When a user creates a file it is stored on the hard drive, or removable media. This is given a human name, for example ricki.doc. From here the programme is run by the operating system searching for a particular attribute. How does it do this? Well, it needs a SFID. An SFID is system file identification. Every file or folder is given one of these when created on the system. This allows the operating system to associate every file that’s created with a number accordingly.
File Structure
Every file has to have a certain structure; this structure will therefore contain certain properties. Every file contains data; this is in the form of binary but is not measure in bit form, only in byte form. This is then stored in the summary and this is where all file information can be found.
Each file type also as its own structure and some are more complex than others. Complex file structures actually consist of certain file formats. This can range from .doc to .xcl. There is no limit to word file and can be of any size, this makes it a variable file length. This is not true for excel, .xcl so this is a fixed length.
File formats can also be convertible, they are said to be portable formats. They can be used from one operating system to another, for example an .exe can be used on a Linux machine, and this is called a re-locatable object file. This compatibility is all decided when the operating system is designed and therefore cannot change unless it is updated.
File Attributes
A file can have many properties that make it individual; these are called its attributes. For example, a file can have the following attributes;
- Name
- Identifier – this is its unique SFID
- Type
- Location
- Size
- Protection – this can be read only etc.
- Time and date and the creators identification
Naming Files
The SFID is always given to a file when it is created, this SFID is always declared as a whole number, and thus the integer data type is given to it. When I open a file the dynamic linking system needs to locate from physical to virtual address. To do so it must use the SFID.
When a file is to be run it is stored in to what is known as a Meta data table this can be seen below;
With a file the user should be able to do the following tasks;
- Create a file
- Write a file
- Read a file
- Reposition within a file
- Delete a file
- Compress a file
It’s now important to consider why we name files. Naming files is used to locate important documents with ease. This saves time when lots of documents are stored. How do we do this if for example, on a college network two people save the same file name? This is overcome by using what is known as partitioning. Partitioning allows multiple users to use the same file names without corrupting individual SFID’s. Below we can see an examp,e of the level one directory, this is an example of one person with differing files saved in the dame location. The level 1 directory has the following problems, names of files and folders would need to be different in order for there to be no conflict in the SFID.
The level two directories differ from the one level directory in the fact that I can possess features of differing users. To avoid issues with SFID we can have partitioned spaces on the non volatile storage space to avoid clashes. Here we can see an example;
The two level directories has four user names in the image above and this means that people users do not have the issue of saving files as the same name and possibly effecting the SFID.
User file Identification – UFID
User file identification is used when there are multiple folders, files open. Let’s say for example there are 5 files open within the operating system, this will open separate SFID’s for all of them. To avoid problems each programme is given a UFID, normally using numbers, in this case one to 5 and is stored in the R.A.M. This is only a temporary measure and is deleted when the programme is closed. This makes switching and changing between documents far more efficient.
