PKZIP “Implode” compression oddity

When I was researching old versions of PKZIP, I found that modern unzip programs aren't able to unzip the PKZIP v1.01 distribution file. Three of the member files inside the self-extracting ZIP file fail to decompress correctly. /cygdrive/c/dosprogs/ZIPTEST/101 $ unzip ../DIST/PKZ101.EXE exploding: README.DOC warning: 475 bytes required to uncompress to 873 bytes; supposed to require …

An un-unzippable ZIP file

Apparently, I'm writing a series of articles about ZIP format. Here's a curiosity. I'm going to use the Info-ZIP zip program to make a ZIP file that the Info-ZIP unzip program cannot unzip. I'll only do things that should be benign, i.e. things that a normal user might conceivably do. This article assumes you have …

Survey of ZIP APPNOTE documents

[2019-09-21: I've edited this document to include some new discoveries.] I already wrote an article about the version history of the PKZIP software, so now it's time for one about the version history of the ZIP format. That sounds like it should be easy, but… not so much. I think the format versions are more …

Survey of PKZIP versions for MS-DOS

I wanted to know exactly what versions of the old PKZIP compression software were publicly released for MS-DOS, and some basic characteristics about them, particularly what compression methods they used when compressing files. Sure, Wikipedia has a list, but it wasn't quite what I wanted, and it omitted at least one version I was pretty …

Will the real PKZ110.EXE please stand up?

I've been researching the version history of PKZIP, the once-popular compression software that gave us the still-popular ZIP file format. There are two important MS-DOS versions of it: v1.10, released in March 1990, which was the latest official version for more than 2.5 years, until v2.04c(?) was released in December 1992.v2.04g, released February 1993, which …

Survey of PackBits-like RLE compression schemes

This post is a survey of some run-length encoding (RLE) data compression formats, most of which are used for bitmapped graphics. One such format is PackBits. It's sort of the granddaddy of them, or the canonical example of such a format. Sometimes, formats similar to it are said to be "PackBits-like", though the term doesn't …

Analyzing some Windows Write files

Write is the word processing program that came with Microsoft Windows 3.x (and earlier versions of Windows, but it's 3.x that I'm concerned with here). Many thousands of free and shareware programs were written for Windows 3.x, and a lot of them include some documentation in Write's WRI file format. Objectively, most of them should …