In the

x86 assembly language x86 assembly language is a family of Low-level programming language, low-level programming languages that are used to produce object code for the x86 class of processors. These languages provide backward compatibility with CPUs dating back to th ...

, the TEST instruction performs a

bitwise AND In computer programming, a bitwise operation operates on a bit string, a bit array or a binary numeral (considered as a bit string) at the level of its individual bits. It is a fast and simple action, basic to the higher-level arithmetic operat ...

on two

operand In mathematics, an operand is the object of a mathematical operation, i.e., it is the object or quantity that is operated on. Unknown operands in equalities of expressions can be found by equation solving. Example The following arithmetic expres ...

s. The

flag A flag is a piece of textile, fabric (most often rectangular) with distinctive colours and design. It is used as a symbol, a signalling device, or for decoration. The term ''flag'' is also used to refer to the graphic design employed, and fla ...

s SF, ZF, PF are modified while the numerical result of the

AND And or AND may refer to: Logic, grammar and computing * Conjunction, connecting two words, phrases, or clauses * Logical conjunction in mathematical logic, notated as "∧", "⋅", "&", or simple juxtaposition * Bitwise AND, a Boolean oper ...

is discarded. The OF and CF flags are set to 0, while AF flag is undefined. There are 9 different

opcode In computing, an opcode (abbreviated from operation code) is an enumerated value that specifies the operation to be performed. Opcodes are employed in hardware devices such as arithmetic logic units (ALUs), central processing units (CPUs), and ...

s for the TEST instruction depending on the type and size of the

s. It can test 8-bit, 16-bit, 32-bit, or 64-bit values. It can also test registers and memory against registers and immediate values.

TEST opcode variations

The TEST operation clears the flags CF and OF to zero. The SF is set to the

most significant bit In computing, bit numbering is the convention used to identify the bit positions in a binary numeral system, binary number. Bit significance and indexing In computing, the least significant bit (LSb) is the bit position in a Binary numeral sy ...

of the result of the

. If the result is 0, the ZF is set to 1, otherwise set to 0. The parity flag is set to the

bitwise In computer programming, a bitwise operation operates on a bit string, a bit array or a binary numeral (considered as a bit string) at the level of its individual bits. It is a fast and simple action, basic to the higher-level arithmetic operat ...

XNOR The XNOR gate (sometimes ENOR, EXNOR, NXOR, XAND and pronounced as exclusive NOR) is a digital logic gate whose function is the logical complement of the exclusive OR ( XOR) gate. It is equivalent to the logical connective (\leftrightarrow) fr ...

of the

least significant byte In computing, bit numbering is the convention used to identify the bit positions in a binary number. Bit significance and indexing In computing, the least significant bit (LSb) is the bit position in a binary integer representing the lowes ...

of the result, 1 if the number of ones in that byte is even, 0 otherwise. The value of AF is undefined.

Examples

; Conditional Jump test cl,cl ; set ZF to 1 if cl

0 jz 0x8004f430 ; jump if ZF

1 ; Conditional Jump with NOT test cl, cl ; set ZF to 1 if cl

0 jnz 0x8004f430 ; jump if ZF

0 ; or test eax, eax ; set SF to 1 if eax < 0 (negative) js error ; jump if SF

1 ; regular application test al, $0F ; set ZF if "al AND $0f = 0" (here: address-align test for 16b) jnz @destination ; jump if eax IS NOT "MODULO 16=0"

References

X86 instructions {{software-stub