[fpc-other] Yet another mainframe emulator

[Mark Morgan Lloyd]

Saunders, Rich wrote:
> Deeply weird is right!
> 
> I fondly remember the Burroughs series. I had to port a very large 
> FORTRAN system to it for our client, the Quaker Oats Company. They used 
> a set of these mainframes for their main systems.
> 
> I loved the idea of the descriptor-oriented architecture. Each file was 
> tagged with attributes,

The major use of the term "descriptor" was a data structure which 
pointed to a variable. If you told the hardware to put something on the 
stack and gave it the address of a descriptor it would dereference it, 
and if you gave it the address of a descriptor which pointed to another 
descriptor (etc.) it would follow the entire chain: the compiler quite 
simply didn't have to generate code to make this work.

Later implementations of the architecture added tag bits to each word in 
memory (something like 48 bits data, parity, 3 bits tag, ECC) which 
couldn't be manipulated by a user-mode program. That was obviously a 
major advance in robustness and security.

> There were security attributes as well.

The idea of code signing originated (I believe) on this architecture: 
you could compile a compiler from source, but unlike application 
programs it had to be "blessed" by somebody traceable before it could 
generate executable code. Larry Wall (Perl) at one point worked for 
Unisys (Burroughs' successor company) and refers to "blessing" data 
structures into objects: he's been harshly criticised for that by people 
who claim that he is insinuating his christian convictions into the 
programming language and its infrastructure.

In any event, the Large System architecture is dead as a native 
platform: for the last decade or so it's been available in emulated form 
on Unisys's Xeon and Itanium systems, and The Register reports that the 
Xeon systems can now run the emulation faster than the available 
specialist CMOS processors.

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

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