[mit 6.s081] LEC 3: OS organization and system calls notes

Lec 3: OS organization and system calls

• Ref: https://github.com/huihongxiao/MIT6.S081/tree/master/lec03-os-organization-and-system-calls
• Preparation: xv6 book Chapter 2

Topic

• Isolation Isolation,
• Kernel/User mode kernel / User mode
• System Call system call
• xv6 Realization

Isolation Isolation,

Unix Interface - Realize reuse and physical memory isolation :

• abstract the HW resource Abstract hardware resources
• process: instead CPU process : The application communicates with the CPU In exchange for , Abstract the CPU
• exec: instead of memory Load the image , Memory isolation
• files: instead of disk block file system : Abstracted disk blocks

Defensive Defensive

App cannot crash the OS
App cannot break out of its isolation
Strong isolation through hardware Strong Isolation between apps + OS
typical: HW support:

• user/kernel mode
• virtual memory

user/kernel mode

user: unprivileged instructions - ADD, SUB, JRC, BRANCH
kernel: privileged instructions - setup page table Set the page table register , Turn off clock interrupt

virtual memory

page table: virtual addr -> physical addr
process has own page table
memory isolation

User/Kernel mode Switch

RISC-V: ECALL Instructions : Control is transferred from the user to the kernel

Each application initiates a system call through syscall function

Kernel = Trusted Computing Base(TCB) The kernel is called a trusted computing space

• Kernel must have no bugs
• Kernel must treat processes as malicious

Monolithic Kernel vs Micro Kernel Macro kernel vs Microkernel

• Macro kernel : All operating system services are in kernel mode
  • shortcoming : There is a lot of code in the kernel , Bug hidden danger
  • somewhat : Sub modules in the same program , Good performance
• Microkernel : The kernel has only a few modules
  • advantage : Less Bug
  • shortcoming : Performance loss of user mode and kernel mode switching ; Sharing difficulties

xv6: Macro kernel

Kernel compilation

1. Makefile Read C file , call gcc compiler Generate .s Of RISC-V Assembly language files
2. Call the assembly interpreter to generate .o Binary file
3. All kernels C The document repeats 1, 2 The operation of
4. System loader (Loader) Collect all .o file , Link together to generate kernel files
   

Pass to QEMU Several parameters of ：

• kernel： What is passed here is the kernel file （kernel In the catalog kernel file ）, This is what will happen in the future QEMU Program files running in .
• m： The message here is RISC-V The amount of memory that the virtual machine will use
• smp： What is passed here is what the virtual machine can use CPU Check the number
• drive： What is passed is the disk drive used by the virtual machine , What's coming in here is fs.img file
  

XV6 The boot process

kernel/entry.S -> kernel/start.c-start() -> kernel/main.c-main()-> kernel/proc.c-userinit() The first user process , Used to interact with the operating system

userinit() Will perform initcode Code , The system call will be executed SYS_exec, And execute user mode init Program (user/init.c), In the end shell Run up .