[ih] NCP and TCP implementations

[Steve Crocker]

To create the TIP, a second bank of memory was added to the 316 and some
interrupts and modes were added to enable switching back and forth between
banks.  It was a bit of a kludge with some unexpected interactions.  The
BBN crew finally sorted out the details and wrote a delightful titled "It's
Amazing That it Works at all."

On Tue, Mar 10, 2020 at 1:55 PM Bernie Cosell via Internet-history <
internet-history at elists.isoc.org> wrote:

> On 10 Mar 2020 at 13:23, Steve Crocker via Internet-hi wrote:
>
> > The TAC was an extension of the IMP.  The original IMP was built on
> > the
> > Honeywell 516 (and later 316) platform, which was a 16 bit twos
> > complement
> > computer.  I assume Hinden's reference to 15-bit arithmetic reflected
> > the
> > fact that the arithmetic was signed.
>
> I honestly cannot remember what the TAC was!!   Was that the TIP?
>  Regardless,
> yes, the x16s had 16-bit signed arithmetic with 10 bit addressing 9 bits
> of page
> address, 1 bit of "this page" or the 0 page, 16Kwords of memory.
>
> Things got more complicated with the 316 -- it supported 32K words.  What
> we
> did for the TIP [and maybe the TAC, whatever that was] was to keep the IMP
> *unchanged* in the bottom 16K, and then in the upper 16K we wrote a
> self-contained "host". There was some [small!] hack to fake interrupts and
> input/output to this host but to the IMP it thought it was just another
> NCP
> connected host.  It'd set up a host output buffer and instead of doing a
> hardware
> "send" it'd pass control to the upper 16K.  Similarly [at least for the
> TIP], when it
> got something in from a terminal it'd copy it into a host-input buffer and
> then
> issue an "interrupt" down to the IMP.   Worked quite well.
>
>   /Bernie\
>             Bernie Cosell
>        bernie at fantasyfarm.com
> -- Too many people; too few sheep --
>
>
>
>
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