Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!think.com!mintaka!spdcc!iecc!johnl
From: johnl@iecc.cambridge.ma.us (John R. Levine)
Newsgroups: comp.arch
Subject: Re: skip instructions
Message-ID: <1991May05.174307.8952@iecc.cambridge.ma.us>
Date: 5 May 91 17:43:07 GMT
References: <1182@opus.NMSU.Edu>
Distribution: comp
Organization: I.E.C.C.
Lines: 25

In article <1182@opus.NMSU.Edu> jthomas@nmsu.edu (James Thomas) writes:
>Is there a reference (or more) describing why SKIP instructions have mostly
>disappeared from instruction sets?

It seems to me the real reason was variable length instructions.  DEC
machines up through the PDP-10 all had fixed length single word
instructions, so it was obvious what skipping one would mean.  Ditto the IBM
scientific machines up through the 7094, and their clones such as the GE
635.  The IBM 360 and PDP-11 both had variable length instructions and
condition codes.

Variable length instructions make skips a lot less appealing.  I once used a
machine that had single word skips but a combination of one and two word
instructions (Varian 620, for you historians) and some impressively strange
bugs resulted from skipping into the middle of a two-word instruction.

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.

-- 
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.