There are many models for accessing memory: PRAM, Actor Model, Petri Nets, Tuple Spaces, Scoop, Process calculi, etc.

Since we are using the PRAM (= Parallel Random Access Machine) model this will be further discussed:

• Processes can only communicate with each other by accessing shared Memory
• MAU connects the Processes with Shared Memory

Also, it needs to be defined if reading/ writing should be Exclusive or Concurrent. Therefore, at least four ways of implementation are possible.

• Exclusive Read Exclusive Write (EREW) → only one Process can read/ write at the same time
• Exclusive Read Concurrent Write (ERCW) → only one Process read at the same time but many Processes can write at the same time
• Concurrent Read Exclusive Write (CREW) → only one Process write at the same time but many Processes can read at the same time
• Concurrent Read Concurrent Write (CRCW) → many Processes can read/ write at the same time

The netx90 uses shared Memory. Both ARM Cortex-M4 Processors can read/ write on Memory simultaneous. Therefore, the netx90 has a CRCW implementation.

Semaphores

Semaphores are used to control access to resources by multiple processes in a concurrent system. A trivial semaphore is a plain variable that is changed depending on programmer-defined conditions. 

"Add UML Diagram here" see uni ulm presentation page 16

Heap/ Stack

The program’s machine instructions (e.g. executable code) are stored in the text area. Global and static data are stored in initialized/ uninitialized data. The “malloc” operator allocates memory on the heap, released by the “free” operator. Therefore, memory allocated on the heap is always where space is left on the heap. Memory holes are the consequence. The stack contains all the automatic, non-static variables and is implemented as “least in first out” structure. The operator “push” allocates memory on the stack, “pop” releases the last allocated element from stack.

Stack-overflow

If the stack is full and a push operation is executed, the stack is considered to be in an overflow state and may override data on the heap.