Technology & Computing

What is test in assembly?

By: Nick HunterUpdated: December 18, 2020

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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. There are 9 different opcodes for the TEST instruction depending on the type and size of the operands.

Just so, what is Movzbl?

1 Answer. Specifically, the movzbl instruction fetches the byte stored at the sum of the two parameters, zero pads it, and stores it into eax . The movsbl instruction takes the lowest byte of eax, sign extends it, and stores the result back in eax.

Secondly, what does test EAX EAX do?

The test eax, eax is necessary to make the jne work in the first place. And jne is the same as jnz , just as je is the same as jz . Both act based on the ZF (zero-flag) value. The jne branch will be taken if ZF=0 and therefore whenever strcmp returns a non-zero value (i.e. strings not equal).

What is the difference between and & test instruction?

3.4. 6 Test Instruction. TEST (Test) performs the logical "and" of the two operands, clears OF and CF, leaves AF undefined, and updates SF, ZF, and PF. The difference between TEST and AND is that TEST does not alter the destination operand.

What will happen if you do XOR EAX EAX?

xor eax, eax is a faster way of setting eax to zero. This is happening because you're returning zero. The in instruction is doing stuff with I/O ports. Basically reading a word of data from the port specified dx in and storing it in al .

Related

What does not do in assembly?

The NOT instruction implements the bitwise NOT operation. NOT operation reverses the bits in an operand. The operand could be either in a register or in the memory.

What is JS Assembly?

Description. The js (or jne) instruction is a conditional jump that follows a test. It jumps to the specified location if the previous instruction sets the Sign Flag (SF).

What is logical instruction?

Logical instructions are the instructions which perform basic logical operations such as AND, OR, etc. In 8085 microprocessor, the destination operand is always the accumulator. Here logical operation works on a bitwise level.

How does CMP work in assembly?

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

How do you check if a number is even or odd in assembly language?

If you want to check whether a given number is odd or even, a simple test would be to check the least significant bit of the number. If this is 1, the number is odd, else the number is even.

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 does LEAQ do in assembly?

The LEA (Load Effective Address) instruction is a way of obtaining the address which arises from any of the Intel processor's memory addressing modes. it moves the contents of the designated memory location into the target register.

How do I remove bits in assembly language?

You can clear bits by using the AND -operation. So you can use this operation to mask and flip bit regions in a register. Pass in a constant as the second argument which has bits flipped up you want to keep up. AL is the lowest byte portion of EAX -register so you can do it this way.

What is JG Assembly?

Description. The jg instruction is a conditional jump that follows a test. It performs a signed comparison jump after a cmp if the destination operand is greater than the source operand.

What is an assembly instruction?

assembly instructions. The instructions included with a product to show visually and with words and text how to assemble the product. Assembly instructions are a vital part of a product that requires self assembly e.g. self assembly furniture.

What does zero extend mean?

In a move or convert operation, zero extension refers to setting the high bits of the destination to zero, rather than setting them to a copy of the most significant bit of the source.