My discussion was about the architecture. The fact that it wasn't supported by the OSes of the day may be true, but that doesn't mean it was "prevented", or *couldn't* be used. If customers thought it was useful, they could have asked IBM to support it, or implemented it themselves. The salient fact, according to the article, is that no one ever did.
I'm aware that -0 means first edition in IBM manual numbering, but they occasionally change the manual number and start over. Anyway, it appeared to be the oldest one on bitsavers, which is where I got it. sas
