[Simh] VAX/VMS

[Dave Wade]

> -----Original Message-----
> From: Simh [mailto:simh-bounces at trailing-edge.com] On Behalf Of Wilm
> Boerhout
> Sent: 16 February 2016 11:58
> To: simh at trailing-edge.com
> Subject: Re: [Simh] VAX/VMS
> 
> Johnny Billquist schreef op 16-2-2016 om 12:49:
> >
> > No, it is not. Talk to IBM about S/360... :-) And there are some VAXen

S/360 compatibility is only forward, and only to a certain point. S/360 and S/370 are both 24-bit addressing and fairly compatible, but S/370 (Mostly) has Virtual Memory as standard. 

Then came the "great divide" S/370XA. XA mode has 31-bit addressing and different I/O instructions. Some of the XA boxes will work is S/370 mode, but many won't.

More recently IBM moved to 64-bit hardware. Again some will boot in 31-bit mode but more recent boxes need a 64-bit OS.

So the earliest incarnations of "OS", which were I guess "MFT" which is basically a fixed number of partitions will run on later machines until you get to systems which will only run in 31bit mode. (XA Mode).  

OS/VS2 and its siblings MVS (This is the free version), MVS/SP (The paid for version) will only run on S/370 or later, not on 360, as they need Virtual Memory and it stops working at the same point as MFT when 31 bit only machines appear. There are also issues of Virtual Memory Page size which may stop MVS (the free version working) working on some hardware (there are patches to work round this).

You also have issues over disk (DASD in IBM speak) support. So whilst MFT was written for a 1996 S/360 it would in theory run on an P390E from 1996 so 30 years of computability. However, it would need older disks, which the P/390E cannot support.

Of course these changes are really only to do with programs that run in supervisor state. User mode programs generally will run unchanged from 1966 through to the present day, and the latest zOS a descendant of MVS will still run 24-bit applications.  I am pretty sure that until a few years many commercial sites, so mostly Cobol, still used the older "free" Fortran-66 compiler for the odd Fortran job.

> > on which V7.3 will definitely not run. How about rtVAX for example.
> >
> I stand corrected. Please note that I had a marketing job once. It sticks...

... I also believe that some of the in-compatibility in IBM kit is to drive the hardware->software->Hardware->Software upgrade chain and keep the dollars rolling in...

Dave G4UGM 




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