The zero flag is a single bit flag that is a central feature on most conventional
CPU architectures (including
x86
x86 (also known as 80x86 or the 8086 family) is a family of complex instruction set computer (CISC) instruction set architectures initially developed by Intel based on the Intel 8086 microprocessor and its 8088 variant. The 8086 was intr ...
,
ARM
In human anatomy, the arm refers to the upper limb in common usage, although academically the term specifically means the upper arm between the glenohumeral joint (shoulder joint) and the elbow joint. The distal part of the upper limb between th ...
,
PDP-11,
68000
The Motorola 68000 (sometimes shortened to Motorola 68k or m68k and usually pronounced "sixty-eight-thousand") is a 16/32-bit complex instruction set computer (CISC) microprocessor, introduced in 1979 by Motorola Semiconductor Products Secto ...
,
6502
The MOS Technology 6502 (typically pronounced "sixty-five-oh-two" or "six-five-oh-two") William Mensch and the moderator both pronounce the 6502 microprocessor as ''"sixty-five-oh-two"''. is an 8-bit microprocessor that was designed by a small te ...
, and numerous others). It is often stored in a dedicated register, typically called
status register
A status register, flag register, or condition code register (CCR) is a collection of status flag bits for a processor. Examples of such registers include FLAGS register in the x86 architecture, flags in the program status word (PSW) register in ...
or
flag register, along with other flags. The zero flag is typically abbreviated Z or ZF or similar in most documentation and assembly languages.
Along with a
carry flag
In computer processors the carry flag (usually indicated as the C flag) is a single bit in a system status register/flag register used to indicate when an arithmetic carry or borrow has been generated out of the most significant arithmetic l ...
, a
sign flag
In a computer central processing unit, processor the negative flag or sign flag is a single bit in a system status (flag) register used to indicate whether the result of the last mathematical operation produced a value in which the most significant ...
and an
overflow flag
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 r ...
, the zero flag is used to check the result of an arithmetic operation, including bitwise logical
instructions. It is set to 1, or true, if an arithmetic result is zero, and reset otherwise. This includes results which are not stored, as most traditional instruction sets implement the
compare instruction as a subtract where the result is discarded. It is also common that processors have a bitwise AND-instruction that does not store the result.
The logical formula of the zero flag for a twos-complement binary operand is NOT(OR(all bits of the operand in question)).
In most processors, the zero flag is mainly used in conditional
branch
A branch, sometimes called a ramus in botany, is a woody structural member connected to the central trunk of a tree (or sometimes a shrub). Large branches are known as boughs and small branches are known as twigs. The term ''twig'' usually ...
instructions, which alter control flow on previous instruction results, but there are often other uses as well.
In some instruction sets such as the
MIPS architecture
MIPS (Microprocessor without Interlocked Pipelined Stages) is a family of reduced instruction set computer (RISC) instruction set architectures (ISA)Price, Charles (September 1995). ''MIPS IV Instruction Set'' (Revision 3.2), MIPS Technologies, ...
, a dedicated flag register is not used; jump instructions instead check a register for zero.
{{X86 assembly topics
Computer arithmetic