Question

Consider the following program with one mystery immediate 1-byte hexadecimal value 0??h:

mov ax, 00035h
mov bl, 0??h
movsx bx, bl
add ax, bx
movzx eax, ax
call print_int
call print_nl
This program prints "65462". What is the mystery value? Explain your work/reasoning.

Answer 1

The mystery value is 03h, which corresponds to the ASCII code of the character '3'. The program adds numbers and ASCII characters to generate the output '65462'.

Step-by-step explanation:

The mystery immediate 1-byte hexadecimal value in the program is 03h. This value corresponds to the ASCII code of the character '3'. Let's analyze the program step by step to understand why this value leads to the output '65462'.

1. The program starts with the initial value of 0 in register A.
2. It then adds 12 to register A, resulting in A = 12.
3. The next instruction adds the mystery value (03h) to A, giving A = 15.
4. Register D is then assigned the value of register A, so D = 15.
5. Now, A is updated to A + 13, resulting in A = 28.
6. Register B is assigned the value of A, so B = 28.
7. Finally, register A is added to D, giving A = 43.

Since A is the ASCII code of the character '3', the program prints '3'.

Now, let's move to the second part of the program. The value of A is 43, which represents the ASCII code of the character '+'. The program then concatenates the string '43+' to another string, resulting in '6543+'. Finally, it concatenates the string '+13', giving the output '65462'.