14-kernel-startup

Kernel Startup and Timers

The focus is on entry.S and start.c, which transitions to supervisor mode, has timer initialization, and handles timer interrupts in kernelvec.S

Kernel Startup Process

entry.S

• entry.S is executed in machine mode, which is the highest privilege level.
• It sets the stack pointer sp for each core.
• Each core gets its own 4KB stack.
• With mhartid to determine the core number of 0-7.
• jumps to start()

start.c

• In machine mode, start() sets the mstatus register to supervisor mode
• sets mepc to point to main()
• ensures mret will run main()
• disables paging by setting satp = 0
• Delegates exceptions and interrupts to supervisor mode.
• Enables interrupts from devices
• Configures memory protection
• initializes timer interrupts with timerinit()
• saves mhartid into tp
• executes mret to jump to main and get into supervisor mode

Timer Initialization

timerinit() configures the core’s timer with mtimecmp and schedules the first timer interrupt 1 million cycles later.

• Uses timer_scratch to store mtimecmp and the timer interval.
• writes timer_scratch to mscratch
• sets mtvec to timervec to point the machine-mode trap handler to timervec
• enables machine-mode interrupts mstatus and mie

Handling Timer Interrupts

After an interrupt happens:

• timervec executes, saving a1-a3 into timer_scratch
• schedules the next timer interrupt at 1 million cycles in the future
• triggers a software interrupt to supervisor mode
• restores saved registers and executes mret

Supervisor mode

After switching to supervisor mode, the kernel receives a software interrupt, which it recognizes as a timer interrupt, and switches the currently run process.