CS 452/652 Winter 2022 - Lecture 4

Programming

• use unit testing where possible
• use assertions generously!
• availability of resources: track < ARM < developer < PC
• hardware emulation, such as QEMU → unclear cost/benefit
• important principle: KISS
  • related: "premature optimization is the root of all evil" (Donald Knuth)
• think strategically about
  • functionality: start simple, add functionality in small steps
    • test (and commit) at each step
  • data structures: encapsulate, start simple, exchange later
  • bugs: not every bug is a show-stopper

Optimization

Processor Configuration

• ARM 920T has instruction- and data-cache → enable!
  • RedBoot disables before starting program with 'go'
• Related features (not relevant for CS 452/652)
  • lockdown mode: lock specific code/data into fast cache memory
  • Thumb mode: use simpler/smaller instruction set to reduce memory footprint
• MMU enabled with default setting → no need to tinker with
• details: see control register CP15 in EP93XX and ARM documentation

Compiler Optimization

• code without side effects might be removed
• repetitions might be removed
• loops unrolled
• code reordered based on performance of microarchitecture
• compiler understands data dependencies in synchronous control flow
• C/C++ keyword "volatile"
  • tells compiler about unspecified side effects related to a variable or memory location
  • compiler avoids reordering around and removal of volatile accesses
  • example declaration of memory-mapped register

    volatile unsigned int* timer1value = (volatile unsigned int*)0x80810084u;
    

• goal is complete understanding of what we are doing ⇒ enable optimization!

Runtime Optimization

• pipelining and out-of-order execution
• very important challenge in other areas!
• old processor with one core → not relevant for CS 452/652
• Java keyword "volatile" handles this (different from C/C++)

Kernel Overview

• microkernel → device drivers outside kernel
• normal scope: memory, threading, communication
• here: tasks and message passing
  • do not use shared memory between tasks
  • hardware protection optional
• entry/exit between kernel and user-level tasks
  • software interrupt - system call
  • hardware interrupt - device activity

Generic Kernel Loop

void kmain() {
  initialize();  // includes starting the first user task
  for (;;) {
    currtask = schedule();
    request = activate(currtask);
    handle(request);
  }
}