AH = 3Ch
CX = attributes : any combination of:
   = 0000h : ATTR_NORMAL
   = 0001h : ATTR_READONLY
   = 0002h : ATTR_HIDDEN
   = 0004h : ATTR_SYSTEM
   = 0008h : ATTR_VOLUME
   = 0020h : ATTR_ARCHIVE
DS:DX -> filename : zero term. ascii string

CF = 0 : no error
  AX = handle
CF = 1 : error
  AX = 3 : ERROR_PATH_NOT_FOUND
     = 4 : ERROR_TOO_MANY_OPEN_FILES
     = 5 : ERROR_ACCESS_DENIED

• If the file already exists, it is opened and the file length is truncated to 0.
• When DOS creates a file, it is opened with read-and-write access.
  If ATTR_READONLY is specified, it takes effect after the file is closed.
• If ATTR_VOLUME is specified, filename is a volume label for the current medium.
  The new label is created only if the current medium does not have an existing volume label.

>4300 : Get File Attributes
>4301 : Set File Attributes
>5A : Create Temporary File
>5B : Create New File
>6C : Extended Open/Create

Create the file "test.txt" - filename is in the data segment

filename db "test.txt", 0 ;zero-term. string handle dw 0 ;------------------- mov dx, offset filename mov cx, 0 ;ATTR_NORMAL mov ah, 3Ch int 21h jc error_handler mov handle, ax