Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.2 9/18/84 exptools; site ihwpt.UUCP
Path: utzoo!watmath!clyde!cbosgd!ihnp4!ihwpt!knudsen
From: knudsen@ihwpt.UUCP (mike knudsen)
Newsgroups: net.micro.6809
Subject: Re: Don't use C "register"
Message-ID: <602@ihwpt.UUCP>
Date: Tue, 3-Dec-85 19:15:10 EST
Article-I.D.: ihwpt.602
Posted: Tue Dec  3 19:15:10 1985
Date-Received: Thu, 5-Dec-85 06:14:43 EST
References: <593@ihwpt.UUCP> <1130@gitpyr.UUCP>
Distribution: net
Organization: AT&T Bell Laboratories
Lines: 24

>   How do you figure?  The 'tfr' instruction takes 6 machine cycles, and
> a load or store using direct addressing averages 5 cycles.  Using even
> the simplest addressing mode, direct, is going to cost you on the average
> of 4 cycles more than 'tfr', and the other addressing modes (inherent
> excluded) are even more expensive.

Okay, I admit to ambiguity in my posting, which was done under adverse
conditions.  I meant the ROUND-TRIP timing: it is STILL faster to
LDX from RAM, twiddle it, and STX back to RAM,
than to TFR U,X, twiddle, and TFR X,U.

Also, TFR takes 8 cycles for doulbe-byte regs; 6 cycles is for A and B.

>   The 'cwai', 'swi2', and 'swi3' instructions are tied for being the
> slowest on the 6809; they all consume 20 cycles.

Of course you're right.  I meant "slowest of normally used 
instructions."  CWAI is used to go to sleep, so doesn't really count.
I'm afraid that under OS-9, SWIs are pretty "normal" too.

I still say: Don't use TFR if you can help it,
and don't declare register in MW C.
	mike k