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From: beemster@fwi.uva.nl (Marcel Beemster)
Newsgroups: comp.arch
Subject: Re: RISC hard to program?
Message-ID: <1145@carol.fwi.uva.nl>
Date: 18 Jul 90 08:27:36 GMT
References: <40088@mips.mips.COM> <2162@opus.cs.mcgill.ca> <3648@auspex.auspex.com> <2163@opus.cs.mcgill.ca>
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peterd@opus.cs.mcgill.ca (Peter Deutsch) writes:
>[I want no alignment restrictions] (Sorry Peter, I had to shorten it a little.)

But do you also want to pay for it? As an example take the 68040.
Suppose it has to load the following misaligned Long Word:

	Address		Data
	$0		xLLL
	$4		Lxxx

To load this Long Word starting at address $1, it actually does 3 memory
cycles (assuming cache miss). The first one to load:
	$0		xLxx,	Reading a byte.
The second one loads:
	$0		xxLL,	Reading a word.
And the third one to load:
	$1		Lxxx,	Reading the last byte.

If you don't believe me, check Section 8.2.1, page 8.8, Fig 8.4 of the
user manual. I guess they couldn't afford the smarter way (out of
microcode or something like that). And no, the second read of $0 will
not be a cache hit.

Now the question is, if you as a programmer know how misaligned access
is going to cost precious memory cycles, are you going to avoid them?
I know I do, and find the (software development) cost not that high.

++marcel					beemster@fwi.uva.nl
"So, they destroyed their planet. Why?" "That was better for the economy."