How do I ignore line breaks in input using NASM Assembly?

Question

Learning NASM Assembly, I am trying to make a program that reads two one-digit number inputs.

I have two variables declared in the .bss:

num1 resb 1
num2 resb 1

Then, I ask the user to write the numbers like this:

; Get number 1
mov EAX,3
mov EBX,1
mov ECX,num1
mov EDX,1
int 0x80

; Get number 2
mov EAX,3
mov EBX,1
mov ECX,num2
mov EDX,1
int 0x80

Since I am only interested in one-digit number inputs, I set EDX to 1. This way, whatever the user types, only the first character will be stored in my variable (right?).

The problem is that everything that follows after that first character will be used for the future reads. If you type 5 and then press ENTER, the ASCII code 53 will be stored in num1 just fine, but the line break you generated by pressing ENTER will carry on to the next read instruction, which will be stored in num2. Clearly that's not what I was intending. I want the user to type a number, press ENTER, type another number, and press ENTER.

I am not entirely sure how to work around this in the simplest way possible.

The dumbest idea was to put a "dummy" read instruction between num1 and num2, which will capture the line break (and do nothing with it). This is obviously not good.

Answer 1

Here's a very basic way of reading input until you get digits you want. It will skip anything but digits. This approach is fine if it provides the functionality you want. If you need different behavior depending upon other non-numeric input, then you need to specify that behavior. Then that behavior can be programmed as well.

    ; Get number 1
    mov   ECX,num1
    call  GetNumber

    ; Get number 2
    mov   ECX,num2
    call  GetNumber
    ...

GetNumber:
    pusha              ; save regs
get:
    mov   EAX,3        ; system call for reading a character
    mov   EBX,0        ; 0 is standard input
    mov   EDX,1        ; number of characters to read
    int   0x80         ; ECX has the buffer, passed into GetNumber
    cmp   byte [ecx],0x30
    jl    get          ; Retry if the byte read is < '0'
    cmp   byte [ecx],0x39
    jg    get          ; Retry if the byte read is > '9'

    ; At this point, if you want to just return an actual number,
    ; you could subtract '0' (0x30) off of the value read
    popa               ; restore regs
    ret