Thanks Marcos
I have make changes in my code as u said
now the COM file is created. But the file is currupt it display simbles as a pdf file is opened in note pad

I m posting my new code

_code segment
assume cs: _CODE, ds: _CODE
ORG 100H
start: jmp skip

msg db 'hello ashwani', '$'
ORG 100H
skip: mov ax, cs
mov ds, ax

mov ah, 09h
mov dx, offset msg
int 21h

mov ah, 4ch
mov al, 00h
int 21h

_CODE ends
end start