ASM Instructions

Question 1

MOV

Answer 1

Instruction copies a data item from one location to another.

Question 2

ADD/SUB op1, op2

Answer 2

Add or subtract two operands, storing the result in the first operand. These can be registers, memory locations (limit of one) or constants. For example, ADD EAX, 10 means add 10 to the value of EAX and store the result in EAX

Question 3

XOR EAX, EAX

Answer 3

Performing an ‘exclusive or’ of a register with itself sets its value to zero; an easy way of clearing the contents of a register

Question 4

INC/DEC op1

Answer 4

Increment or decrement the value of the operand by one

Question 5

CMP op1, op2

Answer 5

Compare the value of two operands (register/memory address/constant) and set the appropriate EFLAGS value.

Question 6

Jump (JMP) and conditional jump (je, jz, etc)

Answer 6

As the name implies these instructions allow you to jump to another location in the execution flow/instruction set. The JMP instruction simply jumps to a location whereas the conditional jumps (je, jz, etc) are taken only if certain criteria are met (using the EFLAGS register values mentioned earlier). For example, you might compare the values of two registers and jump to a location if they are both equal (uses je instruction and zero flag (zf) = 1).

Question 7

ADD DWORD PTR [X] or MOV eax, [ebx]

Answer 7

When you see a value in brackets it is referring to the value stored at memory address X. In other words, EBX refers to the contents of EBX whereas [EBX] refers to the value stored at the memory address in EBX.

Question 8

Relevant size keywords: BYTE = _ byte, WORD = _ bytes, DWORD = _ bytes.

Answer 8

1,2,4