8 Dec
2016
8 Dec
'16
8:55 p.m.
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/