Stack size allocation in FreeRTOS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this FreeRTOS stack size allocation code?

Consider the following FreeRTOS task creation snippet. What is the stack size allocated to the task in bytes?

FreeRTOS

xTaskCreate(
  vTaskCode,       // Task function
  "Task1",        // Task name
  100,             // Stack size
  NULL,            // Parameters
  1,               // Priority
  NULL             // Task handle
);

A 400 bytes

B 100 bytes

C 100 words (400 bytes on 32-bit system)

D 100 bits

💡 Hint

Remember that FreeRTOS stack size is specified in words, not bytes.

FreeRTOS stack size parameter is the number of words, not bytes. On a 32-bit system, one word is 4 bytes, so 100 words equal 400 bytes.

🧠 Conceptual

intermediate

Why is stack size specified in words in FreeRTOS?

FreeRTOS stack size is given in words, not bytes. Why is this design choice made?

A Because the stack grows in word-sized units and it is more efficient to count words

B Because bytes are not supported in embedded systems

C Because FreeRTOS only supports 16-bit processors

D Because specifying bytes would cause stack overflow errors

💡 Hint

Think about how the CPU accesses memory and stack alignment.

Stack memory is accessed in word-sized chunks (e.g., 4 bytes on 32-bit systems). Counting stack size in words matches CPU architecture and is efficient.

🔧 Debug

advanced

Identify the error in this FreeRTOS stack size allocation

What is wrong with this task creation code regarding stack size?

FreeRTOS

xTaskCreate(
  vTaskCode,
  "Task2",
  512,  // Stack size
  NULL,
  2,
  NULL
);

A Stack size is too large and causes memory overflow

B Stack size is specified in bytes but FreeRTOS expects words

C Stack size is specified in words but the value is too small

D No error, this is a valid stack size allocation

💡 Hint

Consider the meaning of the stack size parameter and typical stack sizes.

The stack size parameter is in words. 512 words is a valid stack size (2048 bytes on 32-bit system). This is not an error.

❓ Predict Output

advanced

What happens if stack size is set too small in FreeRTOS?

Given a task with stack size set to 10 words, what is the most likely outcome when the task runs?

A Stack overflow occurs causing unpredictable behavior or crash

B The task runs normally without issues

C The task stack automatically expands to fit needs

D The task is rejected by the scheduler at creation

💡 Hint

Think about what happens if the stack is too small for task needs.

If the stack size is too small, the task will overflow its stack, causing memory corruption and unpredictable behavior or system crash.

🚀 Application

expert

Calculate total RAM used by three FreeRTOS tasks with different stack sizes

You create three tasks with stack sizes 50, 100, and 150 words respectively on a 32-bit system. How much total RAM in bytes is allocated for their stacks?

A 12000 bytes

B 1200 bytes

C 120 bytes

D 1200 words

💡 Hint

Remember to convert words to bytes and sum all stacks.

Total words = 50 + 100 + 150 = 300 words. On 32-bit system, 1 word = 4 bytes. Total bytes = 300 * 4 = 1200 bytes.