Hello to the Shellcode

Method one, with MOV, code:

Hello_mov.asm
section. Data
        msg db "Hello, world!, 0xA
        len equ $-Msg section
. Text
Global _start< C5/>_start:
        mov edx, len
        mov ecx, msg
        mov ebx, 1
        mov eax, 4
        int 0x80

        mov ebx, 0
        mov eax, 1< C13/>int 0x80

Method Two, with XOR, code:

Hello_xor.asm
section. Data
        msg db "Hello, world!, 0xA
        len equ $-Msg section
. Text
Global _s Tart
        _start:
        xor eax,eax
        xor ebx,ebx
        xor ecx,ecx
        xor Edx,edx

        mov dl, len
        mov ecx, MSG
        mov bl, 1
        mov al, 4
        int 0x80

        mov bl, 0
        mov al, 1
        int 0x80

After compiling the connection, then disassemble and look at the

[Root@localhost shellcode]# nasm-f elf hello_mov.asm
[root@localhost shellcode]# 
[root@localhost shellcode]# Ld-o Hello_mov hello_mov.o
[root@localhost shellcode]# 
[root@localhost shellcode]#] [ 
root@localhost shellcode]#/hello_mov 
Hello, world!
. [Root@localhost shellcode]# objdump-d Hello_mov

Hello_mov:     file format elf32-i386


disassembly of section. Text:

08048080 <_start>:
 8048080:       ba 0e          mov    $0xe,%edx
 8048085:       B9 A4    $0x80490a4,%ecx
 804808a:       bb          mov    $0x1,%ebx
 804808f:       B8          mov    $0x4,%eax
 8048094:       cd                   int    $0x80
 8048096:       BB 00 00 80          mov    $0x0,%ebx
 804809b:       b8          mov    $0x1,%eax
 80480a0:       CD                   int    $0x80

[Root@localhost shellcode]# nasm-f elf hello_xor.asm [root@localhost shellcode]# ld-o hello_xor hello_xor.o [root@local
Host shellcode]#./hello_xor Hello, world! [Root@localhost shellcode]# objdump-d hello_xor hello_xor:file format elf32-i386 disassembly of section. text:0 8048080 <_start&gt: 8048080:31 c0 xor%eax,%eax 8048082:31 db x or%EBX,%EBX 8048084:31 C9 xor%ecx,%ecx 8048086:31 D2 xor%e Dx,%edx 8048088:b2 0e mov $0xe,%dl 804808a:b9 9c mov $0x804909c, %ECX 804808F:B3 mov $0x1,%bl 8048091:b0 mov $0x4,%al 8048  093:CD int $0x80 8048095:B3 mov $0x0,%bl 8048097:b0 mov $0x1,%al 8048099:cd int $0x80 [root@localHost shellcode]#  

Compared to the following:

MOV ecx,0
is 5 bytes, while
xor ecx,ecx

is 2 bytes

So, I see the general Shellcode code, are using XOR eax,eax instead of MOV eax,0