Day 8 was about Process. Though, modern operating systems use the concept of threads to provide execution context. We treat processes like individual threads being run for our implementation.

First we define a structure for PCB (Process Control Block). We first define PROC_UNUSED and 0 and PROC_RUNNABLE as 1. Then we create a structure for a unit process with the following slots:

• Process ID as an Integer.
• State (PROC_UNUSED or PROC_RUNNABLE) as an Integer as defined before.
• Stack Pointer as a vaddr_t (Virtual Address Type).
• Kernel Stack as a list of unsigned 8bit Integers.

The Kernel Stack is essential for saving the registers while context switching.

Then, we defined the switch_context function that does the context switching. It takes the previous and next stack pointer as arguments and then switches them during execution. It saves the callee-saved registers into the stack, switches the stack pointer then restores the callee-saved registers from the stack. The execution context is saved as a temporary local variable on the stack.

Then, we work on process creation. The process creation function create_process takes in the entry point of the process as a parameter and then returns the process struct.

The delay function is also created to act as a sleep style function which just does nop (Nothing) for 30000000 clock pulses.

Then a scheduler is made to make the context switching more autonomous. A scheduler is basically a Kernel program which decides the next process.

Then, in the exception handler, we make it so that each process has it's own independent kernel stack. While switching, the contents of sscratch are switched too to resume the execution of process from where it was interrupted as if nothing had happened.