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I'm new to assembly language and my faculty gave an assignment which I cannot figure out how to do. Please help me understand the code....

The assignment as follows: Take 4 digits from user as string(alphanumeric to integer), convert them into a number and store in AX

if user input is '1','2','3','4' the output should be: 1234

P.S. I don't want the code, I'm having trouble understanding the procedure

Michael
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user5006384
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    We're not here to do your homework for you... – Marc B Jun 24 '15 at 19:31
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    @ Marc B I'm not asking for the code, I just need to understand the process. – user5006384 Jun 24 '15 at 19:33
  • To capture digits as chars and convert them into a whole number you need a procedure that we may call "string2number", here it is = http://stackoverflow.com/questions/30243848/assembly-x86-date-to-number-breaking-a-string-into-smaller-sections/30244131#30244131 . To convert the number back to string you will need "number2string" (don't forget to upvote the answer if it's useful!). – Jose Manuel Abarca Rodríguez Jun 24 '15 at 19:41
  • True, but I don't think his assignment needs to go the other way, number2string. It's good practice anyway. :-) – donjuedo Jun 24 '15 at 19:44

2 Answers2

2

1. Ask for input

Your goal here is to get a series of characters, these can be taken together as a whole string or one by one. If you take a whole string then you have to read it char by char, if you take one char at a time you are already done.

2. Convert numerical digits to digits

The chars you read are glyphs, symbols. Symbols in computer are represented (as everything else) with numbers. The way this mapping is done is your (charset) encoding. Without entering the vast realm of encoding you can rely on the fact that the glyphs for the decimal digits are encoded with the same numbers in the majority of western encoding, in particular you can assume ASCII encoding.
If you look at an ASCII table you see this

Character 0    1    2    3    4    5    6    7    8    9
Encoding  30h  31h  32h  33h  34h  35h  36h  37h  38h  39h

So if the user input 1234 you have these bytes in the string 31h 32h 33h 34h.
Your goal is to convert from the encoding to the digit, for example the char '4' has encoding 34h and you want to transform this into the value 4h.
For base from 2 to 36 this can be done with basic arithmetic, more fancy bases may need look up tables.
Don't forget to validate the input! Check the string for valid characters!

3. Compute the number from the digits

Now you have a sequence of digits, let's call them dn, ..., d3, d2, d1, d0 where dn is the first digit the user inputted and d0 the last one. You want to create a number from this digits. You know that a number like 1234 actually means 1*10^3 + 2*10^2 + 3*10^1 + 4.

So what you have to do is simply:
dn*10^n + ... + d3*10^3 + d2*10^2 + d1*10^1 + d0
(that explains my choice of subscripts)

Considering that n is varying (min is 1 max is 5 for 16 bit numbers) you can simplify this computation by accumulating the partial result.
This means that you have a partial result temp initially 0. Each time you have a new digit di you perform
tmp = tmp*10 + di
Convince yourself that this effectively compute the sum above. This method brings for free the nice feature that you compute the low 16 bit of the number which usually is a graceful degradation for numbers that can't fit a WORD.

4. Pseudo algorithm

n = 0;
t = 0;
while (1)
{
   c = get_char_from_input();
   if (end_of_input(c) || max_digits_read(n))
      break;

   d = get_digit_from_char(c);
   t = t * 10 + d;
   n = n + 1;
}
return t;

Up to you the task to develop an efficient assembly version.

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Break down the coding steps according to exactly how you would do it by hand. It helps to imagine you are explaining the process to someone else, to do by hand.

Without giving away too much detail, you will need to convert ASCII characters into integers, and you will have to process one digit at a time, from one end of the string to the other. Making function calls for such subtasks will help your organization.

donjuedo
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