Question

Each of two robot arms is programmed to lift assemblies from a conveyor belt, test them for tolerances, and place them in one of two bins depending on the results of the test. The assemblies arrive one at a time, with a sufficient interval between them. To keep both arms from trying to grab the same assembly, the computers controlling the arms share a common memory cell. If an arm is available as an assembly approaches, its controlling computer reads the value of the common cell. If the value is nonzero, the arm lets the assembly pass. Otherwise, the controlling computer places a nonzero value in the memory cell, directs the arm to pick up the assembly, and places the value 0 back into the memory cell after the action is complete. What sequence of events could lead to a tug-of-war between the two arms?

Short answer

A tug-of-war occurs if both arms read, write nonzero, and attempt to grab the assembly simultaneously.

Step-by-step solution

Understanding the Setup

We have two robotic arms, and both are programmed to lift, test, and sort assemblies into bins. They share a common memory cell to coordinate so that both do not grab the same assembly at the same time.

Function of the Memory Cell

The memory cell holds either a zero or a nonzero value. If it contains a nonzero value, it indicates that an arm is using the assembly. A zero value means the assembly is free for either arm to pick.

Decision Process for Arms

When an assembly approaches, an available arm checks the memory cell. If the cell is zero, the arm writes a nonzero value to claim the assembly and proceeds with its task. After completing the task, the arm resets the memory cell to zero.

Condition for Tug-of-War

A tug-of-war occurs if both arms read the memory cell at the same time when it contains zero. Both arms think the assembly is available, both write a nonzero value in the cell, thinking they secured the assembly. This results in both trying to grab the assembly simultaneously.

Conclusion of Sequence of Events

The sequence leading to tug-of-war is: Both arms read the memory cell simultaneously when it is zero, both write their nonzero value simultaneously, and both attempt to grab the assembly at the same time.

Key concepts

Shared Memory

In the context of computer systems, shared memory refers to a memory resource that different processors or components can access. In this exercise, the shared memory cell is used by the two robot arms to avoid conflicts when trying to grab the same assembly.

By sharing a memory cell, the arms coordinate which one will interact with the approaching assembly. The memory cell can either be free (indicated by a zero) or in use (indicated by a nonzero value). This shared memory is crucial for ensuring efficient manufacturing operations without overlap.

Use of shared memory is common in computing to facilitate communication and data exchange between different processes without requiring complex messaging systems. It's a direct way to ensure resources are managed and utilized effectively.

Synchronization

Synchronization in computing is the process of ensuring that two or more concurrent processes do not interfere with each other. In this exercise, synchronization is required to ensure that only one robot arm picks up an assembly at a time.

By using the shared memory cell, synchronization is achieved by writing a nonzero value into the cell when an arm claims an assembly. This prevents the other arm from grabbing the same piece because it will see the nonzero value and let the assembly pass.

Synchronization mechanisms help manage the timing of processes and their access to resources. They are vital for maintaining order in systems where multiple actions occur simultaneously.

Race Condition

A race condition occurs when two parallel or concurrent processes attempt to change or access shared data simultaneously, leading to unexpected or incorrect outcomes. In the robotic arm problem, a race condition could cause both arms to think they can grab the same assembly.

This exercise highlights a classic race condition scenario: both arms reading the shared memory cell at the same time when it is zero. Without proper synchronization, both might write a nonzero value simultaneously, attempting to claim the assembly.

Addressing race conditions typically involves implementing control measures like locks, which regulate access to the shared resource and ensure that only one process interacts with it at a time.

Mutual Exclusion

Mutual exclusion is a principle that ensures that a resource can be accessed by only one process at a time. It is critical in preventing race conditions. In this exercise, mutual exclusion is meant to prevent both robot arms from using the shared memory cell simultaneously.

To achieve mutual exclusion, a protocol is typically used to lock the shared resource when a process is using it. The process then unlocks it once done, allowing other processes to access it.

Implementing mutual exclusion helps in maintaining system stability, as it guarantees that processes do not interfere with each other's operations, especially when they interact with shared resources.