Re: where the PDP-8 came from, not The joy of FORTRAN

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