[Simh] VAX physical address space (was: Re: More VAX Simulators)

[Bob Supnik]

If you're talking about the NVAX chip and its derivative systems, 
including the 6600 and 7000, the answer is, no, it doesn't support a 34b 
physical address space. NVAX has a 32b physical address space. Despite a 
25b page frame number (PFNs) in the PTE, the translation buffer has only 
23b for the PFN = 4GB physical address space (see the KA680 Technical 
Manual).

The preferred mode of physical addressing was 32b one-to-one, but a mode 
switch allowed for 30b with sign extension. The Qbus systems, for 
backward compatibility, used 30b with sign extensions and limited the 
physical address space to 1GB (512MB memory, 512MB IO), like in the 
earlier VAX chips.

While NVAX+ was interface compatible with EV4, for interchangeability in 
the 7000, the guts were the same as NVAX, with a 23b PFN.

/Bob

On 4/21/2019 12:00 PM, simh-request at trailing-edge.com wrote:
> Message: 5
> Date: Sun, 21 Apr 2019 17:04:32 +0200
> From: Johnny Billquist<bqt at softjar.se>
> To:simh at trailing-edge.com
> Subject: Re: [Simh] More VAX Simulators
> Message-ID:<1e40709f-d44c-bb20-4d75-b9442fa389e6 at softjar.se>
> Content-Type: text/plain; charset=utf-8; format=flowed
>
> On 2019-04-21 16:13, Timothy Stark wrote:
>> Wow!  I try that new simh emulator soon.
> Definitely sounds interesting, yes...
>
>> Have you try implementing VAX 3100 M96/M98, VAX 4000 M106/M108, and VAX
>> 4000 M90 yet?
>>
>> How about VAX 66x0 that provides full 32-bit addressing for up to 3.5 GB
>> main memory?
> You know that technically, it supports 34-bit physical addresses, right?
> However, I'm not sure DEC ever built any VAX model capable of using it,
> nor do I know if VMS supports it.
>
> Virtual address space would still be 32 bits, of course.
> And as far as I know, only NVAX supported the 34-bit physical address
> space, and it does require changing some registers before the MMU page
> address registers go to 25 bits. But it might be that you also need to
> go to that mode in order to use 3.5G of physical memory. I can't
> remember for sure...
>
> The 66x0 machine I'm pretty sure didn't allow for more than 32-bit
> physical addresses. But I'm wondering about the 7000 models...
> Not sure if you could even go to 3.5G on the 66x0 machines. The CPU can
> do it, but I wonder about the bus structure.
>
>     Johnny