CS 452/652 Winter 2025 - Lecture 1

Introduction and Overview

Jan 7, 2025 next

Course Overview

• real-time systems
  • software that interacts with the real world
  • model/control physical world, communication, timing
  • robustness against defects (train, switch, sensor)
• real-time systems
  • small operating system, plus train control application
  • start from scratch, get a better understanding of things you may take for granted
    • no memory translation/protection, no file system
    • micro-kernel with send-receive-reply (SRR) communication
    • multi-threading; concurrency & non-determinism
    • device interaction; interrupt handling
• few guard rails; room to roam
  • design flexibility in assignments
  • you define and implement a project

Infrastructure

RPi Boot Process

• bootloader loads start4.elf and other files (config.txt)
• bootstraps VideoCore (VC) GPU and loads kernel8.img
• jumps to 0x80000 to hand control to kernel
• VideoCore does other things behind the scenes

Development Overview

• basic procedure
  1. build and create .img file on dev box
  2. upload .img to boot server (cs452.student.cs.uwaterloo.ca)
  3. reboot / power on RPi (output to gtkterm on test PC)
  4. RPi automatically loads image for its MAC address

Sample Code

• iotest
  • very simple, does console input/output using channel 1
  • use it to try the dev/test process
• development tools
  • cross-compiler and other usual tools (linker, readelf, etc)
  • on linux.student.cs.uwaterloo.ca, tools are in /u/cs452/public/xdev/bin
  • on personal computer, install your own

Development Basics

• freestanding code
  • no operating system, no libraries (including libc)
  • explicit linking rules
• load and execute: ELF/image files
  • at runtime, program will have code, data, stack (no heap)
  • build process creates ELF file
    • text, data, bss sections
    • start of the text section is the entry point
  • img file is produced from ELF - includes text and data (not bss)
    • bss variables need to be initialized by the program
• libc (newlib) is part of compiler package, but no system calls
  • can't rely on underlying OS to provide services, like I/O, memory allocation
• MMU is disabled
• FP/SIMD
  • kernels do not typically use FP operations, but modern compilers/libc use SIMD
  • SIMD instructions work, but unaligned accesses fail without MMU
    • -mstrict-align compiler flag
    • problem with precompiled libc, etc.
    • SIMD test in Makefile → -mgeneral-regs-only compiler flag
  • options:
    • don't use libc (like iotest)
    • use only non-problematic bits of libc (see check in iotest Makefile)
    • or?

Assignment 0

• due in one week! start now!
• polling loop, checking track state, clock, user input

  for (;;) {
    if (c1) f1();
    if (c2) f2();
    ...
  }