Programming: Operating Systems

Tutorials

rust OS tutorials

https://github.com/rust-embedded/rust-raspberrypi-OS-tutorials

Component Overview

System Bus/Switch

Middleman between CPU and all system devices.
Brokers communication between devices and CPU, or with each other.

Devices

Communication

Interrupt Signals

wikipedia

https://en.wikipedia.org/wiki/Interrupt

   Operating System
     /\  |
      |  |
      |  \/
    Device Driver
     (software)
     /\  |
      |  |
   (via Interrupts)
      |  |
      |  |
      |  \/
   Device Controller
      (hardware)

For example, a keypress.

• device controller loads info into it's registers describing action to take (ex: read a char form keybard)
• device controller populates a local buffer with data, and informs driver that data is available to be read
• driver passes info to OS

TODO:
who initiates this? hardware? os? either?

Flow:

• Interrupts are signals sent (normally) to the system bus.
• When received, CPU stops current task, caches any state
• Interrupt signal number is then looked up in interrupt vector (array mapping signals to their routines)
• routine is processed
• CPU restores state, continues previous task

Terminology:

• device controller raises interrupt
• CPU catches interrupt
• CPU dispatches to interrupt handler
• CPU clears interrupt

Notes:

• Modern CPUs prioritize interrupts using 2x interrupt-wires to the cpu (maskable, and critical/non-maskable)
• Computers often have more devices than interrupt vector can hold, so entries are usually pointers to other datastructures that can lookup interrupt destination

NOTE:
Different than unix signals, which behave similarly but are processed by the operating system, and affect the processing of a process

Direct Memory Access (DMA)

Interrupt-driven events is innappropriate for larger/slower non-volatile-storage.
DMA uses a single interrupt-per-block (size determined by filesystem) instead of per bit.
Aside from that, I/O communicates directly between storage device, and RAM.

Memory

Some types:

• RAM (random access memory) is volatile (non-persistent) memory (generall DRAM)
• EEPROM (electronically eraseable programmable read only memory) is persistent, but slow. infrequently written to. It is responsible for loading your bootloader, which bootstraps your boot manager, and eventually operating system.
• HDD (hard disk drive)
• NVM (non volatile memory) flash

Structure:

• Memory is composed of an addressed array of bytes (8-bits)
• Two operations are used on memory

* load copies data from itself into CPU register * store saves data to itself

CPU

• CPUs have registers of a specific word-size. A 64-bit processor uses a word-size of 8-bytes (8*8bits==64)
• Devices may have their own microprocessors (ex: keyboards have a microprocessor that converts keypresses into codes sent to the actual CPU).

+------------------------------+
|        Processor             |
|+-------------+ +------------+|
||  Core-1     | | Core-2     ||
||    registers| |   registers||
||    L1 Cache | |   L1 Cache ||
|+-------------+ +------------+|
|        |            |        |
|        +-----+------+        |
|              |               |
|              \/              |
|        +----------+          |
|        | L2 Cache |          |
|        +----------+          |
+------------------------------+
              |
              \/
+------------------------------+
|          Memory              |
+------------------------------+

Startup

Bootstrap

Your motherboard's firmware/BIOS is installed onto EEPROM. It uses a system like efi or bios to find and load your operating system.

• BIOS (old) loaded OS bootloader from first sector on disk
• EFI (new) finds bootloaders on disks, using an entire dedicated partition rather than just first sector

Kernel

Kernel Event Queue

See kernel event queue.