Context is a repository for me to explore how a multi-process system would work on a single core microcontroller. The name "context" comes from the crucial component to this experiment: the "context switch". This is where your CPU is running some code, and then for some reason needs to shelve what it's working on and switch to a new task. This is common when servicing interrupt handlers.
When an interrupt comes in, the CPU jumps to an address saved in the mvec register (in RISC-V) and begins running code setup there. The programmer will have setup the code there to save the environment before the interrupt: CPU registers, the old Program Counter (PC), which in RISC-V is obtained after the interrupt in the mepc register. Then the interrupt handler runs. After it runs the old running environment - or context - is loaded back out of memory restoring the old CPU state. This means execution before the interrupt continues merrily as if nothing happened.
This is not dissimlar to switching Tasks in a multi-process system. How does that actually get set up? How does one actually make a multi process system? This is what this repo is here to help do.
Currently this is technically working, but not well. For example, when two tasks are using the UART to print text to the host terminal at the same time there's a chance both tasks deadlock. In part this is due to having no mediation and both tasks running in Machine mode are contesting the shared resource.
- Run tasks at a lower priv level
- Shared resources should be accessed only through a basic kernel, which mediates access
- Understand the IllegalInstruction Exceptions that sometimes occur