Computer File Systems provide a mechanism to store, find, and modify various kinds of data. These computer file systems are typically presented to users as “folders” that contain both files and other folders. This presentation gives an overview of common concepts in computer file systems.

Almost everything that we care about on our computers is stored in what we call a “file”. A “file” is really just an organized collection of 0’s and 1’s that represent something. For example, the upper case letter “M” is stored in a computer as “01001101” (this is the “ASCII” code for “M”), and the upper case “C” is stored as 01000011. So the club’s initials (MMCC) would be stored in the computer as:

01001101010011010100001101000011

Similarly, a black and white picture might be stored with the number “0” representing black, and the number “1” representing white. So this picture …

… might be represented with a bunch of 1’s and 0’s like this:

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
1 1 1 1 1 1 0 0 0 0 0 0 1 1 1 1 1 1
1 1 1 1 0 0 1 1 1 1 1 1 0 0 1 1 1 1
1 1 1 0 0 1 1 1 1 1 1 1 1 0 0 1 1 1
1 1 0 0 1 1 1 1 1 1 1 1 1 1 0 0 1 1
1 1 0 1 1 1 0 1 1 1 1 0 1 1 1 0 1 1
1 0 1 1 1 1 0 1 1 1 1 0 1 1 1 1 0 1
1 0 1 1 1 1 0 1 1 1 1 0 1 1 1 1 0 1
1 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 1
1 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 1
1 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 1
1 0 1 1 1 0 1 1 1 1 1 1 0 1 1 1 0 1
1 1 0 1 1 1 0 1 1 1 1 0 1 1 1 0 1 1
1 1 0 0 1 1 1 0 0 0 0 1 1 1 0 0 1 1
1 1 1 0 0 1 1 1 1 1 1 1 1 0 0 1 1 1
1 1 1 1 0 0 1 1 1 1 1 1 0 0 1 1 1 1
1 1 1 1 1 1 0 0 0 0 0 0 1 1 1 1 1 1
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

Of course, this is a very low resolution “image” (only 18 x 18). But larger images are stored in a similar manner using millions of these “bits” of data.

These 1’s and 0’s are called “bits” as a contraction of “binary digits”.

The computer needs a place to store all of these bits. That’s where the term “hard drive” comes in. A classic “hard drive” is made up of one or more spinning circular plates with a special magnetic surface that can be magnetized in different directions to represent a “1” or a “0”. The following picture shows what a typical “hard drive” looks like:

Hard drives have been in common use for home computers since the 1980’s, but recently, a purely electronic kind of “hard drive” is replacing the spinning disks. These come in a several sizes as shown here:

But regardless of how the bits are physically stored, we can still view entire file system (“hard drive”) as a long series of 1’s and 0’s.

So a “hard drive” is made up of some total number of bits that defines its total capacity. It is a fixed number just as shown in the previous picture. But this “sea of bits” can be organized to make things easier to find and update. This is where the concept of a “directory” (or “folder”) comes in. Some groups of these bits are used to store the locations of logical “files” which, in turn, store things like pictures or documents or music. In the following colored drawing, the black sections represent the groups of bits that keep track of the other “files”. In other words, these black sections might store the file names and file start locations and file lengths of the other files. In other words, the black areas are like a “directory” of the other files on the drive. So as files are added or removed or resized, these “directories” are updated to keep track of them. Also, as shown in the following diagram, there can be multiple directories on a drive. There is usually a directory at the start of the drive to map out where some of the other files are located. But some of those “other files” could also be directories (as shown in black) which point to other files and possibly other directories.

While this looks very messy on our “hard drive”, we can represent all of these directories and files more logically (for us) as a series of folders inside folders. The entire file system (or hard drive) may be thought of as one large folder:

That large folder contains files (images, documents, music, etc), and it can also contain other folders.

And those other folders can contain even more files and folders as shown here:

Most modern operating systems have some way of showing you the file system as a picture. They will typically draw a “directory” as a “folder”. And clicking on a directory (folder) will show the files and other folders inside of it. The following 3 pictures show how the file systems are displayed on “Windows”, “Mac OS”, and “Linux”:

You’ll notice that while these do look different, they all share the same representation of folders that hold files and other folders. This is often called a “tree” structure because there is one “root” that may contain several branches which in turn contain even more branches and so on. It’s important to understand this and to be comfortable moving around in this “tree of files”. The next few pictures demonstrate how folders can be used to organize (group) files by both date and subject.

The previous discussion focused on the actual storage abstraction common to all mass storage devices (hard drives, flash drives, SSD drives, etc). However, some programs can work “on top” of the underlying file system. For example, photo organizing software can sometimes provide an abstraction of your files (photos in this case) that does not even resemble the actual storage structure of your drive. These abstractions can often be more convenient to use. For example photos can be organized by grouping them according to their common features (date, size, content, etc). This can be convenient when viewing the photos with that particular software. But it can also make it harder to find those same photos outside of that particular software. So it’s always helpful to know a little bit about the underlying file system.

This MMCC presentation was shared with the club on July 1, 2021. Here are the slides: