[ih] IBM ASCII, was Re: Impact of history on today's technology [was: why did CC happen at all?]

[John Levine]

>That's loony, but reality seems to be even weirder! From the article below
>it looks like they shifted the top two bits up one and put a copy of the
>top bit in the gap.

It's all documented in the IBM 360 Principles of Operation.  And yes, it's loony.

>I get the impression from http://www.bobbemer.com/P-BIT.HTM that it mostly
>would not have worked.

I'm not aware of any software, ever, that used the 360's ASCII mode
bit other than diagnostics.  The 370 reused the bit to turn on virtual
memory, which in view of how carefully they preserved upward
compatibility everywhere else, strongly suggests that literally nobody
used it.

In the late 1960s when IBM supported ASCII labeled mag tapes (ANSI
X3.27) they were normal 7 bit ASCII and the system translated as
necessary to and from EBCDIC.

R's,
John