CODEWITHSUNDEEP
assemblyx86-16
g0dafk
g0dafk

Reputation: 31

Print little or big endianness in assembly 8086

I want to check the endianness of my computer in an assembly program ( 8086 ) and to print it.

I know what endianness is and how you find it. It is about the way data is stored in memory. For eg if you store 1234 it will be stored 3412 ( little ) or 1234 ( big ). But i don't know how to do it in assembly language. I'm thinking about storing something in memory than load it and compare it with the original. It should be exactly as the theory says. If i store 1234 i will load it as 3412 or 1234. I have a problem when i'm trying to create an if statement.

litte db "little$" 
big db "big$"  

mov ax, var1
cmp ax, var2
je equal
jmp notequal


equal:
lea dx, big
notequal:
lea dx, little

I expect it to print little or big, but no matter what values i put in var1 and var 2 it always prints little, because somehow the code is executed line by line. Shouldn't it go to only one label, "equal" or "notequal"? I want something like:

if var1==var2
print big
else
print little

Upvotes: 2

Views: 1201

Answers (1)

paxdiablo
paxdiablo

Reputation: 882686

This segment will always print "little" since, even if you begin execution at equal, it will carry on through notequal:

equal:
    lea dx, big
notequal:
    lea dx, little

What you would need is something like:

equal:
    lea dx, big
    jmp done
notequal:
    lea dx, little
done:
    ; carry on

Additionally, you should store the word 1234h into some memory location y, then read the byte back from that same location.

If 12h then you are big-endian. The value 34h means little-endian.

Anything else means you have a memory problem :-)

By the way, I'm pretty certain all x86 CPUs are little-endian so, if you're writing this in x86 assembly language, you probably don't need to check.

By way of example, you can use Coding Ground with the following code to see it in action:

section .text
    global _start

_start:
    mov  ax, 0x1234        ; load up 1234 hex
    mov  [myword], ax      ; store that word to memory
    mov  al, [myword]      ; get first byte of that
    cmp  al, 0x12          ; 12 means big endian
    je   big
little:
    mov  edx, l_len        ; prepare for little message
    mov  ecx, l_msg
    jmp  print
big:
    mov  edx, b_len        ; prepare for little message
    mov  ecx, b_msg

print:
    mov  ebx, 1            ; file handle 1 = stdout
    mov  eax, 4            ; 'syswrite' function call
    int  0x80

    mov  eax, 1            ; 'exit' function call
    int  0x80

section .data

myword   dw   0

l_msg    db   'Little endian', 0xa
l_len    equ  $ - l_msg

b_msg    db   'Big endian', 0xa
b_len    equ  $ - b_msg

The output window shows, as expected:

$ nasm -f elf *.asm; ld -m elf_i386 -s -o demo *.o
$ demo
Little endian

Upvotes: 3

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