Question

If this program below has an initial Stack Pointer value of (ESP=2000 2090h), what is the value of the Stack Pointer (ESP) after the last instruction (pop ax) is finished.

     mov bx, 09h

     mov ax, 0h   

     push bx

     pop ax   

Question 3 options:

	ESP = 2000 2094h
	ESP = 2000 2090h
	ESP = 2000 2092h
	ESP = 2000 208Eh

Answer 1

EXPLANATION:

• mov instruction is used for moving the date from source to destination, in which a source could be a register, immediate value or a register containing memory location, and destination could be a register. In the provided assembly program, the source is the immediate value(09h and 0h) and the destination is a register(bx and ax)
• push instruction is used for transferring the two bytes of data from the register to the stack memory. push instruction first decrements the Stack Pointer(ESP) value by 2 bytes and then transfers the data.
• pop instruction performs the opposite operation of push instruction, it fetches the 2 bytes of data from stack memory locations and stores it in the register, and then it increments the Stack Pointer(ESP) value by 2 bytes.

INSTRUCTIONS:

mov bx, 09h ; Moving the 09h value in b register ,  (ESP=2000 2090h)

     mov ax, 0h ; Moving the 0h value in a register ,  (ESP=2000 2090h)

     push bx ; Pushing b register value in Stack Pointer , (ESP=2000 208Eh)

     pop ax ; Popping the 2 bytes values from Stack to a register,  (ESP=2000 2090h)

ANSWER:

Hence the value of the Stack Pointer (ESP) after the last instruction (pop ax) is finished will be ESP=2000 2090h