linux - Assembly language registers changes

I am running assembly language 64bit code on ubuntu 14.04 LTS 64bit

global _start

section .text
_start:
    ;display on screen
    mov rax, 1
    mov rdi, 1
    mov rsi, message

    mov rdx, length
    syscall

    ;exit gracefuly
    mov rax, 60
    mov rdi, 11
    syscall

section .data
    message: db 'Hello world!',0xa

    length: equ $-message

But when i disasseble it using gdb, it is dumping eax,edi,edx,eax and edi register insted of rax,rdi,rdx,rax and rdi register respectively...

Dump of assembler code for function _start:
   0x00000000004000b0 <+0>:     mov    eax,0x1
   0x00000000004000b5 <+5>:     mov    edi,0x1
   0x00000000004000ba <+10>:    movabs rsi,0x6000d8
   0x00000000004000c4 <+20>:    mov    edx,0xd

   0x00000000004000c9 <+25>:    syscall 
   0x00000000004000cb <+27>:    mov    eax,0x3c
   0x00000000004000d0 <+32>:    mov    edi,0xb
   0x00000000004000d5 <+37>:    syscall 
End of assembler dump.

Why this is happening ?