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

According to Peter Flass  <peter_flass@yahoo.com>:

Now, for *real* fun, try the PDP-8, where you don't even have a
subtract instruction...!

 
Two's complement to the rescue.... :)

>
That’s why add is TAD.

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

The PDP-5, of which the PDP-8 was an improved reimplementation was a cut down
PDP-4. The DEC computer engineering book says so and it's clear if you compare
the artchitectures.

Just like the -5 and -8, the -4 had a single accumulator and a link bit, single
level indirect addressing, and an instruction set that included JMS JMP TAD ISZ
AND instructions just like the PDP-5/8, a very similar microcoded operate
instruction, and auto-index locations 10-17. Interrupts did a JMS 0. The I/O
instructions had a similar structure.

The 18 bit machines could address all of memory directly so the current
page/page 0 addressing hack was new for the PDP-5.  It is impressive
that they managed to cram so much of their 18 bit machine into a tiny
12 bit design.

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