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From: Rich Alderson <news@alderson.users.panix.com>
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
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Lawrence D'Oliveiro <ldo@nz.invalid> 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