From: Paul Ruizendaal <firstname.lastname@example.org>
To: "email@example.com" <firstname.lastname@example.org>
Subject: [TUHS] CRC calculation in the 1980s
Date: Tue, 19 Sep 2023 01:02:21 +0200 [thread overview]
Message-ID: <87246EC9-0AC8-404E-A8DD-508AB046018B@planet.nl> (raw)
When looking at old xmodem code I noticed that it calculated its CRC bit-by-bit, switching to byte-wise using a table in the late 80’s. It never seems to have used the byte-wise, “on-the-fly” algorithm. This seems to match a pattern: I often come across bit-wise and table implementations, but rarely on-the-fly implementations if any. The on-the-fly algorithm was known at least since 1983, following a paper by Perez: http://www.bitsavers.org/components/fairchild/_appNotes/Byte-wise_CRC_Jun83.pdf
The paper was noted, for example it is on the citation list of RFC1134, describing the PPP protocol. Today, a wikipedia page gives implementations for various polynomials: https://en.wikipedia.org/wiki/Computation_of_cyclic_redundancy_checks#Parallel_computation_without_table
Now, it would seem to me that on memory constrained personal computers and PDP-11’s the “on-the-fly” algorithm would have been a good choice, being just a few lines of code and maybe 30-50% slower than table lookup. The tables aren’t big, but a kilobyte is a lot when you only have 64.
Any suggestions as to why the on-the-fly algorithm did not catch on more in the 1980’s? Maybe it was simply less well known than I think?
next reply other threads:[~2023-09-18 23:02 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-09-18 23:02 Paul Ruizendaal [this message]
2023-09-18 23:34 ` [TUHS] " Robert Brockway
2023-09-19 0:02 ` segaloco via TUHS
2023-09-19 15:34 ` Paul Winalski
2023-09-19 15:50 ` Dan Cross
2023-09-19 16:22 ` Paul Winalski
2023-09-19 14:10 [TUHS] " Paul Ruizendaal
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