NIU Department of Mathematical Sciences
Downloading PC files

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Binary vs. ASCII

Computer files come in two flavors: ASCII (pure text) files in which the highest (8-th) bit of each byte should be set to zero, and binary files, in which all 8 bits in each byte are important.

When transferring files between similar systems the distinction is not crucial; but on different machines the format of text files is slightly different. This makes it necessary to use the right transfer mode - text or binary. Text files sent in binary mode might have incorrect newline characters and other "gremlins" in them, while binary files sent in text mode will most likely be unuseable because all the highest bits will be stripped off. This applies to network transfers (FTP) and ordinary modem ones (Kermit, Zmodem etc.) Consult the documentation for a specific program to see how the transfer mode can be set.

Recognizing the type of various files is largely a matter of experience. To help us in this, files are customarily given names with extensions which indicate what the file contains. Executable programs (.EXE), image files (.JPG, .GIF), sound data (.SND, .AU), most word processor documents (.DOC, .W51) are all binary files. Text (.TXT, .ASC), PostScript (.PS), TeX (.TEX) are all ASCII. Here is a more complete list.

You can avoid most of this hassle by using a modern Web browser to access FTP archives. The file type will be automatically recognized in most cases. When you see a link which represents something that you want to download to local disk rather than view on the screen, simply press one of the control keys while clicking on the link. The key differs from system to system and between browsers. For example, in Netscape under Unix hold down Shift; on a Mac press Option; under Windows the Alt key should work.

Archived files

Most software packages consist of several files and directories. To simplify things they are distributed in "archived" or "wrapped up" form. The resulting archive can be downloaded as a single file, but it must then be "unwrapped" at its destination.

Some such archives are "self-extracting": they will typically have the extension .EXE and they should be run like any other program. It's best to do that in a newly created empty directory.

Other archives require a special "unpacker" program. The most common of these have the extension .ZIP and can be unpacked using the shareware utility pkunzip, available here. If you'll be searching for PC software on-line, you won't go far without it.

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Last modified: 6/5/97 by webmaster@math.niu.edu