Architecture

Overview

bedrock[RTOS] is a nanokernel real-time operating system. It is a platform, not a library — the kernel owns the boot process, the scheduler, and the hardware abstraction layer. Application code runs as tasks managed by the kernel.

Directory Structure

bedrock-core/
├── kernel/              Scheduler, task management, time, IPC
├── include/bedrock/     Public API headers
├── arch/                HAL implementations per architecture
│   └── arm-cortex-m/    ARM Cortex-M3 reference port
├── boards/              Board-specific linker scripts and configs
│   └── qemu-cortex-m3/
├── lib/                 Static pool allocator, utilities
├── examples/            Application templates
├── docs/                Documentation (EN / RU)
├── 3rd/tools/chorus     Build system (git submodule)
├── Kconfig              Kernel configuration
└── chorus.build         Build configuration

Kernel Components

Scheduler (kernel/br_sched.c)

Priority-based preemptive scheduler with round-robin at equal priority levels.

• CONFIG_NUM_PRIORITIES separate ready queues (default 8), priority 0 is highest
• Tasks at the same priority are scheduled round-robin (FIFO within queue)
• Scheduler lock/unlock mechanism for nested critical sections
• Context switch triggered via br_hal_context_switch() — on Cortex-M this pends PendSV

Task Management (kernel/br_task.c)

Static TCB pool — no dynamic allocation.

• Up to CONFIG_MAX_TASKS tasks (default 16)
• Each task has: ID, name, priority, stack, entry point, state
• Task states: INACTIVE, READY, RUNNING, BLOCKED, SUSPENDED
• Idle task is created automatically at lowest priority during br_kernel_init()

Time Services (kernel/br_time.c)

Tickless design — no periodic timer interrupts.

• 64-bit microsecond timestamps (br_time_t = uint64_t, ~584,000 years range)
• Sleep list sorted by wake time (ascending)
• Hardware alarm reprogrammed only when needed
• br_time_alarm_handler() wakes expired tasks and reschedules

IPC (kernel/br_ipc.c)

Three synchronization primitives:

• Semaphore — counting semaphore with configurable max count and wait queue
• Mutex — binary lock with priority inheritance to prevent priority inversion
• Message Queue — fixed-size ring buffer with separate send/receive wait queues

All wait queues are priority-ordered.

Hardware Abstraction Layer

The kernel never accesses hardware directly. All platform interaction goes through the HAL interface defined in include/bedrock/br_hal.h.

Adding a new architecture requires implementing these functions in arch/<arch_name>/. Zero kernel changes needed. See the Porting Guide.

Memory Model

• Zero malloc in kernel — all structures are statically allocated
• TCB pool: fixed array of CONFIG_MAX_TASKS entries
• Stacks: caller-provided buffers passed to br_task_create()
• IPC objects: caller declares them as variables (stack or global)
• /lib pool allocator: optional, for application-level fixed-size block allocation

Boot Sequence

1. Reset_Handler — copies .data, zeros .bss, calls main()
2. main() calls br_kernel_init():
   • Zeros TCB pool
   • Calls br_hal_board_init() and br_hal_timer_init()
   • Initializes scheduler
   • Creates idle task
3. Application creates tasks via br_task_create()
4. br_kernel_start() — picks the highest-priority ready task and starts it (never returns)

Context Switch (ARM Cortex-M)

• br_hal_stack_init() builds an initial exception frame: xPSR, PC, LR, R12, R3–R0 (hardware frame) + R4–R11 (software-saved)
• br_hal_context_switch() pends PendSV
• PendSV_Handler saves/restores R4–R11 and swaps PSP between tasks
• br_hal_start_first_task() sets PSP and switches to thread mode