Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!usc!ucla-cs!ucla-ma!euphemia!pmontgom
From: pmontgom@euphemia.math.ucla.edu (Peter Montgomery)
Newsgroups: comp.sys.mips
Subject: Re: double -> int
Message-ID: <1991Jun13.035702.2567@math.ucla.edu>
Date: 13 Jun 91 03:57:02 GMT
References: <1991Jun12.172740.22629@cs.cornell.edu>
Sender: news@math.ucla.edu
Organization: UCLA Mathematics Dept.
Lines: 36

In article <1991Jun12.172740.22629@cs.cornell.edu> charney@cs.cornell.edu (Mark Charney) writes:
>
>Can someone give me the documentation for trunc.w.d?
>
>I'm trying to figure out what is going on with DOUBLE->INT conversion
>on my mips chip.  I've included a C program, the associated assembly 
>language file and a disassembly. 
>
> #   5	  c = (int) a ;
>	trunc.w.d	$f6, $f4, $14
>	swc1	$f6, 4($sp)
>
>  [con.c:   5] 0x4001b0:	444ef800	cfc1	t6,$31
>  [con.c:   5] 0x4001b4:	444ef800	cfc1	t6,$31
>  [con.c:   5] 0x4001c0:	35c10003	ori	at,t6,0x3
>  [con.c:   5] 0x4001c4:	38210002	xori	at,at,0x2
>  [con.c:   5] 0x4001c8:	44c1f800	ctc1	at,$31
>  [con.c:   5] 0x4001d0:	462021a4	cvt.w.d	$f6,$f4
>x [con.c:   5] 0x4001d8:	e7a60004	swc1	$f6,4(sp)
>  [con.c:   5] 0x4001dc:	44cef800	ctc1	t6,$31
>---------------------------------------------------------

	I have Gerry Kane's "mips RISC Architecture", Prentice-Hall, 1989.

	Since register t6 is register 14 (p. D-2), I assume that the
$14 on the trunc.w.d macro specifies which scratch register to use.
I don't know why the cfc1 t6,$31 apears twice, but the code reads
the Control/Status register FCR31 (p. 6-5) into t6 and changes
its bottom bits to 01 binary before writing it back.  Those
are the RM (rounding mode) bits.  The 01 says round to zero (p. 6-7).
After the cvt.w.d, FCR31 is reset to its saved value from t6.
--
        Peter L. Montgomery 
        pmontgom@MATH.UCLA.EDU 
        Department of Mathematics, UCLA, Los Angeles, CA 90024-1555
If I spent as much time on my dissertation as I do reading news, I'd graduate.