[Simh] Semi-OT: 7-bit binaries

[Timothe Litt]

On 01-Feb-17 08:54, Lars Brinkhoff wrote:
> 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:
Different controllers did different things; the TM10 replicates/ors the
bits.  The Massbus tapes (TU45/77/78/79 on TM02/TM03) and the STC (TU7x
on DX10/DX20) wrote zeros.  The HSC drives (TAxx) also wrote zeros.  I
don't believe they or'd on read, but I'd have to check.  The proper
thing for normal software to do is to ignore the redundant bits.  But I
did write some tape identification software that used them as one of its
clues.

The later controllers also implemented high-density modes (which the
TM10 doesn't).  Two words in 9 frames.

There are also differences in noise record detection; the TM10 could
deal with short records that the Massbus tapes (obeying the ANSI
standard) ignored.  This created issues for some weird tape formats,
e.g. DOS magtapes with 14 byte header records (which is below the noise
record threshold) in industry mode...

Tapes are fun.

>> 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.
>
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