1988 Netnews posting about 1977 VAX perceptions

Here are some latter-day thoughts about the description I wrote of VAX more than 10 years ago, from a pre-announcement briefing on it.

Although the instruction set didn't work out as nicely as it seemed it would, even from the CISC viewpoint, it holds up pretty well. I certainly don't gush as strongly today, mostly because it isn't quite as orthogonal as it seemed it would be. There are still plenty of special cases lurking in the instructions and address modes C wants to compile, for example. The call instructions do indeed do just what you want, but are too slow.

Of course the real critique arises not from the details, but from the RISC insight. I take this be, in essence, that those address syllables are expensive to decode, and that the complicated instructions are bad not because of complication alone, but because they practically insist on a microcode implementation that RISC is able to dispense with altogether.

The analysis of the mapping is correct, and picks out the architectural weakness of the scheme (no attractive way to do segments) but misses the small page size. As I noted in the text, we didn't hear or weren't told what it was. Today, 512 bytes is much too small. Then, I probably wouldn't have thought so. It's interesting that the small size persists, though. DEC must feel that the software couldn't cope with a change.

My impressions of the software situation were entirely correct.

The main fact about the machine that we missed out on altogether was the IO architecture. Even the VAX-11/780 had a messy collection of odd bus adaptors glued on it, and of course the situation has never really become any better.

Dennis Ritchie
dmr@research.att.com [1999 address: dmr@bell-labs.com]