Question

Draw the stack (use, word/pdf) before every instruction that is marked red is executed to show your understanding of the call and return functions.   Use N/A to represent unpredictable values.

            
            Main Proc
            4040018          mov ecx, 0000000Ch
            404001C          mov ebx, 0000000Bh
            4040020          call FADD
            4040026          mov eax, ebx
                      …
                      …                    

            Main EndP

            FADD  PROC
            4041040          Push ecx
            4041044          Push ebx
            4041048          mov eax, edx
                      …
                      …
            404A060         Pop ebx
            404A062         Pop ecx
            404A064         ret
            FADD EndP

Answer 1

1). ANSWER :

GIVENTHAT :

Main program start at location 4040018

1)CALL FADD

Call sub procedure at location 4040020

The CALL instruction is used whenever we need to make a call to some procedure or a subprogram. Whenever a CALL is made, the following process takes place inside the microprocessor:

• The address of the next instruction that exists in the caller program (after the program CALL instruction) is stored in the stack.
• The instruction queue is emptied for accommodating the instructions of the procedure.
• Then, the contents of the instruction pointer (IP) is changed with the address of the first instruction of the procedure.
• The subsequent instructions of the procedure are stored in the instruction queue for execution.

The Syntax for the CALL instruction is as follows:

    CALL subprogram_name

FADD/FADDP/FIADD — Add

Adds the destination and source operands and stores the sum in the destination location. The destination operand is always an FPU register; the source operand can be a register or a memory location. Source operands in memory can be in single-precision or double-precision floating-point format or in word or doubleword integer format.

• FADD, given one operand, adds the operand to ST0 and stores the result back in ST0. If the operand has the TO modifier, the result is stored in the register given rather than in ST0.

FADDP performs the same function as FADD TO, but pops the register stack after storing the result.

2)move eax,ebx

Mov instruction:

· mov — Move (Opcodes: 88, 89, 8A, 8B, 8C, 8E, ...)

· The mov instruction copies the data item referred to by its second operand (i.e. register contents, memory contents, or a constant value) into the location referred to by its first operand (i.e. a register or memory). While register-to-register moves are possible, direct memory-to-memory moves are not. In cases where memory transfers are desired, the source memory contents must first be loaded into a register, then can be stored to the destination memory address.

Syntax
mov <reg>,<reg>
mov <reg>,<mem>
after execution of this instruction copy the value in ebx into eax

ebx=0000000bh

After execution eax=0000000bh

3)push ecx

ecx contain 0000000Ch

PUSH - stores 16 bit value in the stack.

Syntax for PUSH instruction:

PUSH REG
• A push operation decrements the stack pointer by 2 or 4 (depending on operands) and copies a value into the location pointed to by the stack pointer.

After execution of push ecx

contain 0000000Ch goes to location 4041040

4) push ebx

ebx contain 0000000Bh

After execution of push ebx

contain 0000000Bh goes to location 4041044

5)move eax,edx

after execution of this instruction copy the value in edx into eax

edx is N/A

6)POP ebx

POP - gets 16 bit value from the stack.

Syntax for POP instruction:

POP REG

• Store original value of the register in stack (using PUSH).
• Use the register for any purpose.
• Restore the original value of the register from stack (using POP).

· pop — Pop stack

· The pop instruction removes the 4-byte data element from the top of the hardware-supported stack into the specified operand (i.e. register or memory location). It first moves the 4 bytes located at memory location [SP] into the specified register or memory location, and then increments SP by 4.

POP ebx:

After execution of POP ebx pop contain 0000000Bh at location 4041044

    7)POP ecx

After execution of POP ecx ,pop contain 0000000Ch at location 4041040

8)RET

The RET instruction stands for return. This instruction is used at the end of the procedures or the subprograms. This instruction transfers the execution to the caller program. Whenever the RET instruction is called, the following process takes place inside the microprocessor:

• The address of the next instruction in the mainline program which was previously stored inside the stack is now again fetched and is placed inside the instruction pointer (IP).
• The instruction queue will now again be filled with the subsequent instructions of the mainline program.

The Syntax for the RET instruction is as follows:

            RET

After execution of RET instruction The address of the next instruction in the mainline program that is 4040026 which was previously stored inside the stack is now again fetched and is placed inside the instruction pointer (IP).