Odd "endianness" [was Re: RE: Base 64 posts to the list]

Rich Alderson RichA at livingcomputers.org
Wed Dec 7 14:46:32 CST 2016

From: Chuck Guzis
Sent: Monday, December 05, 2016 6:15 PM

> On 12/05/2016 01:09 PM, Lars Brinkhoff wrote:

>> As Charles wrote, the PDP-10 commonly uses 7-bit bytes for ASCII
>> text, but that's only part of the truth.  The architecture is quite
>> byte size agnostic.  There are instructions to operate on any byte
>> size from 1 to 36 bits, at any position inside a word.  (Well, a
>> later extension to the architecture restricted this a bit.)

The restriction Lars mentions only applies to what are referred to as
One-Word Global Byte Pointers (OWGBPs), which encode the divisions of
a 36-bit word into 6, 7, 8, 9, or 18 bit bytes into a 6-bit value in
the high-order bits of a word with a 30-bit address filling the rest.

There are also Two-Word Global Byte Pointers (which I've never seen
abbreviated) which carry the standard "any size byte at any position"
in the first word, with a zero address in the right half, and the
30-bit extended address (with 0's in the 6 high-order bits) in the

> I've seen PDP-10 9-track tapes done two ways--one character per frame
> and then 4 frames (36 bits) with 5 7-bit characters and the sign bit
> left over.

Neither of those is entirely accurate.  9-track tapes on the PDP-10 used
one of the following encodings:

1. Core-Dump:  4 frames of 8 bits, 5th frame with 4 leading 0's (or 0100
   on one type of controller) and the last 4 bits.
2. Industry-Compatible:  4 frames of 8 bits, and ignore the low order 4.
3. ANSI-ASCII: 4 frames of 7 bits padded with a leading 0, 5th frame with
   low order bit (B35) followed by the remaining 7 bits.  In this case,
   B35 is usually 0, but in the case of line-numbered files B35 = 1 is
   the indicator that the 5 ASCII digits are a line number (and the
   parity bit is set incorrectly on the tape).
4. High-Density: 4 frames of 8 bits, 5th frame has low order 4 bits of
   the 1st word in its high order bits + high order 4 bits of the 2nd
   word in its low order bits, then 4 frames of 8 bits finishing up the
   2nd word.

I've been dealing with PDP-10 tapes for 40 years now.


Rich Alderson
Vintage Computing Sr. Systems Engineer
Living Computer Museum
2245 1st Avenue S
Seattle, WA 98134

mailto:RichA at LivingComputerMuseum.org


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