1. Consider the following program segment given below:
ORG 0x0400
Address
0400h MOV SP, 0460H
MOV AL, 1
MOV BX, 0
MOV CX, 7
AGAIN: CALL SHERIDAN
0420H SHL AL, 1
OUT 0, AL
LOOP AGAIN
INT3
SHERIDAN:
PUSH CX
MOV CX, AABBH
LOOP $
POP CX
RET
In executing the above program, the Microprocessor is using the stack. In that respect, draw the stack along with its content for the duration of the program execution.
During the program execution, the stack is primarily used for storing variables and return addresses.
Initially, the stack pointer (SP) is set to 0460h. The stack grows downwards, so the top of the stack starts at address 0460h.
At the beginning of the program, the stack is empty.
Stack Contents:
SP (0460h): Empty
As the program executes, the following changes occur to the stack:
1. Before calling the SHERIDAN subroutine:
- PUSH CX: Pushes the CX register value onto the stack.
- Stack Contents:
SP (045Eh): CX
2. Inside the SHERIDAN subroutine:
- MOV CX, AABBH: Loads AABBH value into the CX register.
- LOOP $: Loops back to the SHERIDAN label until CX becomes zero.
- As the loop executes, the CX value is decremented, and the return address is pushed onto the stack.
- Stack Contents (after each loop iteration):
SP (045Ch): Return Address (Address where LOOP $ is executed)
SP (045Ah): Return Address (Address where LOOP $ is executed)
...
SP (0440h): Return Address (Address where LOOP $ is executed)
SP (043Eh): Return Address (Address where LOOP $ is executed)
- Once the CX becomes zero, the LOOP $ instruction is not executed anymore, and the CX value is restored using POP CX.
3. After returning from the SHERIDAN subroutine:
- POP CX: Pops the return address from the stack and restores the CX register value.
- Stack Contents:
SP (0440h): Return Address (Address where LOOP $ is executed)
SP (043Eh): Return Address (Address where LOOP $ is executed)
...
SP (045Ah): Return Address (Address where LOOP $ is executed)
SP (045Eh): Empty
4. The program execution continues:
- SHL AL, 1: Shifts the AL register value to the left by 1.
- OUT 0, AL: Outputs the AL register value to port 0.
- LOOP AGAIN: Loops back to the AGAIN label until CX becomes zero.
- As the loop executes, the CX value is decremented, and the return address is pushed onto the stack.
- Stack Contents (after each loop iteration):
SP (043Ch): Return Address (Address where LOOP AGAIN is executed)
SP (043Ah): Return Address (Address where LOOP AGAIN is executed)
...
SP (0420h): Return Address (Address where LOOP AGAIN is executed)
SP (041Eh): Return Address (Address where LOOP AGAIN is executed)
- Once the CX becomes zero, the LOOP AGAIN instruction is not executed anymore.
5. After the LOOP AGAIN instruction:
- INT3: Generates a software breakpoint interrupt.
- Stack Contents:
SP (0420h): Return Address (Address where LOOP AGAIN is executed)
SP (041Eh): Return Address (Address where LOOP AGAIN is executed)
...
SP (043Ah): Return Address (Address where LOOP AGAIN is executed)
SP (043Ch): Return Address (Address where LOOP AGAIN is executed)
At the end of the program, the stack still contains the return addresses pushed during the SHERIDAN and AGAIN loops.
can you draw your answer please
Certainly! Here is a visual representation of the stack along with its content for the duration of the program execution:
```
| Higher Memory Addresses |
| |
| ... |
| Return Address 1 | <-- SP (0460h)
| Return Address 2 |
| AABBH |
| Return Address 3 |
| . |
| . |
| Return Address N-2 |
| Return Address N-1 |
| Lower Memory Addresses |
```
Note: The stack grows downwards, so the top of the stack is located at the higher memory address (0460h in this case). The content of the stack shown above is based on the description provided in the previous answer.
To draw the stack along with its content during the program execution, we need to consider the instructions that manipulate the stack pointer (SP), as well as the instructions that push or pop values onto or from the stack.
Here's how the stack would look like for the given program segment:
Initial Stack:
--------------
Empty
Execution Steps:
----------------
1. MOV SP, 0460H (Moves the stack pointer, SP, to address 0460H)
Stack after MOV SP:
---------------------
Address | Content
---------------------
0460h | Empty
2. MOV AL, 1 (Moves the value 1 into register AL)
Stack after MOV AL:
---------------------
Address | Content
---------------------
0460h | Empty
3. MOV BX, 0 (Moves the value 0 into register BX)
Stack after MOV BX:
---------------------
Address | Content
---------------------
0460h | Empty
4. MOV CX, 7 (Moves the value 7 into register CX)
Stack after MOV CX:
---------------------
Address | Content
---------------------
0460h | Empty
5. CALL SHERIDAN (Calls the SHERIDAN subroutine)
Stack after CALL SHERIDAN:
---------------------
Address | Content
---------------------
0460h | Return Address (040Bh) <- SP
-------------
040Bh | Empty
6. SHL AL, 1 (Shifts the value in register AL to the left by 1 bit)
Stack after SHL AL:
---------------------
Address | Content
---------------------
0460h | Return Address (040Bh) <- SP
-------------
040Bh | Empty
7. OUT 0, AL (Outputs the value in register AL to port 0)
Stack after OUT 0, AL:
---------------------
Address | Content
---------------------
0460h | Return Address (040Bh) <- SP
-------------
040Bh | Empty
8. LOOP AGAIN (Decrements the CX register by 1 and jumps to the label "AGAIN" if CX is not zero)
Stack after LOOP AGAIN:
---------------------
Address | Content
---------------------
0460h | Return Address (040Bh) <- SP
-------------
040Bh | CX (6)
9. CALL SHERIDAN (Calls the SHERIDAN subroutine again)
Stack after CALL SHERIDAN:
---------------------
Address | Content
---------------------
0460h | Return Address (040Bh) <- SP
-------------
040Bh | CX (6)
-------------
040Bh | Return Address (0412h)
10. (Inside SHERIDAN subroutine)
PUSH CX (Pushes the value of CX onto the stack)
Stack after PUSH CX:
---------------------
Address | Content
---------------------
0460h | Return Address (040Bh) <- SP
-------------
040Bh | CX (6)
-------------
040Ch | CX (6)
11. MOV CX, AABBH (Moves the value AABBH into register CX)
Stack after MOV CX:
---------------------
Address | Content
---------------------
0460h | Return Address (040Bh) <- SP
-------------
040Bh | CX (6)
-------------
040Ch | CX (6)
12. LOOP $ (Decrements the CX register by 1 and jumps back to the start of the LOOP instruction if CX is not zero)
Stack after LOOP $:
---------------------
Address | Content
---------------------
0460h | Return Address (040Bh) <- SP
-------------
040Bh | CX (5)
-------------
040Ch | CX (6)
13. POP CX (Pops the value from the stack into register CX)
Stack after POP CX:
---------------------
Address | Content
---------------------
0460h | Return Address (040Bh) <- SP
-------------
040Bh | CX (5)
-------------
040Ch | Empty
14. RET (Returns from the subroutine)
Stack after RET:
---------------------
Address | Content
---------------------
0460h | Return Address (040Bh) <- SP
-------------
040Bh | Empty
-------------
040Ch | Empty
15. (Back to the main program)
LOOP AGAIN (Decrement CX by 1 and loop to label "AGAIN" if CX is not zero)
Stack after LOOP AGAIN:
---------------------
Address | Content
---------------------
0460h | Return Address (040Bh) <- SP
-------------
040Bh | CX (4)
-------------
040Ch | Empty
The program execution continues in a similar fashion until the end.