kernel

a basic kernel made to x86

Boot

Prekernel

Kernel

PML4

• 4 KiB pages (PTE) for each section (.text, .rodata, .data, .bss etc).

ISRs

keyboard (ps1)

• minimal structure
• fill keyboard buffer with scancodes
• spinlock or disable IRQ while writing to the buffer (avoiding race condition)

VGA

• 0xb8000 as always
• due to VA == PA -> VGA is virtually mapped to ~0xffffffff000b8000

Threading

• kernel thread creation
• cooperative (non-preemptive) round-robin scheduling
• context switching in System V ABI–oriented (x86_64)
• wait queues
• counting semaphores
• a dedicated idle thread
• all threads share the same address space and heap

Thread States

• UNUSED -> slot available
• RUNNABLE -> eligible to run
• RUNNING -> currently executing
• BLOCKED -> sleeping on a wait queue
• ZOMBIE -> finished execution, pending cleanup

Run queue

• O(1) enqueue
• O(1) next-thread selection
• TODO priority handling
• cooperative only
• every thread yields explicitly via kthread_yield()

Context Switching

• made by void context_switch(uint64_t** old_sp, uint64_t* new_sp)
• push callee-saved registers
• store current RSP into *old_sp
• load new_sp into RSP
• restore registers
• ret into the new thread context

Thread Stack Layout

top
r15
r14
r13
r12
rbx
rbp
return -> kthread_entry
arg
fn
bottom

• SP is aligned to 16 bytes

Devices

TTY

• (console driver) vga_putc() e vga_popc() -> who actually handles with the screen
• (keyboard driver) consumes the scancodes from the keyboard buffer populated by the ISR
• ldisc_input applies terminal methods (echo, canonical/non-canonical, backspace, etc) and handles with ASCII
• holds the input buffer that will be consumed by stdin (when tty->line_ready == true)

stdin

• fd 0
• bufferized -> ring buffer
• free for read(0)
• is connected to TTY -> sleeps and reads the input buffer when it is woken up

stdout

• fd 1
• bufferized -> ring buffer