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

On Thu, 27 Feb 2025 01:39:07 -0000 (UTC), John Levine wrote:

If you look at the 18 bit PDP-4 you'll find one's complement ADD and
two's complenent TAD.

Just in the middle of reading that DEC “Computer Engineering>" book.
Gordon Bell admitted that having both 1’s-complement (a hangover from the
PDP-1) and 2’s-complement arithmetic in the PDP-4 was a mistake; later 18-
bit machines dropped the 1s-complement support.

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.