XinuPi: Porting a Lightweight EducationalOperating System to the Raspberry Pi

Eric Biggers1 Farzeen Harunani2 Tyler Much2

Dr. Dennis Brylow2

1Macalester College

2Marquette University

October 3, 2013

Outline

Overview of Embedded Xinu and Raspberry Pi

Goals and purpose of porting Embedded Xinu to theRaspberry Pi

Challenges, hardware, and technical details

Summary and future work

Overview of Embedded Xinu and the Raspberry Pi

Embedded Xinu

Small, simple framework forteaching embedded operatingsystem design

Nexos project (WESE 2008)

Xest (WESE 2010)

Figure : WRT54GL: One platformEmbedded Xinu currently runs on

Raspberry Pi

Inexpensive single-boardcomputer

Intended for educational use

Figure : Raspberry Pi Model B(Cost: $35)

Goals and Purpose

Create functional Raspberry Pi port of Embedded Xinu tofacilitate hands-on operating systems and embedded softwarecurriculum

Write documentation and educational modules

Why Embedded Xinu? Why Raspberry Pi?

Code written “forstudents by students”

Demonstrated educationalsuccess (WCAE ’07,SIGCSE ’08, SPLASH’10, SIGCSE ’11)

Inexpensive

Rich set of peripherals

Popular (1 million+ sold)

Strong community

Integration of embedded systems concepts throughoutcurriculum

Concepts covered before the actual embedded systems course:

Preemptive multitaskingProgramming in assembly languageHardware interruptsResource-constrained programmingDirect hardware communicationRemote target software developmentCross-architecture developmentPacket construction and transmission over real networks

Marquette University’s embedded systems course focuses onmore advanced concepts with a variety of platforms

Overview of Raspberry Pi Hardware

BCM2835 SoCCPU (ARM1176JZF-S)GPU (Broadcom VideoCore)Memory (256MB or 512MB)Various memory-mapped peripherals

ARM1176JZF-S CPU

ARM memory GPU memory . . . Peripherals memory . . .

0x0 0x08000000 0x10000000 0x20000000

Physical memory

. . . System Timer . . . Interrupt Controller . . . Mailboxes . . . PL011 UART . . . USB Controller . . .

Figure : Logical view of system from ARM CPU

Interrupt Controller (for CPU)

Allows enabling and disabling specific IRQ lines

IRQ line Device1 System Timer (1)3 System Timer (3)9 USB Controller

55 PCM sound57 PL011 UART62 SD card

Pending0

+0x00

Pending1

+0x04

Pending2

+0x08

FIQControl

+0x0c

Enable1

+0x10

Enable2

+0x14

Enable0

+0x18

Disable1

+0x1C

Disable2

+0x20

Disable0

+0x24

Figure : BCM2835 interrupt controller (for CPU) registers

System Timer

64-bit 1 MHz free-running counter

Four independent interrupts: one used by XinuPi, tworeserved by GPU

CS

+0x00

CLO

+0x04

CHI

+0x08

C0

+0x0c

C1

+0x10

C2

+0x14

C3

+0x18

Figure : BCM2835 system timer registers

USB (Universal Serial Bus) support

Raspberry Pi relies on USB to attach keyboards, mice, andeven the built-in Ethernet controller

Raspberry Pi has nonstandard, undocumented USB HostController (made by Synopsys Inc.)

Basic USB 2.0 support essentially complete

Code is several thousand lines due to high complexity of USB,but still a fraction of Linux code size

Ethernet support

Raspberry Pi Model B has integrated USB Ethernet Adapter(SMSC LAN9512)

Device is not documented, so we had to gleam hardwaredetails from the Linux driver

Our driver for this is about 1/4 the size of the correspondingLinux driver

Operation: Packets are sent and received via USB bulktransfers

SMSC LAN9512

USB

USB Host Controller

Memory

CPU

Network

Graphics support

BCM2835 includes integrated VideoCore GPUCPU and GPU initialize communication via hardware-based“mailbox” system, then share configurable depth RGBA linearframebuffer in memoryGPU is very powerful but not well documentedFramebuffer console and turtle-graphics application

Summary

Goal: Port Embedded Xinu toRaspberry Pi

Promising and viable platform foroperating systems and embeddedsoftware education

Completed work Future work

Preemptive multitasking

Basic USB support

Networking support

Network bootloader

Basic graphics support

Partial documentation

Lesson plans

Classroom trials beginningSpring 2014

Improved documentation

Additional drivers (USBkeyboard, sound, SDcard)

Acknowledgments

Marquette University MSCS REU Program

National Science Foundation (funding from grantCNS-REU-1063041)

Raspberry Pi Foundation (for designing the Raspberry Pi)