A small AArch64 real-time operating system built on top of the mini-arm-os foundation. Adds task switching, schedulers, and synchronization primitives — the missing pieces between a bare-metal kernel and a usable embedded runtime.

What's new vs mini-arm-os

Subsystem	Mini-arm-os	Mini-rtos
Boot + UART + heap	✅	✅
Interactive shell	✅	—
Task switching (callee-saved reg save/restore)	—	✅
Round-robin scheduler	—	✅
Priority scheduler	—	✅
Cooperative yield()	—	✅
sleep_ms() (timed blocking)	—	✅
Preemptive tick via system counter	—	✅
Counting semaphores with FIFO waiter queue	—	✅

Architecture

mini-rtos/
├── boot/startup.S           # _start: stack + bss + → kmain
├── kernel/
│   ├── kernel.c             # demo: 3 tasks sharing a UART semaphore
│   ├── task.[ch]            # Task + Context structs, static stacks
│   ├── sched.[ch]           # RR / priority pickers, context_switch glue
│   ├── context_switch.S     # 13-reg save/restore in pure assembly
│   └── sync.[ch]            # Counting semaphore with up to 8 waiters
├── drivers/
│   ├── uart.[ch]            # PL011 PL011 console
│   └── timer.[ch]           # CNTPCT_EL0 → tick events
├── include/types.h
└── linker.ld

Context switch

context_switch.S is the heart of the kernel. It saves the AArch64 ABI's callee-saved registers (x19–x28, fp, lr) plus sp into the outgoing task's Context, then loads them from the incoming task and returns. Because we restore lr, the first time a task runs it returns straight into its entry function.

context_switch:
    stp x19, x20, [x0,  #0]    // save 13 regs
    ...
    ldp x19, x20, [x1,  #0]    // restore from new task
    ...
    ret                         // returns into new task's lr

Build & run

brew install qemu aarch64-unknown-linux-gnu     # macOS
# or: sudo apt install qemu-system-arm gcc-aarch64-linux-gnu   (Linux)
./run.sh

You should see three tasks interleaving:

mini-rtos — AArch64 cooperative+preemptive kernel
══════════════════════════════════════════════════
[boot] 3 tasks created — starting scheduler

  [blinker] tick #1
  [counter] reached 100000
  [blinker] tick #2
  [counter] reached 200000
  ...

── Task table ──
ID  Name        Prio  State    Ticks
0   blinker     2     BLOCKED  14
1   counter     3     READY    62
2   monitor     1     RUNNING  3

Exit QEMU with Ctrl-A then x.

Switching the scheduler policy

In kmain():

sched_init(SCHED_ROUND_ROBIN);   // round-robin
sched_init(SCHED_PRIORITY);      // priority (lower number = higher priority)

License

MIT