On Mon, 5 Oct 2026, [email protected] wrote:
Christian Corti wrote:
It would be interesting to know if the memory layout with 48 bits plus
tag/parity bits was inspired from the TR 4.
Almost certainly not. The B6500/6700 was the successor to the Burroughs
B5500, which had a word consisting of 48 bits plus parity. 48 bits was
not an uncommon word size in the early '60s, e.g., Atlas, CDC 1604,
Datamatic 1000, KDF9, Honeywell 800, as it allowed 8 six-bit characters
per word.
The B5500 distinguished data words from control words by means of the
high-order bit in the word, known as the "flag bit." Alas, embedding the
flag bit within the 48 bits turned out to be a lousy idea for character
manipulation, and the machine ended up with an entirely separate mode of
operation to handle character strings, which ignored the flag bit.
In designing the B6500, a big priority was to do something about the
flag bit, and the obvious thing was to move it outside the 48-bit data
word. But there were other bits in the B5500 control words that
indicated the type of control word, so those bits were combined with the
flag bit in a three-bit external tag field. Modern successors to the
B6500/6700 have four tag bits.
Yes, much like the additional four bits of the TR 4 and its successor, the
TR 440. Two bits for the four possible data types, and two bits for the
Dreierprobe (sort-of parity bits). The Burroughs is the only machine with
a similar approach, and since the TR 4 was before the B5500 I was thinking
if Burroughs had a look over to Telefunken. At least, Tfk marketed their
fixed-head disk modules as TSP 500. So there were possibly more and deeper
cooperations between the two.
Just thinking loudly ;-)
Christian
