A good starting point for your program is the toupper.asm program shown in class. It already queries the user for input and sets up a loop that looks at each character of the input. The source code for toupper.asm 

The bytes entered by the user are ASCII characters. You should convert this to a numerical digit by subtracting the character '0'.

Recall that the last character might be an X and represents the value 10. You will need to special case this. For simplicity, let's not worry about an X appearing in the middle of the input.

You should set up your loop to iterate 10 times and NOT for the length of the string (as is done in toupper.asm) because that will include the newline character at the end of the user input.

Since all the numbers are small, you can use the 8-bit portions of the general purpose registers: AH, AL, BH, BL, CH, CL, DH and DL. This is convenient because you have more registers to play with and because you won't have to mix 8-bit and 32-bit arithmetic.

Develop your program incrementally. After each step, use the debugger to check that you have accomplished the desired goal.

Set up the loop to iterate 10 times, each time storing the next character in an 8-bit register (say AL).

Convert AL into a "number".

Add the special case where AL might be X.

Compute t in another 8-bit register.

Compute t % 11.

Compute sum % 11 in another 8-bit register.

Print out the correct message to the user.

Extra Credit

For 10 points extra credit, submit a separate assembly language program that prints out the value of the check digit given the first 9 digits of an ISBN number. A sample run of your program might look like:

linux2% ./a.out

Enter first 9 digits of ISBN #: 320154197

Check digit: 4

linux2%

linux2% ./a.out

Enter first 9 digits of ISBN #: 045777370

Check digit: 7

linux2%

linux2% ./a.out

Enter first 9 digits of ISBN #: 044101125

Check digit: X

linux2%