[fpc-other] Yet another mainframe emulator

[Mark Morgan Lloyd]

Saunders, Rich wrote:
> Thanks for the correction about descriptor architecture. I thought you 
> were referring to the file system tagging stuff.
> 
> I would hope that someday the good ideas of the past would find their 
> way back into general use.
> 
> I also remember some really awesome attributes of VMS that have been 
> discarded. At least HP has been keeping OpenVMS alive so people can 
> actually see what it can do. Probably the most secure and reliable OS in 
> general use today. 17 year cluster uptime? Amazing.

I came across this over the weekend, which illustrates some of the odder 
properties of the descriptor architecture:

"Part of the idea was that when you fetch something, the processor 
wouldn't know until it hit the word it was fetching whether there was 
going to be data there [0 in the MSB] or whether it was going to be an 
indirect reference [1 in the MSB] to code which would be executed to 
furnish the data. It made it a very clean way of implementing ALGOL name 
call expressions because the basic code inside the procedure wouldn't 
have to do anything special to access a call-by-name parameter. With 
access just like a call-by-value parameter." 
http://conservancy.umn.edu/handle/107105

There's suggestions in the paper that Intel's aborted iAPX432 
architecture borrowed ideas, and I think I'd say that the protected-mode 
architecture of the '286 (and successors) is at least an example of 
convergent evolution. But in practice, after 50 years of competition, 
this type of architecture is thoroughly dead with the dominant 
philosophy being much more IBM-like.

There appears to be a lot of interest at the moment in resurrecting old 
architectures and finding their system software. However I think the 
only one which has any "survival value" is the IBM S/370 and its 
successor machines.

-- 
Mark Morgan Lloyd
markMLl .AT. telemetry.co .DOT. uk

[Opinions above are the author's, not those of his employers or colleagues]