Question

I am working through some simple problems for practise, and one of the problems is to read in a number, and then output that number plus 1.

For example:

Please enter a number: 4
5

Now, with that input and output, it should be easy. But I want to make my program capable of processing multiple digit numbers. And that is where I am stumped. Because if I was given the input of 999, how could my program know to make the last 9 a 0, and add 1 to the tens column? And then, how could it know to make the number into 1000 by adding another digit?

Here is my code so far:

SECTION .data
msg        db "Please enter a number: ",0
msglen     EQU $-msg
sz         EQU 32
NUL        EQU 0
len        EQU 32

SECTION .bss
num        resb len

SECTION .text
GLOBAL _start
_start:

Msg:                             ; display the message
mov        eax, 4
mov        ebx, 1
mov        ecx, msg
mov        edx, msglen 
int        80h              

Read:                            ; read in the number
mov        eax, 3
mov        ebx, 1
mov        ecx, num
mov        edx, len
int        80h


Length:                          ; Find length of string
mov        eax, num
mov        ebx, 0
loop:
cmp        BYTE[eax], NUL
je         Set 
inc        eax
inc        ebx
jmp        loop                               

Set:                             ; set up the registers and variables
mov        ecx, num

Print:                           ; main section - prints out the number plus 1
cmp        BYTE[ecx], NUL
je         Exit 
mov        eax, 4
mov        ebx, 1
mov        edx, 1            ; NOTE- does not yet add 1 to the number
int        80h

inc        ecx
jmp        Print 

Exit:                            ; exits safely
mov        eax, 1
mov        ebx, 0
int        80h  

Could someone show me how to do this problem? I just can’t figure out the adding part. I can print out multiple digits, up to 32 bits, but the adding is just a mystery.

Thanks in advance,

Rileyh

Answer 1

How about using atoi() to convert it into the integers and then printf() to output the result? There are some sample codes about it:

http://cs.lmu.edu/~ray/notes/nasmexamples/

As for your integer-adding problem, if I were you, I’d start by tearing the problem down to “add one”. You’ll notice you’re following this algorithm there:

1. Set the pointer to the last character in the string.
2. Increase the character by one. If the character’s below ASCII ‘9’, return.
3. Else, set the pointer to the previous character. If its number is not -1, jump to 2.
4. If we’re trying to overwrite the character number -1, allocate n+1 bytes for the new string, copy the old one’s contents to a new one and add ‘1’ to as the first character.

From this point, you’ll only need to figure out what changes if you want to add more than one digit of the input.