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Re: Larger memory option for VAXstations
On 2015-08-19 19:39, Mark Pizzolato - Info Comm wrote:
On Wednesday, August 19, 2015 at 8:11 AM, Johnny Billquist wrote:
Yes, through VMB. And VMB probes the available memory. So just having
memory expanded do not need a different firmware. VMB should already
handle things. The possible issue would be if VMB on the 4000 always stops
at (say) 128M, since it would "know" that there can't be any more.
Well, yes VMB (in the ROM) does it and then produces the bitmap which describes the available memory in the RPB to the boot process all the way through to the OS.
When I added extended memory support to the simh VAX 3900 simulator, I discovered that the ROM VMB knows about the absolute memory limits and how to probe memory on that platform. It also knows about maximum expected memory AND it also performs other memory initialization tasks. One of these is to initialize the Qbus scatter/gather map. As it turns out, this map is provided by using 32KB of RAM at a constant physical address. The bitmap produced by VMB does two things: 1) it marks the Qbus Map region as not available memory, and 2) it uses RAM immediately before the map to contain the available memory bitmap. The location of the physical memory used for the Qbus map is derived from the maximum RAM the original hardware could have. When simulated system RAM exceeds the original 64MB limit, there wasn't room at the original bitmap location to describe more memory. To support extended memory size the ROM VMB had to be patched in numerous places to build the available R
beyond the Qbus map instead of before it.
Similar issues may also have existed in the 4000 ROM based VMB.
That is what might throw a wrench in here, yes.
I'm not sure what you're referring to about "change the mode of the CPU though"...
As someone else already pointed out, the mode I'm talking about only
apparently only came with the NVAX+.
In "normal" mode, the VAX Have a 9 bit field which gives an address
within a page. The rest of the virtual address is used with the page
table for the address translation. The translation table have a 23 bit
field for the physical address, which gives a total physical address
space of 30 bits.
The NVAX (or if's actually the NVAX+), can change mode, which changes
the interpretation of the page table, and adds four more bits to the
physical address, giving you a total addressable physical memory space
of 16G. That is the mode change I was talking about. Not sure if you've
seen it. Doubt simh have support for it (yet).
I have a VAX7000-720 with 1.25 Gig of memory. Not possible with the old
page table layout...
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