CODEWITHSUNDEEP
ubuntuassemblyx86
bigz90
bigz90

Reputation: 89

Assembly 8086 - Read a value from keyboard and compare it to another value

I wrote a program which ask to insert a value: if the value is less or equal to 100 the program exit without errors; else it prints an error message and exit. Unfortunatly,it never prints the error message when I insert a value greater than 100 so maybe there's some problems in the conversion of input sting but I don't understand where! Here's the code:

.section .data
 str:
    .ascii "Insert value:\n"
 str_len:
    .long . - str   
 err:
    .ascii "Error\n"
 err_len:
    .long . - err       
 car:
    .byte 0     
 .section .text
    .global _start  

 _start:
    movl $4, %eax     #print string with sys read
    movl $1, %ebx
    leal str, %ecx
    movl str_len, %edx
    int $0x80
    xorl %eax, %eax

 again:
    pushl %eax          #save eax on stack
    movl  $3, %eax      # read from keyboard with sys write
    xorl  %ebx, %ebx    
    leal  car, %ecx     
    mov   $1, %edx      # read 1 char at the time
    int   $0x80         
    cmp   $10, car      # if it reads '\n' then end
    je    end
    subb  $48, car      # else it converts ASCII code of char to correspondent values
    popl  %eax
    movl  $10, %ebx
    mull  %ebx          # eax = eax * 10 + car  
    addl  car, %eax 
    jmp   again 

end:
    popl %eax  
    cmpl $100, %eax
    jg error

error:
    movl $4, %eax       #print error message
    movl $1, %ebx
    leal err, %ecx
    movl err_len, %edx

exit:  
    movl $1, %eax
    xorl %ebx, %ebx
    int $0x80

Upvotes: 1

Views: 366

Answers (1)

Michael
Michael

Reputation: 58427

There are a number of problems in your code:

This instruction will result in a 32-bit comparison (cmpl):

cmp   $10, car

So you need to specify that you want an 8-bit comparison by using cmpb.

In the error part of your program you've forgot the int $0x80, so it won't print anything.

You've got an jg error, but then the error: label is on the very next line, so you'll end up there regardless of whether that jump is taken or not. That jump should be changed to jng exit.

Upvotes: 2

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