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

Gottfried Specht Gottfried.Specht at
Thu May 5 13:55:55 CDT 2016

Thanks, Erik.

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. That learning stuck ...

Kind regards,
Gottfried Specht | Gottfried at |  +49 211 151695+49 151 2911 2915 

-----Ursprüngliche Nachricht-----
Von: cctalk [mailto:cctalk-bounces at] Im Auftrag von Erik Baigar
Gesendet: Donnerstag, 5. Mai 2016 10:36
An: General Discussion: On-Topic and Off-Topic Posts
Betreff: Re: AW: When did Memory- and IO Protection Emerge (Esp. in Minis)?

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,


> -----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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