Odd "endianness" [was Re: RE: Base 64 posts to the list]
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
I've been dealing with PDP-10 tapes for 40 years now.
Vintage Computing Sr. Systems Engineer
Living Computer Museum
2245 1st Avenue S
Seattle, WA 98134
mailto:RichA at LivingComputerMuseum.org
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