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Re: Larger memory option for VAXstations
On 2015-08-20 19:58, Mouse wrote:
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.
According to the VARM I have a scan of (revision J, of December 1989),
there are no 23-bit PFNs. It documents 21-bit-PFN and 25-bit-PFN PTE
formats, but none with 23-bit PFNs.
Argh! Simple math gives the same... With a 9 bit field for the address
within a page, and a total address size of 30 bits, the PFN is
(obviously) 21 bits and not 23. Today I suck at math... :-)
It is true that PTEs on a 21-bit-PFN system have two additional bits
off the high end of the PTE which are documented as "Reserved for
DIGITAL software", presumably really meaning "reserved for the
operating system", but then there are two more such bits beyond those,
and one MBZ bit, before getting to the modify bit.
Is there an even more recent VARM out there? Or is this a retcon on a
previous version which _did_ have 23-bit PFNs? I have a paper VARM
from years earlier (1982?), but it's still packed away from my move, so
I can't easily check it.
No. You are reading it right. I just can't count. :-)
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