[Info-vax] In memoriam: 10 years since Alpha's passing away.

[Johnny Billquist]

On 2011-07-06 15.39, Bob Koehler wrote:
> In article<c842666c-614c-44d8-b5e2-2d62c55cd129 at e21g2000vbz.googlegroups.com>, Hans Vlems<hvlems at freenet.de>  writes:
>> On 5 jul, 11:34, Michael Kraemer<M.Krae... at gsi.de>  wrote:
>>> urbancamo schrieb:
>>
>> The VAX was a _true_ 32 bit architecture, right?
>
>     The original VAX hardware architecture was spec'ed to match the 30
>     bit SBI on the 11/780.  For a long time VAXen had 30 bit or narrower
>     busses.  Although laid out as a 32 bit architecture in most ways, it
>     didn't become true 32 bit until an ECO to the spec several years later,
>     when VAXen were designed with wider busses.

Not really, actually. The hardware address of the original VAX design is 
actually only 30 bits. The page address register in the MMU have 21 bits 
for the physical address, while the virtual address provides the 9 low 
bits, which means you can never get more than 30 address bits from the 
architecture.
This had nothing to do with the SBI, which, by the way, limited things 
even more, if I remember right.

This was eventually addressed in the NVAX, which added a second mode to 
the MMU page address registers, where 4 previously unused/undefined bits 
were added to the address part, making the actual physical address 
formed to be 34 bits on the VAX. But that requires you to set up the MMU 
in a different mode.

>     None of which mattered to us programmers, who had 31 bits of process
>     specific virtual address space and no programs to fill it up.

Indeed.

>     Although I'm sure others did, I never actually saw any VAX with
>     SYSGEN parameters set up to map as many as 24 bits of virtual
>     address space.

It's not a SYSGEN issue, as it is not a software thing. It's a hardware 
limitation which is very hard.

	Johnny