of executable programs by detecting buffer...
subtraction, and no overflowflag. The Z80 complemented this with the ADC HL,rr and SBC HL,rr instructions, which sets the new overflowflag accordingly. (The...
(i.e., it is larger than the ALU's output word size), an arithmetic overflowflag will be set, influencing the next operation. Hardwired into a CPU's...
instructions usually set different CPU flags such as the carry flag for unsigned arithmetic and the overflowflag for signed. Those values can be taken...
differences between the behavior of the two cores with regard to an undefined overflowflag in certain bitwise operations. Alder Lake's CPU topology has performance...
In software, a stack buffer overflow or stack buffer overrun occurs when a program writes to a memory address on the program's call stack outside of the...
the lions of the flag of the Seigneur of Sark in the original grant protrude outside the canton and overflow the red cross. The flag was also referred...
Auxiliary carry flag (AF), Zero flag (ZF), Sign flag (SF), Trap flag (TF), Interrupt flag (IF), Direction flag (DF), and Overflowflag (OF). Also referred...
flag, where ADC as a signed addition may set both overflow and carry flags, and as an old-style x86 instruction also reset the rest of the CPU flags....
Virginia, a U.S. state. The state flag of Virginia consists of the obverse of the seal against a blue background. A state flag was first adopted at the beginning...
certain combinations were supported. The FLAGS register contains flags such as carry flag, overflowflag and zero flag. Finally, the instruction pointer (IP)...
instruction always takes multiple clock cycles. On the x86, which provides overflowflags and conditional moves, very simple branch-free code is possible. Although...
flag. Set to 1 when an arithmetic result is zero. N Negative flag. Set to a copy of the most significant bit of an arithmetic result. V Overflowflag...
registers (X and Y), 7 processor status flag bits (P; from bit 7 to bit 0 these are the negative (N), overflow (V), reserved, break (B), decimal (D), interrupt...
Global Information