boot_design

Boot Design

The boot program is the one that is responsible for :

1. Setup the environment for the kernel to run in machine mode
2. Transfer the control to kinit() (kernel running in machine mode)
3. Designate a specific place where the CPU will jump to if a trap occurred when kernel is in either machine mode or supervisor mode.
4. Initiate an environment for the kernel to run in supervisor mode.

• the boot code is the one that contains the _start function. _start function is the entry point for the whole kernel image. So it is the first place the CPU points to in the RAM after the Qemu firmware has done its thing.

Bird's View (ideal but not as implemented)

 stateDiagram
    load_firmware
    load_bootloader
    load_kernel
    run_kernel
    shutdown

    state look_for_bootloader <<choice>>
    state look_for_kernel <<choice>>

    [*] --> load_firmware
    load_firmware --> look_for_bootloader
    look_for_bootloader --> load_bootloader : Bootloader is found
    look_for_bootloader --> shutdown : Bootloader is missing

    load_bootloader --> look_for_kernel
    look_for_kernel --> load_kernel : Kernel image is found
    look_for_kernel --> shutdown : Kernel image is Missing

    load_kernel --> run_kernel
    run_kernel --> shutdown

    shutdown --> [*]

Assembly Glue

Instead of the boot code being a program that gets called once(as seen above), it becomes the glue code that gets called whenever kinit and kmain go out of scope. It encompases the entire kernel. It acts as glue code.

Detailed Program flow

graph TD
    Qemu_virtual_firmware -->|Loads the kernel image|Booze
    subgraph Booze
        Start --> _choose_bootloading_HART
        _choose_bootloading_HART --> C1{is it the wrong HART?}
        C1 -->|Yes| Shutdown_specific_HART
        Shutdown_specific_HART -->|probe next HART| C1
        C1 -->|No| clear_BSS_section

        clear_BSS_section --> _initialize_registers_for_kinit
        _initialize_registers_for_kinit -->|Jump_to_kinit| kinit

        kinit -->|returns back to Booze Glue| Glue
        Glue --> _initialize_environment_for_kmain
        _initialize_environment_for_kmain -->|initialization includes switching to S Mode| kmain

        kmain -->|Happens ONLY if trap happens| Trap_handler
        Trap_handler -->|handles trap, returns control| kmain

        kmain -->|Happens only if kmain has finished all execution instructions| shutdown
    end