Question

x86 MASM

Write a program that will use bit-wise instructions to take an inputted binary value and reverse it.

Example:

Input: 000000000011
Output after instruction: 110000000000

Answer 1

Let us assume the 16 bit number is stored at memory location 2450 and 2451

Algorithm –

1. Load contents of memory location 2450 in register AL
2. Load contents of memory location 2451 in register AH
3. Assign 0004 to CX Register Pair
4. Rotate the contents of AL by executing ROL instruction using CX
5. Rotate the contents of AH by executing ROL instruction using CX
6. Store the content of AH in memory location 2450
7. Store the content of AL in memory location 2451

Explanation for the logic–

1. MOV AL, [2450]: loads contents of memory location 2450 in AL
2. MOV AH, [2451]: loads contents of memory location 2451 in AH
3. MOV CX, 0004: assign 0004 to CX register pair
4. ROL AL, CX: rotate the content of AL register left by 4 bits i.e. value of CX register pair
5. ROL AH, CX: rotate the content of AH register left by 4 bits i.e. value of CX register pair
6. MOV [2450], AH: stores the content of AH in 2450 memory address
7. MOV [2451], AL: stores the content of AL in 2451 memory address
8. HLT: stops executing the program