[Simh] [SimH] VAX/VMS

[Bob Supnik]

Okay, I'll bite - what's a VXT1200? I see references to it on Google, 
but I can't find a clear description.

Tim Litt said rtVAX was never intended to run VMS, but its history is a 
bit more complicated. The original intent of the uVAX chip program, 
which kicked off in the spring of 1982, was to establish an 
chip-and-software industry standard in the IBM PC sense, at the 32b 
level. The uVAX program office under Roy Moffa had a clearly stated and 
approved goal to sell uVAX as a chip, a board, and a system, to all 
comers, with appropriate licensing for VMS. As a result, the definition 
of the chip was limited in three ways, to minimize the competitive 
impact on DEC's own products.

1. The chip would implement only a subset of the instructions (also 
needed to get the microcode to fit).
2. The chip would support only 16MB of physical memory.
3. The chip would have physical system space and physical process page 
tables. Because of the physical memory limitation, this would limit the 
size of virtual memory as well.

Dave Cutler's MicroVAX I (code name Seahorse) would be even more 
limited, because it would only support 4MB of physical memory.

As the project went on, the software cost of limitation #3 became 
increasingly clear. In the fall of 1982, Dave Cutler asked that system 
space be made virtual, with an independent mapping enable bit. The uVAX 
team wasn't happy about this but eventually agreed. Then, in the spring 
of 1983, the VMS team concluded that getting VMS to support physical 
process page tables was really, really hard. Dick Hustvedt asked if full 
VAX memory management could be supported. Dave was able to do that for 
MicroVAX I, and after some minimal redesign work, the uVAX team was able 
to support full memory management as well.

But with the virtual memory limitation removed, and evidence that uVAX 
would perform almost as well as a 780 except on COBOL apps, Ken Olsen 
became increasingly unhappy about selling the chip and setting up 
competitors to DEC's own VAX line. In 1984, he reversed the decision to 
sell the chip but allowed the program to sell boards to continue. Then, 
on the day of the MicroVAX II system announcement, /at the announcement 
site/, he reversed the decision about selling boards and had all the OEM 
material removed from the announcement. The chip and board teams were 
devastated.

In order to have <something> 32b to sell in the real-time market, the 
real-time business resurrected the original uVAX definition, now to be 
known as rtVAX. At first, Ken demanded that rtVAX be incapable of 
running Ultrix/Unix as well, but any changes that prevented that also 
made it impossible to run VAXELN, Dave Cutler's real-time OS. 
Eventually, Ken concluded that preventing rtVAX from running VMS was 
sufficient, and it was allowed to proceed. Because changes had to be 
confined to microcode, rtVAX included mapped system space and physical 
process page tables, with a single enable bit.

I don't recall if there was a CVAX real-time variant. There's no 
evidence in the CVAX microcode of alternative memory management flows. 
The 1987 Semiconductor Handbooks don't mention rtVAX at all.

/Bob

On 2/16/2016 8:53 AM, simh-request at trailing-edge.com wrote:
> Message: 7
> Date: Tue, 16 Feb 2016 14:53:40 +0100
> From: Johnny Billquist<bqt at softjar.se>
> To:simh at trailing-edge.com
> Subject: Re: [Simh] VAX/VMS
> Message-ID:<56C329E4.7090007 at softjar.se>
> Content-Type: text/plain; charset=utf-8; format=flowed
>
> On 2016-02-16 13:58, Timothe Litt wrote:
>> >On 16-Feb-16 06:49, Johnny Billquist wrote:
>>> >>More precisely, V7.3 will run on*any*  VAX, including the primal
>>>> >>>VAX-11/780. This level of backwards compatibility is unique.
>>> >>
>>> >>And there are some VAXen on which V7.3 will definitely not run. How
>>> >>about rtVAX for example.
>> >Not fair.  No version of VMS ever ran on rtVAX - it was designed that
>> >way. (For yes, marketing reasons.)
> Oh, I definitely agree that I wasn't fair. But on the other hand, Wilm
> did claim that VMS ran on*any*  VAX.
> I was also tempted to drag up the VXT1200. But since that one does not
> have the name "VAX" in it, it could perhaps technically be disqualified
> on that basis... The rtVAX on the other had is without a doubt a VAX,
> even in name.
>
> 	Johnny
>