AW: AW: When did Memory- and IO Protection Emerge (Esp. in Minis)?

Erik Baigar erik at
Fri May 6 08:45:55 CDT 2016

On Thu, 5 May 2016, Gottfried Specht wrote:

> How do I remember this ca. 40 years later?


> Well, while servicing these systems they would frequently stop with a 
> "Memory Protect Error" (various Operating Systems). Guess what the 
> intuitive action was: Replace the "Memory Protect Board" - which n e v e 
> r fixed the problem. So digging into the technology it became clear, 
> that the Memory Protect Board in these cases had only fulfilled its 
> duty: protect the memory below the fence register from some other piece 
> of hardware (usually a processor or DMA-board) running havoc in memory.

Essentially the board did what it was supposed to do! That is exactly
what the APM is good for in my Rolm 1602: As these machines where
used in applcations where errors in hardware or software running
havoc would have resulted in really severe problems these where
a good idea  ;-)

   Have a nice weekend,


> On Wed, 4 May 2016, Gottfried Specht wrote:
>> I'm not sure whether it qualifies for your full list, but the HP2100A
>> (that came out in 1971) had a "Memory Protect" hardware that
> Hi Gottfried,
> thanks for the excellent answer - yes I think this is exactly what matches my specification! Thanks.
> It is really astonishing how many people know a lot on various machines which is really great. I suspected that HP had something, too.
>> Fence Register: Set under program control; memory below fence is protected.
> This is a clever and somewhat outstanding feature - most others use protection on basis of blocks ar abuse the virtual memory for the purpose  ;-)
>    Best regards,
>       Erik.
>> -----Ursprüngliche Nachricht-----
>> Von: cctalk [mailto:cctalk-bounces at] Im Auftrag von Erik
>> Baigar
>> Gesendet: Dienstag, 3. Mai 2016 17:53
>> An: cctalk at
>> Betreff: When did Memory- and IO Protection Emerge (Esp. in Minis)?
>> Dear Experts,
>> during discussing the Rolms I came accross the following question:
>> What was the first (Minicomputer) architecture which offered
>> memory- and IO protection? I'd define the minimum requirements as:
>>   - Existence of a superuser mode (Rolm calls this Executive mode)
>>   - Existence of a user mode (With at least two users, Rolm offers 4)
>>   - In superuser mode, IO and memory protection for each user can be
>>     set up individually.
>>   - Any access violation is trapped and handeled by superuser code.
>>   - Of course commands for mode switching and setting up the
>>     memory and IO ranges must exist.
>> I have got a real machine (Rolm 1602) having this implemented and dating from 1975. A document on this "Access Protection Module" as Rolm calls it also is dated 1975. It consists of a microcode module which realizes an extension of the 16 bit Nova instruction set and an additinoal CPU module, taking care of the new modes and supervising the IO- and memory accesses.
>> My question is not regarding virtual memory memory, but regarding protection (IO and memory) to ensure capsulation of indivitual processes - not necessarily for multi user environments but e.g.
>> for safety critical applications...
>> Probably OS/2 in 1987 was one of the first home computer OSes to support memory protection (how about IO protection?), BSD on some Digital PDP-* was earlier (1977?) but still after the 1602.
>> Any hints out there on other "Mini" architectures of that era having someting similar?
>>     Erik.

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