Skeleton of a source file in Nasm format:



  bits 16        ;optional
  org 100h       ;code starts at 100h
  section .text
  ;  code here...

   MOV AH, 4Ch   ; "exit to DOS" function as last code in main
   INT 21h

  section .data
  ; data items here (DB and DW variables)
x: DW 0         ;x can be used as operand:  MOV AX,X

  section .bss
  ;  uninitialized data here


Notes:
numbers are decimal unless appended with h or prepended with 0x
  21h  or  0x21

labels are supported.



To assemble from command line:
nasm[w]  myprog.asm -fbin -o myprog.com

or use the nasmide:
Options--Assembler to tell it where the nasm.exe is.
paste from Windows by right click Edit--Paste in window bar.
Options--Environment set to 45/50 lines for more viewing area.


DOS debug can look at the .COM file and debug it.