Week 4.a
CS6640
01/27 2026
https://naizhengtan.github.io/26spring/

□ 0. recap: gdb w/ an example
□ 1. OS organization
□ 2. egos desgin
----

0. gdb

  * gdb basics

    Scenario 1: "what's wrong?"
    - run to failure
    - use gdb to see the final status

    Scenario 2: "I suspect this is wrong"
    - set a breakpoint
    - continue the egos
    - run until the breakpoint
    - single step running & monitoring

  * use cases for gdb

    a) Question: At what memory address does the CPU execute the first kernel instruction?
     -- use gdb

    b) Question: How does printf() work?
      use gdb:
      -- go with code in Lab1
         [see printing a letter at a time]

    c) Question: How does context switching work in Lab 2?

  * a tricky bug in the lab2 example: wrong stack pointer
     [example: stack problem]

1. OS organization:
  part I:  OS architecture
  part II: egos design and implementation

  Expectation:
    after this, you should have a bird view of egos

  Why we have this now, instead of the first class?

  - an analogy of OS organization vs. Software architect

  - for software engineering, one studies
     how to program,
     then algorithms and data structures,
     then design patterns and software engineering (as in management),
     then software architecture
     =>
     low-code programmers,
     programmers,
     software engineers,
     software architect

  - where OS education is the other way around:
    OS users,
    [?]
    OS architect

 A) Part I

  - types of OSes
    [see fig]

    * monolithic OS
    * microkernel OS: 1970s
    * exokernel (LibOS): 1995
    * multikernel: 2009

  - more about these architectures

    * monolithic kernel:
      a single piece of code serving all requests
      high in coupling

    * microkernel:
      kerenl is mostly responsible for IPCs; services are running in user-level
      low in coupling
      IPC can be the bottleneck

      * Improving IPC by kernel design, SOSP 1993

    * exokernel
      kernel only handles multiplexing resources (securely)
      extra performance due to having hardware primitives

      * Exokernel: An Operating System Architecture for Application-Level Resource Management, SOSP 1995

    * multikernel
      target heterogeneous hardware and many-core machines
      replace shared-memory model with shared-nothing model (use message passing)
      treat OS as a distributed system
      * message passing vs. shared memory

      * The multikernel: a new OS architecture for scalable multicore systems, SOSP 2009

  - [story] an anecdote of today's microkernel

   * Microkernel Goes General: Performance and Compatibility in the HongMeng Production Microkernel, OSDI 2024


2. egos design

   * earth, grass, and apps

    [see handout, panel 1]

   * earth interface and syscalls

    [read library/egos.h]

   * two syscalls: sys_send, sys_recv
     (library/libc/syscall.h)

   * system services
     -- sys_proc      (GPID_PROC)
     -- sys_terminal  (GPID_TERMINAL)
     -- sys_file      (GPID_FILE)
     -- sys_shell     (GPID_SHELL)

     They have well-known pids, from 1 to 4.

   * pid to proc_set's idx
     -- 1-to-1 mapping
     -- see [grass/process.c]