27 Jun
2011
27 Jun
'11
11:32 a.m.
From: Chuck Guzis
Sent: Friday, June 24, 2011 7:12 PM
I got the DEC PDP-10 source code for Adventure from a
DEC field
engineer about 1974-75 on a reel of 9-track tape (don't laugh--we
were still heavily into 7-track). I was surprised to see that it was
text packed into binary form--3 7-bit ASCII characters per 36-bit
word (you'd have thought that if it was character data, it'd be one
character per tape character); converted it to 6 bit display code and
unraveled the DEC-10-specific FORTRAN constructs into their CDC 6000
versions (harder than you'd think, particularly with Hollerith
constants); transformed the database to be readable by 6000-series
FORTRAN I/O and I was off and running.
I can guarantee you that the ASCII text was packed 5x7bit into the 36 bit
words. (I had access to the same source as you worked from since 1977.)
Why would anyone waste 25 bits per word to represent ASCII text???? (The
hardware includes byte manipulation instructions which work very nicely.)
And tape frames are 8 bits + parity, which loses a bit per 7-bit character
so why do that? Let the tape controller handle packing a 36 bit word into
5 tape frames, and stop worrying about it. :-)
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.LivingComputerMuseum.org/