08. Freedom from Interference - Spatial

Freedom from Spatial Interference

Mechanisms for Ensuring Freedom From Spatial Interference

There are a few common mechanisms for ensuring freedom from spatial interference like memory protection units and dual storage of relevant data.

MPU is a prevention method because it stops elements from accessing memory to which they should not have access. An MPU can be programmed to set up the proper read, write and execute permissions between software elements.

Dual storage of relevant data like with a 2's complement is a detection method. With a 2's complement, you can detect that the data has changed and is no longer valid. But you can no longer use the data.

Other mechanisms to protect against memory interference include:

• Redundancy checks such as CRC to make sure data does not inadvertently change.
• Micro-controllers with memory error detection and correction capabilities
• Operating systems that allow software units to have their own virtual memory space

One mechanism mentioned was to store a 2's complement of safety relevant data. A 2's complement is a way of represent negative integers in binary.

A CRC (cyclic redundancy check) is a way to check if data has changed during transmission. They work by adding appending a value to the data and then ensuring that the value hasn't changed over the transmission.

Please note that for addressing deadlocks, disabling OS interrupts that would stop process preemption, is inefficient and could compromise the overall response time and system latency. An alternative is a feature that is provided by a Real Time Operating System (RTOS) is a priority ceiling .