PD_ParallelArchitecture TCPP_Architecture systems touch visual

Originally described by Andrew Kitchen, Nan Schaller and Paul Tyman (Kitchen1992).

No link to independent description available. See Details for a synopsis.


The goal of this activity is to help students visualize the difference between execution on a SIMD architecture vs a MIMD architecture. Each student plays a processor and is given a coin.

They are presented with the following algorithm:

1. flip coin.
2. check if heads.
2. If so, raise hand.

The goal is to determine the total number of heads over all the coin flips.

Illustrating MIMD

When the instructor says “go”, each executes the algorithm and raises their hand if they receive heads. The instructor counts the number of raised hands to determine the final result. It should be emphasized to students watching the exercise that each student acts independently and executes the algorithm at their own pace.

Illustrating SIMD

The instructor reads the instructions out loud step by step. As the instructor reads each instruction, the student processors perform the instruction in lock step fashion. Students can only perform their instruction when the instructor reads it aloud. Thus,

At the end, the instructor counts off the number of raised hands to determine the number of heads. This exercise emphasizes to students that SIMD instructions are executed in lock-step fashion, in comparison to MIMD, where each thread has more autonomy to execute their instructions.

CS2013 Knowledge Unit Coverage

PD/Parallel Architecture

Core Tier 2

3. Characterize the kinds of tasks that are a natural match for SIMD machines.

Elective: 4. Describe the advantages and limitations of GPUs vs. CPUs.

TCPP Topics Coverage

Architecture Topics


This activity may be difficult for students who are visually impaired. To enable those students to participate in the exercise, we suggest giving students a larger coin or tile (say a dominos tile or mahjong tile), where the difference between the two sides can be clearly distinguished.


In (Kitchen1992), it is mentioned that a component of this exercise was used at an NSF Undergraduate Faculty Enhancement workshop at Colgate University in the Summer of 1991. A panelist at SuperComputing’91 also mentioned that the SIMD component of this exercise was used successfully in an introductory parallel computing course at Bucknell University. No formal assessment mentioned.