Path: ...!weretis.net!feeder9.news.weretis.net!panix!.POSTED.2602:f977:0:1::5!not-for-mail From: Rich Alderson Newsgroups: alt.folklore.computers,comp.os.linux.misc Subject: Re: where the PDP-8 came from, not The joy of FORTRAN Date: 27 Feb 2025 19:41:55 -0500 Organization: PANIX Public Access Internet and UNIX, NYC Lines: 23 Sender: alderson+news@panix5.panix.com Message-ID: References: <20250225132209.00006cdd@gmail.com> <1191686066.762291713.992426.peter_flass-yahoo.com@news.eternal-september.org> Injection-Info: reader1.panix.com; posting-host="2602:f977:0:1::5"; logging-data="10072"; mail-complaints-to="abuse@panix.com" X-Newsreader: Gnus v5.7/Emacs 22.3 Bytes: 1839 Lawrence D'Oliveiro writes: > On Thu, 27 Feb 2025 01:39:07 -0000 (UTC), John Levine wrote: >> The 18 bit machines could address all of memory directly ... > That would have required 18-bit addresses, leaving no room for any actual > opcodes or addressing modifiers in an 18-bit instruction word. Take a look at the architecture manuals. In the PDP-4, "all of memory" is 4096KW, which is to say, 12 bits worth. The rest of the instruction is a 5 bit opcode and a 1 bit indirection indicator. With the indirection indicator alone one can build a follow on with 8KW (like the PDP-7), and clever programming takes care of things. Later systems have index registers... -- Rich Alderson news@alderson.users.panix.com Audendum est, et veritas investiganda; quam etiamsi non assequamur, omnino tamen proprius, quam nunc sumus, ad eam perveniemus. --Galen