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

[Rich Alderson]

From: Chuck Guzis
Sent: Wednesday, December 07, 2016 10:43 PM

> On 12/07/2016 12:46 PM, Rich Alderson wrote:

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

> The last time that I had to deal with PDP-10 tapes, admittedly also 40
> years ago was essentially core-dump format.  5 7-bit characters per
> word, with one bit unused; words packed end-to-end; i.e. 9 frames for 2
> PDP 10 words.

That sounds more like high density format, if there were 9 frames for 2
words (i.e., 72 bits total).  Core dump format would require 10 frames.

The 5 characters per word is irrelevant to a discussion of tape, whether
9- or 7-track:  That's how ASCII text was represented in memory, on disk,
on DECtape, or on any other word-oriented medium.  Representing the bits
in an ASCII character by the character itself (to make divisions on the
tape more clear), this appears diagrammatically as follows:

Text: HELLOworld

Memory:
HHHHHHHEEEEEEELLLLLLLLLLLLLLOOOOOOO_wwwwwwwooooooorrrrrrrlllllllddddddd_

Core Dump:      High Density:   SIXBIT: (7 track)
HHHHHHHE        HHHHHHHE        HHHHHH
EEEEEELL        EEEEEELL        HEEEEE
LLLLLLLL        LLLLLLLL        EELLLL
LLLLOOOO        LLLLOOOO        LLLLLL
....OOO_        OOO_wwww        LLLLOO
wwwwwwwo        wwwooooo        OOOOO_
oooooorr        oorrrrrr        wwwwww
rrrrrlll        rlllllll        wooooo
lllldddd        ddddddd_        oorrrr
....ddd_                        rrrlll
                                lllldd
                                ddddd_

where _ represents the unused bit 35 in the word and . represents the
don't-care bits inserted by the tape controller/formatter for core dump.

                                                                Rich


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

mailto:RichA at LivingComputerMuseum.org

http://www.LivingComputerMuseum.org/