PendSV and SysTick exceptions in ARM Architecture - Step-by-Step Execution

Concept Flow - PendSV and SysTick exceptions

SysTick Timer Counts Down

↓

SysTick Exception Triggered

↓

SysTick Handler Executes

↓

PendSV Exception Set Pending

↓

PendSV Exception Triggered

↓

PendSV Handler Executes

↓

Context Switch or Deferred Task

↓

Return

SysTick counts down and triggers its exception, which can set PendSV pending. PendSV runs later to handle context switching or deferred tasks.

Execution Sample

ARM Architecture

void SysTick_Handler() {
  // Set PendSV pending
  SCB->ICSR |= SCB_ICSR_PENDSVSET_Msk;
}

void PendSV_Handler() {
  // Perform context switch
  // Context switching code here
}

SysTick handler sets PendSV pending; PendSV handler performs context switch.

Analysis Table

Step	Event	Action	Exception Triggered	Handler Executed	Result
1	SysTick timer reaches zero	SysTick exception triggered	SysTick	SysTick_Handler	PendSV exception set pending
2	PendSV exception pending	PendSV exception triggered after SysTick	PendSV	PendSV_Handler	Context switch performed
3	Context switched	Return to new task context	None	None	System continues with new task

💡 PendSV handler completes context switch; no more exceptions pending

State Tracker

Variable	Start	After Step 1	After Step 2	Final
SysTick_Count	Non-zero	Zero (trigger)	Zero	Reloaded after interrupt
PendSV_Pending	False	True (set by SysTick)	False (handled)	False
Current_Task	Task A	Task A	Task B (switched)	Task B

Key Insights - 2 Insights

Why does SysTick set PendSV pending instead of doing the context switch directly?

SysTick_Handler sets PendSV pending to defer the context switch to a lower priority handler, avoiding interrupt nesting and ensuring safe switching (see execution_table step 1 and 2).

When exactly does PendSV execute after being set pending?

PendSV executes after all higher priority exceptions finish, here after SysTick_Handler completes (see execution_table step 2).

Visual Quiz - 3 Questions

According to the execution_table, what happens immediately after SysTick exception triggers?

A System returns to main task

B PendSV exception is set pending

C Context switch is performed

D SysTick timer reloads

💡 Hint

Look at execution_table row 1, 'Result' column

At which step does the actual context switch occur?

A Step 2

B Step 1

C Step 3

D After Step 3

💡 Hint

Check execution_table 'Handler Executed' and 'Result' columns

If PendSV was not set pending by SysTick, what would change in the variable_tracker?

A SysTick_Count would not reach zero

B Current_Task would switch earlier

C PendSV_Pending would remain False after Step 1

D No change in variables

💡 Hint

See variable_tracker row for PendSV_Pending after Step 1

Concept Snapshot

PendSV and SysTick exceptions work together for task switching.
SysTick triggers regularly and sets PendSV pending.
PendSV runs later at lower priority to switch tasks safely.
This avoids nested interrupts and keeps system stable.
SysTick counts down; PendSV handles context switch.

Full Transcript

In ARM architecture, the SysTick timer counts down and triggers the SysTick exception when it reaches zero. The SysTick handler runs and sets the PendSV exception as pending. PendSV is a special exception designed to run at a low priority. It executes after higher priority exceptions finish, allowing it to perform context switching or deferred tasks safely. This separation avoids nested interrupts and keeps the system stable. The execution flow starts with SysTick triggering, then PendSV pending is set, and finally PendSV runs to switch the current task context. Variables like SysTick_Count, PendSV_Pending, and Current_Task change accordingly during this process.