[Simh] Semi-OT: 7-bit binaries

[Lars Brinkhoff]

Rich Alderson wrote:
> On 9-track tape, in the default mode for all PDP-10 operating systems,
> referred to as core-dump mode, 36 bit words are stored into 5 8-bit
> frames (with logical parity) as follows:
>
> bits 0-7    x x x x x x x x P
> bits 8-15   x x x x x x x x P
> bits 16-23  x x x x x x x x P
> bits 24-31  x x x x x x x x P
> bits 32-35  0 0 0 0 x x x x P

I thought so too.  But I had a reason to look into chapter 3 of the TM10
maintenance manual, and I saw something interesting.

Apparently, the TM10 does a slight variation (or is this the canonical
version?) of the core dump format.  It writes the last *six* bits to the
last frame, so bits 30 and 31 are actually written to two places:

> bits 24-31  x x x x x x y y P
> bits 32-35  0 0 y y z z z z P

When reading, it inclusive-ors the overlapping bits.