DOS 2.0 : Writes the specified number of bytes from a buffer to a file or device.

AH = 40h
BX = Handle of file or device
   = 1 : Standard Output (Screen)
CX = Max number of bytes to write
DS:DX -> Buffer with string characters

CF = 0 : no error
       AX = number of bytes written
CF = 1 : error
       AX = error code
          = 5 : ERROR_ACCESS_DENIED
          = 6 : ERROR_INVALID_HANDLE

• Handle may be a standard device - or a handle created with
  >3C: Create File with Handle
  >3D: Open File with Handle
  >5A: Create Temporary File
  >5B: Create New File
  >6C: Extended Open/Create
• When writing to a file, writing starts at the location of the file pointer. When this function returns, the file pointer points at the byte following the last byte written.
  >42: Move File Pointer
• Writing 0 bytes to a file truncates the file at the position of the file pointer.
• If the number of bytes written is less than the number specified, the destination (disk) is full - note: CF is not set.

Write "Hello" to the screen

msg db "Hello" len equ $ - msg ;msg length mov dx, offset msg mov cx, len mov bx, 1 ;handle: standard out (screen) mov ah, 40h ;Write File or Device int 21h