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

Curious Marc curiousmarc3 at
Wed May 4 15:43:56 CDT 2016

My 1974 HP 21MX, descendant of the HP 2100A, sure inherited this Memory Protect card. One register that you load, prevents access to any memory below the address of the register.

From:  cctalk <cctalk-bounces at> on behalf of Gottfried Specht <Gottfried.Specht at>
Reply-To:  "cctalk at" <cctalk at>
Date:  Wednesday, May 4, 2016 at 9:18 AM
To:  "cctalk at" <cctalk at>
Subject:  AW: When did Memory- and IO Protection Emerge (Esp. in Minis)?


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 

"Gives the security necessary to protect a defined area of memory from alteration by a user program. 
Priority: Second highest priority interrupt (shared with memory parity). 
Operation: Initiated under program control; protects any amount of memory. 
Fence Register: Set under program control; memory below fence is protected.
Interrupt: To trap cell for system routine when user program: 
   a) Attempts to alter a protected location 
   b) Attempts to jump into the protected area 
   c) Attempts to execute an 1/0 instruction 
Violation Register: Contains memory address of violating instruction."

(from the 1972 HP2100A Processor Description)

The predecessor HP2116B (of 1968 vintage!) also had a "Memory Protect" board as an option, I cannot confirm it had the same functionality as the HP2100A above.

IIRC, this was the main mechanism to protect the OS (RTE, DOS) from user code.


More information about the cctalk mailing list