11 Aug
2010
11 Aug
'10
6:35 p.m.
From: Chuck Guzis
Sent: Wednesday, August 11, 2010 5:28 PM
I recall getting the binary DECSystem10 tapes with the
odd packing of
5 7-bit characters in a 36-bit word. I don't recall how the words
were packed onto a 9 track tape, but it required some headscratching.
I do recall writing a bunch of code to unpack and translate to 6-bit
CDC display code. The DEC FORTRAN was close, but a lot of it was 36-
bit dependent, so there was a certain amount of rework there.
What's odd about it? ASCII is a 7-bit code, and if you have 36-bit words
at 5 characters per word you have a maximal use of memory. (There was also
a SIXBIT subset of ASCII which dropped the low 32 and high 32 characters
and packed 6 to a word, but it wasn't used for general computing.)
Depending on what mode tapes were written in, data got mapped into tape
frame differently, but the most common mode, again based on economy of
storage, went as follows:
B0 B1 B2 B3 B4 B5 B6 B7 P
B8 B9 B10 B11 B12 B13 B14 B15 P
B16 B17 B18 B19 B20 B21 B22 B23 P
B24 B25 B26 B27 B28 B29 B30 B31 P
0 0 0 0 B32 B33 B34 B35 P
There was even support for editor line numbers: In the editor, line
numbers are stored as ASCII strings of digits, with bit 35 turned on in
the word. It is set to 0 in each word (5 chars) of text. *On tape*,
the parity bit is forced to the wrong value in the 5th frame of a word
that contains a line number!
FIELDATA, now, *that's* odd. ;-)
Rich Alderson
Vintage Computing Sr. Server Engineer
Vulcan, Inc.
505 5th Avenue S, Suite 900
Seattle, WA 98104
mailto:RichA at vulcan.com
mailto:RichA at LivingComputerMuseum.org
http://www.PDPplanet.org/
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