/BubbleOS

A Real-Time Operating System for RISC-V 32-bit in C

Primary LanguageCMIT LicenseMIT

BubbleOS: A RISC-V Operating System in C

BubbleOS is an operating system for personal educational purposes, aimed at providing insight into the fundamentals of operating system development. os

Overview

This project serves as a practical learning resource for those interested in understanding the essentials of OS development. It offers page memory management, preemptive/cooperative multitasking, trap(interrupts and exception handling), timer, spin lock, and system call.

Future Development

In the future, BubbleOS will undergo a major overhaul and transition to Rust programming language. This transition will introduce advanced features including Virtual Memory support, File Systems implementation, and Network components integration.

Boot

UART

Memory Management

Memory Management

The length of Memory is 128M = 8 managment pages * 4096 bytes * 4096 page size

BubbleOS utilizes a page-based memory management system. The first 8 pages are dedicated to management, followed by real pages for data storage. Each byte within the management pages stores a flag, where the last two bits represent PAGE_LAST (indicating if the page is the last page of an allocation) and PAGE_TAKEN (indicating if the page is currently in use).

For example, a flag value of 0x0000_0001 signifies that the page is currently in use and is not the last page of an allocation.

Cooperative Multitasking

In Cooperative Multitasking, before a new task can be executed, the current one must voluntarily give up the CPU.

  • mscratch: A register in Machine mode to point current context
  • ra(x1): General register to save the return address
  • ret: jump back to ra
  • call: will also store the next address of instruction into ra cooperative There are two tasks, A and B. Task A is running on the CPU. When it calls switch_to to switch to task B, it stores the address of the next instruction of task A in the ra register before entering the switch_to function in BubbleOS. Inside switch_to, it saves the context of task A into memory, then restores the context of task B from memory on top of that in order to execute the instructions for task B it ret(jump to the instruction in B).

Trap and Exception

Trap is Exceptional Control Flow(ECF) in RISC-V.

Trap initialize -> Top Half(Hardware, cannot be controled) -> Bottom Half(Our logic) -> Return

Trap init

  • setting BASE address by mtvec register

Top half

  • copy MIE in mstatus to MPIE and clean MIE to close interrupt
  • setting mepc and mtvec, mepc will be set to the address of next command if trap is caused by interrupt. if trap is caused by Exception, mepc will be set to the address of current command.
  • setting mcause and mtval
  • save previous priviledge to MPP in mstatus and change the current priviledge to Machine

Trap Bottom

  • save context
  • call function trap_handler
  • return
  • restore context

Exit trap

  • execute MRET to restore trap status
    • current privilidge of hart = mstatus.MPP
    • mstatus.MPP = U or M
    • mstatus.MIE = mstatus.MPIE
    • mstatus,MPIE = 1
    • pc = mepc

External Interrupt

  • Interrupt

    • Local interrupt
      • software interrupt
      • timer interrupt
    • Global interrupt
      • externrl interrupt

  • interrupt steps:

    • Copy MIE from mstatus to MPIE and clean MIE to disable interrupt
    • Copy next instruction address to mepc
    • Set PC = mtvec
    • Set mcause and mtval
    • Copy previous to MPP in mstatus and Set current to M

Platform-Level Interrupt Controller

Because every hart only has one external interrupt pin, we need a hub to control all interruptions -- PLIC PLIC

Time interrupt

It is a local interrupt cause by CLINT(Core Local Interrupt)

  1. mtime = 0 and mtiomcmp += INTERVAL
  2. wait for timer interrupt
  3. excute interrupt steps and set mtimecmp += INTERVAL

Acknowledge