Deutsch English Français Italiano |
<vq35t1$cbc$1@gal.iecc.com> View for Bookmarking (what is this?) Look up another Usenet article |
Path: ...!news.iecc.com!.POSTED.news.iecc.com!not-for-mail From: John Levine <johnl@taugh.com> Newsgroups: comp.arch Subject: Re: Why VAX Was the Ultimate CISC and Not RISC Date: Mon, 3 Mar 2025 02:58:41 -0000 (UTC) Organization: Taughannock Networks Message-ID: <vq35t1$cbc$1@gal.iecc.com> References: <vpufbv$4qc5$1@dont-email.me> <2025Mar1.232526@mips.complang.tuwien.ac.at> <vq2dfr$2skk$1@gal.iecc.com> <2025Mar2.234011@mips.complang.tuwien.ac.at> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Injection-Date: Mon, 3 Mar 2025 02:58:41 -0000 (UTC) Injection-Info: gal.iecc.com; posting-host="news.iecc.com:2001:470:1f07:1126:0:676f:7373:6970"; logging-data="12652"; mail-complaints-to="abuse@iecc.com" In-Reply-To: <vpufbv$4qc5$1@dont-email.me> <2025Mar1.232526@mips.complang.tuwien.ac.at> <vq2dfr$2skk$1@gal.iecc.com> <2025Mar2.234011@mips.complang.tuwien.ac.at> Cleverness: some X-Newsreader: trn 4.0-test77 (Sep 1, 2010) Originator: johnl@iecc.com (John Levine) Bytes: 2525 Lines: 27 According to Anton Ertl <anton@mips.complang.tuwien.ac.at>: >>I know ARM was pretty good but VAX >>was fantastic since they sacrified everything else to compact instructions. > >I don't think they did. They spent encoding space on instructions >that were very rare, and AFAIK instructions can be encoded that do not >work (e.g., a consant as destination). The major idea seems to have >been orthogonality, not compactness. It certainly was orthogonal. I was thinking that they had one-, two-, and four- byte offset versions of all of the relative addressing modes, which made the code smaller at the cost of forcing operands to be decoded one at a time since you couldn't tell where the N+1st operand was until you'd looked at the Nth. Nearly all opcodes were one byte other than the extended format floating point instructions so it's hard to see how they could have made that much smaller without making it a lot more complicated. On the other hand, we can compare it to the S/360 instruction set which was fairly compact but a lot easier to decode, e.g., you could tell from the high bits of the first opcode byte how long each instruction was and where the operands were so you could decode the rest and do address calculations in parallel. -- Regards, John Levine, johnl@taugh.com, Primary Perpetrator of "The Internet for Dummies", Please consider the environment before reading this e-mail. https://jl.ly