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:ri...@livingcomputermuseum.org http://www.LivingComputerMuseum.org/