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From: knudsen@ihwpt.UUCP (mike knudsen)
Newsgroups: net.micro.6809
Subject: Don't use C "register"
Message-ID: <593@ihwpt.UUCP>
Date: Sat, 30-Nov-85 15:54:25 EST
Article-I.D.: ihwpt.593
Posted: Sat Nov 30 15:54:25 1985
Date-Received: Sun, 1-Dec-85 03:40:54 EST
Distribution: net
Organization: AT&T Bell Laboratories
Lines: 20


The dhrystone benchmarks just posted
have re-discovered what I've suspected all along:
namely "registers" do no good, and may do harm, in MW C
for the 6809!  Reason: the U Reg is simply substituted for a
RAM location, so the LD's and STD's become TFR's.
TFR is the slowest instruction in the 6809 set.  It is quicker
to move to/from RAM (in almost any addressing mode)
than to TFR between two regs.
If you "reg" a *int, MW C still
TFR's it to X to do the indexing.
(And incrementing, if needed).
Then TFRs back to U.   AAAgh!
No ethinic jokes, please--
but you've been warned!
You want speed -- declare everything "direct extern"
except arrays!
     mike k
PS: My own "sieve" tests last year made me suspicious enough
to look at the assembly code.