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From: jallen@eeserv1.ic.sunysb.edu (Joseph Allen)
Newsgroups: comp.arch
Subject: Re: skip instructions
Message-ID: <1991May10.004650.7258@sbcs.sunysb.edu>
Date: 10 May 91 00:46:50 GMT
References: <1182@opus.NMSU.Edu> <1991May05.174307.8952@iecc.cambridge.ma.us>
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Distribution: comp
Organization: State University of New York at Stony Brook
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In article <1991May05.174307.8952@iecc.cambridge.ma.us> johnl@iecc.cambridge.ma.us (John R. Levine) writes:
>In article <1182@opus.NMSU.Edu> jthomas@nmsu.edu (James Thomas) writes:
>>Is there a reference (or more) describing why SKIP instructions have mostly

>When viewed as a condition prefix on an arbitrary instruction, a skip is a
>pretty nice construct, and I expect would be easy to teach a whizbang
>optimizer to use.  Now that fixed length instructions are coming back, I
>expect that we'll see more skips again.

Yes! lets get rid of those icky condition flags.  How about a "hop"
instruction?   A register to register/immediate compare with condition flags
and a short branch field.  Sounds CISCy but could the compare fit in the time
wasted to dump the pipeline?  I.E., so if it's 2 cycles it wouldn't matter
since it's replacing a compare and a branch.

Also, when are we going to see multiple error return points from OS calls? 
I.E., if no error goto return point+1, if error goto return point+0 which
contains a jump to the error handler.  DEC20 people know what I'm talking
about.

>John R. Levine, IECC, POB 349, Cambridge MA 02238, +1 617 492 3869
>johnl@iecc.cambridge.ma.us, {ima|spdcc|world}!iecc!johnl
>Cheap oil is an oxymoron.

-- 
/*  jallen@ic.sunysb.edu  */     /* Amazing */     /* Joe Allen 129.49.12.74 */
int a[1817];main(z,p,q,r){for(p=80;q+p-80;p-=2*a[p])for(z=9;z--;)q=3&(r=time(0)
+r*57)/7,q=q?q-1?q-2?1-p%79?-1:0:p%79-77?1:0:p<1659?79:0:p>158?-79:0,q?!a[p+q*2
]?a[p+=a[p+=q]=q]=q:0:0;for(;q++-1817;)printf(q%79?"%c":"%c\n"," #"[!a[q-1]]);}