Scheduler_armasm

A minimal preemptive round-robin task scheduler for ARMv7-A, running in Linux user-space via QEMU.

Three tasks print A, B, C in a loop. SIGALRM fires every 2ms via setitimer, triggering a context switch that saves the current task's registers (r0–r12, lr) and restores the next task's.

Implementations

Two versions are available, both doing the same thing:

Directory	Language	Entry point
asm/	Pure ARMv7-A assembly	_start
c_asm/	C + ARMv7-A assembly	main() in scheduler.c, context switch in context_switch.s

The asm/ version handles everything in assembly — task init, signal setup, timer, and context switching. The c_asm/ version moves task setup and signal/timer init into C, keeping only the context switch in assembly.

Requirements

sudo apt install gcc-arm-linux-gnueabi qemu-user

Build & Run

Each implementation has its own Makefile. Navigate to the directory first:

# Pure assembly version
cd asm/
make        # assemble + link
make run    # run under qemu-arm
make debug  # attach QEMU on port 1234 for GDB
make clean

# C + Assembly version
cd c_asm/
make        # compile
make run    # run under qemu-arm
make clean

Expected Output

AAAA...BBBB...CCCC...AAAA...

Tasks interleave as the scheduler switches between them every 2ms.