Hi. I want to use 64-bit registers (processor: Intel Celeron D) under 32-bit
Linux. So I wrote program listed below, compiled and launched:

format ELF executable
entry start
use64

segment readable executable

start:

; mov rbx,msg
; mov rax,[rbx]
; inc rbx
; mov [rbx],rax

mov eax,4
mov ebx,1
mov ecx,msg
mov edx,msg_size
int 0x80
mov eax,1
xor ebx,ebx
int 0x80

segment readable writeable

msg db 'HelloWorld',0xA
msg_size = $-msg

The result was 'HelloWorld'. OK. But try to omit every semicolon, compile
and launch again. 'HFellWorld' will appear on the screen. Why it's not 'HHelloWord' ?

You can't use 64-bit registers under 32-bit environment. The REX byte uses encodings that under 32-bit context are INCs and DECs.

Celeron D didn't have 64-bit GPRs, but maybe you mean SSE registers - there are used by SSE instructions.