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[ih] NCP and TCP implementations

H Steve,

> On Mar 10, 2020, at 11:03 AM, Steve Crocker via Internet-history <internet-history at elists.isoc.org> wrote:
> Argh.  I echoed Leo's use of "TAC."  I read it as referring to the TIP.  If
> I recall correctly, the "TAC" was an access control method on the TIPs.
> "TIP Access Control" I think.

The Terminal Access Controller (TAC) was a TIP like machine that ran TCP/IP instead of NCP.   I think it was done for the DDN, but my memory on that part is hazy.


> Steve
> 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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