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.
Draw the stacks and explain each stack
To draw the stack and explain each stack, we need to analyze the program segment step by step and track the changes in the stack.
1) Initially, the stack pointer (SP) is moved to 0460h. This means the stack starts from address 0460h and grows downwards.
| |
| ... |
| | <- SP (0460h)
+---------+
2) The program then moves the value 1 into the AL register.
3) The program moves the value 0 into the BX register.
4) The program moves the value 7 into the CX register.
5) The program calls the subroutine SHERIDAN.
| ... |
| ... |
| ... |
| ADDR |
+---------+
| ... |
| BBBBh | <- CX
| ... |
+---------+
6) The program shifts the AL register left by 1 bit.
7) The program outputs the value of AL (1) to the port 0.
8) The program loops back to the label AGAIN if CX is not zero. This creates a loop, repeating steps 6-8 seven times.
| ... |
| ... |
| ... |
| ADDR |
+---------+
| ... |
| XXXX | <- CX, where XXXX is the value of CX at the end of the loop
| ... |
+---------+
9) When the loop ends, the program encounters the INT3 instruction, which triggers a software interrupt.
10) The program jumps to the beginning of the SHERIDAN subroutine.
| ... |
| ... |
| ... |
| ADDR |
+---------+
| SHERIDAN | <- ADDR
| BBBBh | <- CX
| ... |
+---------+
11) The SHERIDAN subroutine pushes the value of CX onto the stack.
| ... |
| ... |
| ... |
| SHERIDAN |
+---------+
| ... |
| BBBBh | <- CX
| ... |
+---------+
12) The SHERIDAN subroutine moves the value AABBh into CX.
| ... |
| ... |
| ... |
| SHERIDAN |
+---------+
| ... |
| AABBh | <- CX
| ... |
+---------+
13) The SHERIDAN subroutine loops back to itself.
14) The SHERIDAN subroutine pops the value of CX from the stack.
| ... |
| ... |
| ... |
| ADDR |
+---------+
15) The SHERIDAN subroutine returns.
At this point, the program execution ends, and the stack will look like:
| ... |
| ... |
| ... |
| |
+---------+
To draw the stack and explain its content for the duration of the program execution, we need to analyze the program step-by-step.
Given program segment:
```
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
```
1. At the start, the SP (stack pointer) is initialized to 0460H. This indicates the top of the stack.
2. MOV AL, 1 - This instruction moves the value 1 into AL register.
3. MOV BX, 0 - This instruction initializes BX register with the value 0.
4. MOV CX, 7 - This instruction initializes CX register with the value 7.
5. AGAIN: CALL SHERIDAN - This instruction calls the subroutine SHERIDAN.
6. PUSH CX - This instruction pushes the current value of CX onto the stack. The stack now contains [7].
7. MOV CX, AABBH - This instruction moves AABBH into CX register.
8. LOOP $ - This instruction decrements CX and jumps to the label '$' (which represents the current location) if CX is not zero.
9. POP CX - This instruction pops the value from the top of the stack, which is 7, and stores it back into CX.
10. RET - This instruction returns control to the calling instruction.
Now let's draw the stack and explain its content for each step:
Step 1:
Stack: [Empty]
Step 2:
Stack: [Empty]
Step 3:
Stack: [Empty]
Step 4:
Stack: [Empty]
Step 5:
Stack: [7]
Step 6:
Stack: [7]
Step 7:
Stack: [7]
Step 8:
Stack: [7]
Step 9:
Stack: [Empty]
Step 10:
Stack: [Empty]
So, the stack remains empty except during the subroutine call to SHERIDAN where the value of CX is pushed onto the stack.