1 Oct
2026
1 Oct
'26
6:54 a.m.
I worked on A Series (B6500/e-mode) system software at Burroughs/Unisys in my first job
out of college. I gave a presentation on this stuff at VCF PNW earlier this year.
Among other things, tags that indicate the use of the word is used to make data can’t be
executed as code.
alan
On Oct 1, 2026, at 06:34, Antonio Carlini via cctalk
<cctalk(a)classiccmp.org> wrote:
On 30/09/2026 22:15, Mark Kahrs via cctalk wrote:
Minor point: The Burroughs B6500/6700 has 51 bits
because it had 3 bits of
tag on each word. But the underlying CPU was 48 bits.
I looked through
https://bitsavers/pdf/burroughs/B6500_6700/1043676_B6500_RefMan_Sep69.pdf and round about
p6-5 onwards it goes through a bunch of instruction formats. the tag bits seem to indicate
whether this is a Word Data descriptor or Segment Descriptor or a Mark Stack Control Word
or whatnot. They look like they are meaningful bits rather than say parity or ECC which
are there to catch errors.
It does however say (p2-1 Data Representation) that the "tag bits are inaccessible
to normal state (user programs)".
I know nothing about Burroughs other than what I've just read but the tags seem to be
an intrinsic part of the processing. It sounds like they are not just internal stuff built
by the HW but that a compiler (or whatever created a program on the B6500) had to create
the tags as part of that?
Antonio
--
Antonio Carlini
antonio(a)acarlini.com