42. Identify the condition under which the contents of AX would remain unchanged after execution of the instructions that follow. MOV SHL SHR CL, 4 AX, CL AX, CL

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**Question 42: Analysis of Assembly Language Instructions**

**Objective:** Determine the condition under which the contents of the AX register remain unchanged after executing the following assembly instructions.

**Instructions:**

1. **MOV CL, 4**  
   This instruction moves the value 4 into the CL register. CL is commonly used in various CPU instructions due to its role as a counter in shift and rotate operations.

2. **SHL AX, CL**  
   The Shift Left (SHL) instruction shifts the bits in the AX register to the left by the number specified in the CL register, inserting zeros into the least significant bits. This operation can also be considered as multiplying the AX register's value by \(2^{CL}\).

3. **SHR AX, CL**  
   The Shift Right (SHR) instruction shifts the bits in the AX register to the right by the number specified in the CL register, inserting zeros into the most significant bits. This operation can be seen as an integer division of the AX register's value by \(2^{CL}\).

**Analysis:**

For the contents of AX to remain unchanged after performing both the left shift and the right shift:

- The bits that are shifted out during the SHL operation must be strictly zeros. If any bits that are shifted out are ones, they are lost, and the original value cannot be restored by the SHR operation. Therefore, for this condition to be met, the initial content of AX should have zeros in its least 4 bits, effectively making AX a multiple of \(2^4 = 16\).

**Conclusion:**

The AX register will remain unchanged if, after executing the given instructions, it initially contains a value that, when left and then right shifted by 4 bits, retains its original value. Specifically, the condition required is:  
AX must be a multiple of 16.
Transcribed Image Text:**Question 42: Analysis of Assembly Language Instructions** **Objective:** Determine the condition under which the contents of the AX register remain unchanged after executing the following assembly instructions. **Instructions:** 1. **MOV CL, 4** This instruction moves the value 4 into the CL register. CL is commonly used in various CPU instructions due to its role as a counter in shift and rotate operations. 2. **SHL AX, CL** The Shift Left (SHL) instruction shifts the bits in the AX register to the left by the number specified in the CL register, inserting zeros into the least significant bits. This operation can also be considered as multiplying the AX register's value by \(2^{CL}\). 3. **SHR AX, CL** The Shift Right (SHR) instruction shifts the bits in the AX register to the right by the number specified in the CL register, inserting zeros into the most significant bits. This operation can be seen as an integer division of the AX register's value by \(2^{CL}\). **Analysis:** For the contents of AX to remain unchanged after performing both the left shift and the right shift: - The bits that are shifted out during the SHL operation must be strictly zeros. If any bits that are shifted out are ones, they are lost, and the original value cannot be restored by the SHR operation. Therefore, for this condition to be met, the initial content of AX should have zeros in its least 4 bits, effectively making AX a multiple of \(2^4 = 16\). **Conclusion:** The AX register will remain unchanged if, after executing the given instructions, it initially contains a value that, when left and then right shifted by 4 bits, retains its original value. Specifically, the condition required is: AX must be a multiple of 16.
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