AH = 40h BX = file handle CX = number of bytes to write, a zero value truncates/extends the file to the current file position DS:DX = pointer to write buffer on return: AX = number of bytes written if CF not set = error code if CF set (see DOS ERROR CODES) - if AX is not equal to CX on return, a partial write occurred - this function can be used to truncate a file to the current file position by writing zero bytes