🚀 go-pugleaf

Quadibloc <jsa...@ecn.ab.ca> writes:
>On Saturday, August 11, 2018 at 9:23:08 AM UTC-6, John Levine wrote:
>> In article <1173318279.555679658.757...@news.eternal-september.org>,
>> Peter Flass  <peter...@yahoo.com> wrote:
>> >I was
>> >sorry when it was canceled, although I suspect it would have been difficult
>> >to expand the architecture to larger memory sizes.
>
>> We don't have to guess.  They did expand the addressing for the DEC-20
>> and it was pretty ugly.  The extra address bits were called the
>> section number, with some addressing being within the current section
>> and some being global.  TOPS-20 supported it but I don't recall many
>> programs using it.
>
>The Sigma 9 from Xerox/SDS also took an existing architecture with a full-length 
>address in the instruction, and modified it to use more memory than that address 
>could support.

So did the later generations of the Burroughs B-3500.   Originally,
it had a 6 BCD digit address in the instruction, and the high-order
digit contained the data type and indexing bits (for data, for code
only the indexing bits).  So direct data addresses could only address
100,000 digits; branches could reach 300,000.   Use of an index register
allowed access to the full 1,000,000 digits.

About 1980, work started to extended the architecture to allow a program
to access much more than the 1,000,000 digits by creating a segmentation
scheme and adding seven more base-limit registers (allowing direct access
to 8,000,000 digits via an index register base-selection digit).  The
VEN (Virtual Enter) instruction could either remain in the current
"environment" or could switch to a different environment by reloading
the base-limit registers from a page-table like structure managed by the
MCP; allowing effectively unlimited addressibility, and instructions
were provided to move data between environments.

100% backward compatable with programs compiled for the B-3500.