Reading RX50 contents

This describes some practical issues around retrieving data from the RX50 collection. Readership is public

Data to be retrieved from the RX50 collection falls into these categories:

  1. Files already duplicated elsewhere, so the disk contents just have to be listed.
  2. Files to be sent elsewhere for archiving or other use.
  3. The disk structure itself is significant and should be preserved. This applies to bootable disks and perhaps others.

In the first two cases the disk content often consists of compressed data. Back when the Rainbow was in common use, disk space and transmission bandwidth were both scarce resources and there were many compression utilities to choose from. The one I most commonly used was called ARC, which is the name of the file format and the program most commonly used to manage it. More of this below.

In the last case, it is desirable to archive an image of the disk that can be used to duplicate its contents exactly on a new disk if needed.

Compression formats


ARC format was what I most often used to write data to the RX50s.

This format is not commonly seen nowadays, which is probably due to a dispute that broke out somewhen in 1988 between the purveyors of ARC and another set of utilities called PKZIP and PKUNZIP. This is described in a Wikipedia article.

I seem to remember that I actually sent shareware money to SEA for ARC. I liked the program and its simple command interface for both compression and extraction. I never seriously used the PKWARE utilities on the Rainbow. But for archiving the data, should I convert it to another format?

I decided not. ARC is available under VMS and the source code for it is now freely available under a GPL V2 licence, which means I can build a version for Windows if I need to. I suspect any other format I choose today may seem just as ephemeral to the next generation of data archaeologists as ARC format seems now.


ZIP file format is still very common and the utilities I use for that on various machines come from INFO-ZIP.ORG. This has been ported to many systems of interest to Retrocomputing people and would be my preferred choice now for writing new archives.

It is desirable to have a ZIP format reader available on the Rainbow to make use of various recent software distributions. However, the 16-bit port of Info-ZIP programs to DOS doesn't seem to run on the Rainbow. Perhaps building a version that does would be a suitable future Retrochallenge task for me.


Because of the Info-ZIP issue above, I have installed this on the Rainbow to read recent ZIP files. I did actually have a version already installed, but it was version 1.1, which does not support the more recent compression methods in common use. However, there is a version 2.04g available which does the job. Why this became important is discussed below.


For category 3 disks, I needed software to read and write disk images. This brings me to a very pleasant aspect of this Retrochallenge which was the discovery that I was merely following in the footsteps of another participant. Back in 2009, Printstar's impressive Winter Warmup entry included the creation of just such a utility. So that's the thing to use here.

It was the desire to unpack this kit on the Rainbow itself that led me the the PKUNZIP upgrade mentioned above.

The author's Rainbow-100 web site is an excellent resource.

Writing disks

My hard disk partitions on the Rainbow are almost full, so to stage in RX50 contents for archiving I've needed to free some space by archiving data to more RX50s. I have a supply of these which are preformatted.

What I have found though, is that writing to the preformatted disks often gives rise to errors. Perhaps the disks have degraded or the heads of my RX50 drives are not well aligned to the preformatting. I have a vague memory of observing this same behaviour decades ago, so I'm guessing the problem may be one of alignment.

The outcome is that I always format a new disk before writing to it.


ARC source
The sourceforge ARC project page, from which a package of the entire source may be obtained. This has been actively maintained and the code I retrieved had changes made in 2010.
The home of the Info-ZIP utilities. Kits for many platforms are available through this site.
PKZip 2.04g
From the RAINBOW-100 web site. What you get is a self-extracting archive, which you probably should execute in an empty subdirectory.


This document is maintained by Pute. Comments should be addressed to PNJ at XQWV dot ORG dot UK.
  1. PNJ, 2014-01-29. Original version.

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