Re: Ah, the days before memory protection seemed necessary...
"Of course you had to run an operating system that made use of it, or there may have been bugs in the protection mechanism."
In the late 1960s the English Electric Computers (EEC) System 4 range was IBM 360 compatible - produced from a relationship with RCA. The EEC System 4/50 was pretty much a clone of the RCA Spectra 70/45. EEC produced several OSs for their System 4 range - the top end one being the disk based "J".
EEC matched the spec of the RCA top end Spectra 70/55 - but did their own hardware design by bringing together their merged in-house design talent of the successful KDF9 and LEO 3. This machine was to have two distinct models - the System 4/70 and the 4/75. The latter having virtual memory addressing.
After much engineering debugging the prototype #1 4/70 reached the stage where it could run the first cut of a customised version of the EEC "J" disk OS. For the first stages the memory protection mechanism wasn't activated in the OS
Come the day where it was ready to try user program multi-tasking - now with the memory protection keys. It worked - and then a user program crashed with a memory protection error.
The problem was that the 4/70 cpu microcode used a few words of low memory as transient working storage for some decimal operations. The OS had protected all low memory as part of itself - and the decimal operation was running with the user program protection key.
A quick hardware mod made the relevant low memory unprotected. Problem solved. Except that the key worked on a minimum block size of memory. That area also contained the words (CAW/CCW) used to direct IO controllers. Not really a problem as only the OS could issue IO commands - until someone discovered that a user program could exploit this lack of protection in a very devious manner.
Biting the hand that feeds IT
