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From: earl@mips.UUCP
Newsgroups: comp.arch,comp.lang.c
Subject: Re: String Processing Instruction
Message-ID: <245@gumby.mips.UUCP>
Date: Tue, 31-Mar-87 16:22:37 EST
Article-I.D.: gumby.245
Posted: Tue Mar 31 16:22:37 1987
Date-Received: Thu, 2-Apr-87 04:19:51 EST
References: <15292@amdcad.UUCP> <978@ames.UUCP> <15304@amdcad.UUCP> <15331@amdcad.UUCP>
Distribution: na
Lines: 37
Xref: utgpu comp.arch:729 comp.lang.c:1403
Summary: You win!

In article <15331@amdcad.UUCP>, tim@amdcad.UUCP (Tim Olson) writes:
> > Quiz: what are the magic constants that need to be loaded into t0 and
> > t1 to make this work?
> Let's see... How about
> 	t0 = 0x7efefeff
> 	t1 = 0x81010100
> Is that right?  If so, it appears to work only for 7 bit characters (but
> we admit that such a restriction is reasonable in real life)...

Yes, you've got it.  Congratulations. (Gee, do I owe you lunch or
something now?)  Actually, it works for 8-bit characters in bytes 0,
1, and 2 and a 7-bit one in byte 3 (I'm using little-endian byte
numbers here).  Thus it may accept '\200' as a zero in byte 3.  But
this is easily handled in the follow-on code that has to detect which
byte is nonzero by actually testing byte 3 and branching back to the
loop if it is nonzero.  Actually, here is that code.  Note how after
the word loop the carries also conveniently let you detect the byte.
This part only works for little-endian byte ordering.  (Note: my
personal convention is to indent instructions in branch delay slots
once space):

	## search for word with zero byte
l1:	lw	t2,1(a1)	# read next word
	addu	a1,4
	addu	t3,t2,t0	# cause carry out of any nonzero byte
	xor	t3,t2		# xor to find carry bits
	and	t3,t1		# look at carries out of bytes
	beq	t3,t1,l1	# all bytes carried => none zero
	## found word with zero byte, now find the byte
	 sll	t4,t3,23	# carry out of byte 0?
l3:	beq	t4,0,x0		# if not, go compute length
	 sll	t4,t3,15	# carry out of byte 1?
	bge	t4,0,x1		# if not, go compute length
	 sll	t4,t3,7		# carry out of byte 2?
	bge	t4,0,x2		# if not, go compute length
	 srl	t4,t2,24	# byte 3 zero?
	bne	t4,0,l1		# no, re-enter loop