In this lesson, you will learn how to design and manipulate the Task Control Block (TCB) structure, the fundamental data structure used by an RTOS kernel to keep track of a task’s identity, health, and execution context.
The Role of the TCB
When you have multiple tasks running, the CPU needs a way to "remember" where each task left off and how important it is relative to others. The TCB acts as a task's unique "ID card" and "workspace" combined. Whenever the scheduler decides to switch from Task A to Task B, it essentially pauses Task A, saves its current CPU state into its TCB, and loads the CPU registers from Task B’s TCB.
Structuring the Metadata
A minimal TCB typically resides in RAM and contains the task's current stack pointer, its scheduling priority, and its current state (e.g., Running, Ready, or Blocked). By grouping these fields into a struct, the kernel can easily point to different tasks by simply changing a pointer.
Consider this simplified C representation:
typedef struct tcb {
uint32_t *stack_pointer; // Pointer to the current top of task stack
uint8_t priority; // Scheduling priority level
uint8_t state; // Current lifecycle state
struct tcb *next_tcb; // Link to the next task for the ready list
} TCB_t;
Manipulating the TCB
During a context switch, the kernel performs a "save and restore" operation. First, it pushes the current CPU registers onto the stack of the task being swapped out. Then, it saves that stack address into the stack_pointer field of that task's TCB. To resume a different task, the kernel simply looks up that task's stack_pointer from its TCB, loads it into the CPU's stack pointer register, and pops the saved registers back into the hardware.
By maintaining these blocks in a linked list or an array, the scheduler can quickly traverse them to find the highest-priority task that is currently "Ready," ensuring the system remains deterministic and responsive to real-time events.
Key Takeaway: The TCB is the kernel’s primary data structure for persistent task metadata, serving as the anchor for both execution state and scheduling decisions during a context switch.