Mutlithreading for the Tiva C series

This project is a PoC of an ASM multithreading implementation for ARM Thumb devices.

Project contents:

• Threading (pthread.asm)
  • Start thread: (threadstart)
  • Stop thread: (threadexit)
  • Thread fencing:
    • Lock (threadlock)
    • Unlock (threadunlock)
• Memory management: (malloc.asm)
  • Malloc_init: (minit)
  • Malloc: (gtmalloc)
  • Free: (free)
  • Memcpy: (memcpy)
  • Memset: (memset)

Some notes:

To make use of threading, malloc has to be enabled (to store a thread's stack). To enable malloc, simply call minit. This prepares the first element in the double-linked list to cover the entire 32KiB heap aside from the memory allocated for the stack.

The threading system uses the built-in SysTick system to determine how many instructions each thread is delegated before a thread-switch operation is triggered. The amount of instructions allowed can be set by changing the constant STRST.

Thread fencing allows a thread to request extra time-frames for itself. As it stands at the moment, thread-fencing will always be allowed, since it was designed as a way to making thread-unsafe operations virtually atomic. I would be very hesitant to use thread-fencing, since, if timed incorrectly in, for example an infinite loop, it could cause the thread to inexplicably take full control of the processor by always being fenced when a thread-switch is triggered, thus preventing any switches. This is why I would heavily reccomend manually setting the SysTick timer value to 0 after unlocking: so that a successful thread-switch is attempted (in case one was missed when the thread was locked).

SPECIAL NOTE: The main thread cannot be exited! If you think about it, it would be unreasonable that the main thread should be exited, since there would be nowhere for the thread-switcing routine to return to. As such, the main thread will simply branch to an infinite loop when it calls threadexit. In the future, I hope to implement a flag that declares the main thread as exited, in which case it is skipped until all other threads have exited. If all threads have exited, the threading system should reasonably be disabled and the processor should probably be put into deepsleep indefinitely.

TODOS:

• Automatic thread-switching after unlock if a switch was missed
• Smarter malloc
• Dynamic thread-count updating
• Faster thread-switching
• Runtime thread-instruction variability
• Main-thread exiting (auto dsleep)

Tested platforms:

• Tiva C-series
  • TM4C123GH6PM (Evaluation board: TM4C123GXL)