What is assembly testing?

In the x86 assembly language, the TEST instruction performs a bitwise AND on two operands. The flags SF , ZF , PF are modified while the result of the AND is discarded. The OF and CF flags are set to 0 , while AF flag is undefined.

What is the difference between test and CMP?

This is a reasonable question to ask, because cmp is an arithmetic operation, (it performs a subtraction and discards the result,) while test is a logical operation, (it performs a bitwise AND and discards the result,) so one could reasonably suspect that they may modify the Flags register differently.

What does CMP do assembly?

The CMP instruction compares two operands. It is generally used in conditional execution. This instruction basically subtracts one operand from the other for comparing whether the operands are equal or not. It is used along with the conditional jump instruction for decision making.

What does or EAX EAX do?

EAX is the register used by IA32 calling conventions to either return an interger value or a memory address to the calling routine. By design, strcpy can return either -1,0 or 1 in EAX with 0 indicating both strings match. TEST EAX,EAX tests whether EAX is zero or not and sets or unsets the ZF bit.

What does test %Al %Al do?

The test al,al is a bitwise AND operation between al and itself. If al & al == 0, the zero flag will be set to 1. je (or jz ) instruction will jump to the address of label , if the zero flag is 1. Otherwise, the je will do nothing.

What does JNE do in assembly?

The jnz (or jne) instruction is a conditional jump that follows a test. It jumps to the specified location if the Zero Flag (ZF) is cleared (0). jnz is commonly used to explicitly test for something not being equal to zero whereas jne is commonly found after a cmp instruction.

What is Assembly condition codes?

Condition codes are extra bits kept by a processor that summarize the results of an operation and that affect the execution of later instructions. These bits are often collected together in a single condition or indicator register (CR/IR) or grouped with other status bits into a status register (PSW/PSR).

What does je do in assembly?

A conditional jump instruction, like “je” (jump-if-equal), does a goto somewhere if the two values satisfy the right condition. For example, if the values are equal, subtracting them results in zero, so “je” is the same as “jz”.

What does JG do in assembly?

The command JG simply means: Jump if Greater.

TEST leaves it undefined, but CMP sets it “according to the result”. Since subtracting zero can’t produce a carry from the 4th to 5th bit, CMP should always clear AF. TEST is smaller (no immediate) and sometimes faster (can macro-fuse into a compare-and-branch uop on more CPUs in more cases than CMP).

What is the difference between CMP EAX and test instruction?

BTW, this generates a smaller instruction than cmp eax, 0 which is the reason that compilers will generally do it this way. The test instruction does a logical AND-operation between the operands but does not write the result back into a register. Only the flags are updated.

What is the difference between CMP and Je in disassemblers?

Disassemblers (and typically asm output from compilers) will only ever use one, so the semantic distinction is lost. cmp and sub set ZF when their two inputs are equal (i.e. the subtraction result is 0). je (jump if equal) is the semantically relevant synonym.

What is the use of CMP instruction?

CMP instruction sets status flags according to the comparisons between the arguments. See : wikipedia’s FLAGS page The importance of CMP applies mostly in conditional code execution (Jump – See : assembly_conditions ).