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How to Use Compressed Files and Installers

Most software installers provided by Apple Computer, Inc., Microsoft Corporation, and other third-party companies are provided as compressed files. Compression is used so that installers can be packed into a smaller single file that can be downloaded from the Internet more quickly and easily. Anyone who uses installers or transfers large files regularly and wants to understand more on how compression works and why it is necessary can benefit from this information.

What is File Compression and Why is It Essential?

Compression works by eliminating or minimizing redundancy in a file, therefore making files smaller without losing any information. Every character, letter, digit, and punctuation mark on a computer is actually made up of several characters that make up computer code.

In an effort to make efficient use of disk space and to speed things up, large files that will be shared are often stored as compressed files. File compression reduces the size of a file, and with that, the time it takes to move the file from one location to another.

How Do You Access Compressed Files?

To view compressed files, you need a compatible decompression program that can read the computer code and convert the data back to its original state. There are several different programs available that can do this. See How Can I Automatically Decompress Archives? for additional information.

What Are Some of the Most Common Compression Formats in Use Today?

The most common compression formats used for software installers are listed below. The suffixes shown in parentheses are appended to the name of the compressed file (also referred to as an archive) or disk image.

For example, name.smi indicates a self-mounting image or PrintToPDF.sit indicates a StuffIt archive.

  • StuffIt archive (.sit or .sitx)
  • Zip archive (.zip)
  • Self-extracting archive - Macintosh (.sea)
  • Self-extracting archive - Windows (.exe)
  • Self-mounting image (.smi)
  • OS X Disk image (.dmg or .mpkg)
  • Windows Ghost image (.gho)

What is a Disk Image?

A disk image is an exact binary copy of an entire disk, drive, or drive partition. Disk image files contain all data stored on the source drive, including not only files and folders, but also boot sectors, file allocation tables (FAT), volume attributes, and any other system-specific data. The data in the disk image is stored as a sector-by-sector copy of the disk or drive, not as files or folders.

  • Disk image (.dmg or .mpkg): Compressed files of this type are created with Apple's Disk Utility under Mac OS X. These disk image files are most commonly used as a replacement for CD or DVD installers. With Disk Utility present, double-click the image file(s) to mount the disk image(s).
  • Self-mounting image (.smi): A disk image created with Apple's Disk Copy utility that will mount itself when double-clicked.
  • Ghost image (.gho): A Windows disk or partition image created with Symantec's Ghost software. You must have Symantec Ghost installed to open and install this image file.
  • ISO image (.iso): A conventional internationally recognized disk image standard that is supported by many software vendors. Most commonly associated with Windows, but can be created or read using a Macintosh as well.

How Can You Automatically Decompress Archives?

Macintosh and Windows computers both have the built-in ability to decompress .sit and .zip archive files.

To open an archive on a Macintosh, double-click the archive to decompress original file(s) from the archive.

To open an archive on a Windows computer, right-button click on the archive and select Extract All. The Extraction Wizard will launch and in that window click Next, then Next in the following window, then Finish. A folder will appear in the same place as the .zip or .sit archive and contain the original file(s).

If you will require an archive expander that recognizes other compression formats than .zip or .sit, then you may want to consider using StuffIt Expander, which is available for both Macintosh and Windows computers. StuffIt Expander can be downloaded for free from http://www.stuffit.com/mac-expander.html.

Once installed, most file expansion programs will automatically decompress archives when they are double-clicked. Automatic decompression of archives when they are downloaded is not recommended. This is to avoid inadvertently launching an infected e-mail enclosure or Internet download.

Last Updated: 7/12/12