[Simh] Semi-OT: 7-bit binaries

[Richard Cornwell]

Hi,

   My KA10 implements the TM10 this way. Also it appears that the
   dumps on pdp10.trailing-edge.com etc are written this way.

Rich

> 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.
> 
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Richard Cornwell
rich at sky-visions.com
http://sky-visions.com
LinkedIn:   https://www.linkedin.com/in/richard-cornwell-991076107
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