Bootloader

Bootloader

The entry point of the whole kernel is 0x80000000, which indicates the _start() function inside kernel/src/main.rs.

Because the kernel support multi-threading, each hart would have its specified bootloader stack. In the _start, we correctly setup sp according to mhartid.

After that, we enter rust_start() inside kernel/src/main.rs, which configures the followings:

1. mstatus and mepc for mret to the real main. Here, we need to set mpp in mstatus to supervisor, because privilege changed in mret according to it.
2. pmpaddr0 and pmpcfg0 for physical memory protection. In this toy kernel, I didn't setup delicate protection on memory. Instead, I just allow all address to be accessed.
3. Save mhartid to tp, which is a CPU-specific register.
4. Initialize timer for machine mode timer interrupt, which is introduced later.
5. Delegate all interrupts and exceptions to supervisor mode by setting the mideleg and medeleg registers.

After configurations, we enter the real main function rust_main() by using mret. In this part, different harts have different tasks to do. For example, only the hart 0 would initialize bss, UART, and heap, while all harts would initialize the trap handler, page table, and PLICs. The difference between UART initialization and the PLICs initialization lies in the fact that UART initialization is to setup the external devices right, while the PLICs initialization is to setup the interrupt controller inside the CPU.