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3bOS: A flexible and lightweight embedded OS operated using only 3 buttons

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Ryohei Kobayashi
Ryohei Kobayashi

This presentation describes 3bOS, a simple and customizable embedded operating system that runs on the MieruEMB educational kit using only three push buttons. 3bOS is designed for educational purposes, with around 800 lines of code, making it easy for users to understand and modify. It loads programs from an SD card filesystem and runs them, restoring the operating system interface after they exit. Key aspects of the 3bOS design include its small memory footprint, simple input/output interfaces, and support for executing ELF files from the SD card.

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11:40?12:00, October 24, 2014 ESS2014@NYC Tokyo 3bOS: A flexible and lightweight embedded OS operated using only 3 buttons Encarnacion, Immanuel V? Ryohei Kobayashi? Kenji Kise? ?University of the Philippines, Philippines ?Tokyo Institute of Technology, Japan
This Presentation is about... ! 3bOS " a simple and easily customizable embedded OS " running on the MieruEMB system " using only three push buttons Three push buttons 3bOS running on the MieruEMB system 1
Outline ! Introduction ! Design ! Implementation ! Discussion and Conclusion 2
Outline ! Introduction ! Design ! Implementation ! Discussion and Conclusion 3
The Growth in Popularity of Embedded Systems ! Being used in a lot of fields ! Having skills and knowledge about embedded systems is required 4 Car electronics Medical products Communication devices
The MieruEMB system ! An educational kit " for learning implementation skills and knowledge regarding embedded systems The MieruEMB system 5
The MieruEMB system ! An educational kit " for learning implementation skills and knowledge regarding embedded systems Spartan XC3S500E 512KB SRAM (128 x 128 pixel16-bit color) The MieruEMB system 6 LCD screen SD card slot Three push buttons
Experiments on Computer Science ! view 7
3bOS ! Characteristics " Running on the MieruEMB system " Only three push buttons interface " Simplicity A few # lines of code Flexibility Users can easily implement their desired features ! Features The OS itself is loaded from the SD card Reading FAT-formatted SD card Reading contents in SD card Running ELF files in SD card 8
Outline ! Introduction ! Design ! Implementation ! Discussion and Conclusion 9
ISA ! 32-bit MIPS ! Why MIPS? simplicity and widespread use Lots of information 10
Storage ! The contents of the 3bOS binary file are copied to memory when turned on 11
I/O ! Memory-mapped I/O ! 32-bit address space 12 512KB 1KB 16KB SRAM 0x000000 19-bit Physical Address Space 0x07FFFF General MMIO MMC MMIO 0x800000 0x8003FF MMIO: Memory Mapped I/O MMC : Multi Media Card Video Memory 0x900000 0x903FFF 4GB
File System ! FAT file system (FAT12) ! Why FAT? Simplicity and popularity on many platforms 13
File Explorer ! Features Opening a sub/parent directory Reading contents Running ELF files 14
Outline ! Introduction ! Design ! Implementation ! Discussion and Conclusion 15
Implementation of 3bOS ! The OS is mostly coded in C Containing some assembly code ! Compiled with gcc for MIPS* *Buildroot: http://buildroot.uclibc.org/ 16
Implementation of 3bOS ! Important functions Text and Display drivers SD card driver Execution and Exiting of the ELF files 17
Text and Display Drivers ! Display text (and/or) graphics ! Problems It is hard to use basic libraries(stdio.h, stdlib.h) in MIPS GCC (ver 4.3) printf, malloc, free, memcpy, etc... very limited sprintf 18
Solution: gprintf ! We made own printf ! accepts patterns such as: %d %s %x %b %% etc... ! No compile-time type checking 19
SD Card Driver ! Reads raw data from the SD card ! Sends control signals to read/write from the SD card ! Only a 512-byte sector can be read at a time ! A Minor Problem: Physical and Logical sectors Hardware reads physical sector addresses Windows can only read logical sector addresses Logical Sector #0 = Physical Sector #25 20
Debugging by a hex editor 21
Execution and Exiting of the ELF Files ! A pre-execution menu is first displayed Whenever the user wants to execute an ELF file in the 3bOS ! After the executable exits, 3bOS returns to the pre-execution menu 22
Execution and Exiting of the ELF Files ! The Loading Process of an ELF file and Physical Memory Layout 23
Main Problem ! State is restored imperfectly Processor state Program counter Registers($sp, $gp, $ra, etc...) Video memory Video memory would be overwritten when the new executable runs But, the pre-execution menu should be restored after the executable exits 24
The Wrong Solution ! To save state before the pre-execution menu UI is drawn ! But, the stack in the restored state is different... Pre-execution Menu Please choose how to open the ELF file. ^% *^() )*# An app runs exits 25
The Correct Solution ! No need to save stack or video memory! ! ¡°done¡± is global variable not in the stack 26
Debugging with the IDA Debugger 27
Outline ! Introduction ! Design ! Implementation ! Discussion and Conclusion 28
Discussion ! 3bOS¡¯s Important points Simplicity and Flexibility Just around 800 lines total (about one-eighth the size of FreeRTOS*) Users can easily understand how 3bOS runs on the MieruEMB system Users can easily implement their desired features Low Memory Usage 3bOS leaves around 400KB of usable space for running programs (80% of the total size of the main memory) ! It is suitable to use 3bOS as an educational material *FreeRTOS: http://www.freertos.org/ 29
Conclusion ! 3bOS A simple and customizable embedded OS operated using only 3 buttons Running on the MieruEMB system ! 3bOS is suitable for educational purposes A Few # lines of code Users who want to learn OS programming can easily understand it Its flexibility makes it simple for users to modify, add, or remove features 30

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3bOS: A flexible and lightweight embedded OS operated using only 3 buttons

  • 1. 11:40?12:00, October 24, 2014 ESS2014@NYC Tokyo 3bOS: A flexible and lightweight embedded OS operated using only 3 buttons Encarnacion, Immanuel V? Ryohei Kobayashi? Kenji Kise? ?University of the Philippines, Philippines ?Tokyo Institute of Technology, Japan
  • 2. This Presentation is about... ! 3bOS " a simple and easily customizable embedded OS " running on the MieruEMB system " using only three push buttons Three push buttons 3bOS running on the MieruEMB system 1
  • 3. Outline ! Introduction ! Design ! Implementation ! Discussion and Conclusion 2
  • 4. Outline ! Introduction ! Design ! Implementation ! Discussion and Conclusion 3
  • 5. The Growth in Popularity of Embedded Systems ! Being used in a lot of fields ! Having skills and knowledge about embedded systems is required 4 Car electronics Medical products Communication devices
  • 6. The MieruEMB system ! An educational kit " for learning implementation skills and knowledge regarding embedded systems The MieruEMB system 5
  • 7. The MieruEMB system ! An educational kit " for learning implementation skills and knowledge regarding embedded systems Spartan XC3S500E 512KB SRAM (128 x 128 pixel16-bit color) The MieruEMB system 6 LCD screen SD card slot Three push buttons
  • 8. Experiments on Computer Science ! view 7
  • 9. 3bOS ! Characteristics " Running on the MieruEMB system " Only three push buttons interface " Simplicity A few # lines of code Flexibility Users can easily implement their desired features ! Features The OS itself is loaded from the SD card Reading FAT-formatted SD card Reading contents in SD card Running ELF files in SD card 8
  • 10. Outline ! Introduction ! Design ! Implementation ! Discussion and Conclusion 9
  • 11. ISA ! 32-bit MIPS ! Why MIPS? simplicity and widespread use Lots of information 10
  • 12. Storage ! The contents of the 3bOS binary file are copied to memory when turned on 11
  • 13. I/O ! Memory-mapped I/O ! 32-bit address space 12 512KB 1KB 16KB SRAM 0x000000 19-bit Physical Address Space 0x07FFFF General MMIO MMC MMIO 0x800000 0x8003FF MMIO: Memory Mapped I/O MMC : Multi Media Card Video Memory 0x900000 0x903FFF 4GB
  • 14. File System ! FAT file system (FAT12) ! Why FAT? Simplicity and popularity on many platforms 13
  • 15. File Explorer ! Features Opening a sub/parent directory Reading contents Running ELF files 14
  • 16. Outline ! Introduction ! Design ! Implementation ! Discussion and Conclusion 15
  • 17. Implementation of 3bOS ! The OS is mostly coded in C Containing some assembly code ! Compiled with gcc for MIPS* *Buildroot: http://buildroot.uclibc.org/ 16
  • 18. Implementation of 3bOS ! Important functions Text and Display drivers SD card driver Execution and Exiting of the ELF files 17
  • 19. Text and Display Drivers ! Display text (and/or) graphics ! Problems It is hard to use basic libraries(stdio.h, stdlib.h) in MIPS GCC (ver 4.3) printf, malloc, free, memcpy, etc... very limited sprintf 18
  • 20. Solution: gprintf ! We made own printf ! accepts patterns such as: %d %s %x %b %% etc... ! No compile-time type checking 19
  • 21. SD Card Driver ! Reads raw data from the SD card ! Sends control signals to read/write from the SD card ! Only a 512-byte sector can be read at a time ! A Minor Problem: Physical and Logical sectors Hardware reads physical sector addresses Windows can only read logical sector addresses Logical Sector #0 = Physical Sector #25 20
  • 22. Debugging by a hex editor 21
  • 23. Execution and Exiting of the ELF Files ! A pre-execution menu is first displayed Whenever the user wants to execute an ELF file in the 3bOS ! After the executable exits, 3bOS returns to the pre-execution menu 22
  • 24. Execution and Exiting of the ELF Files ! The Loading Process of an ELF file and Physical Memory Layout 23
  • 25. Main Problem ! State is restored imperfectly Processor state Program counter Registers($sp, $gp, $ra, etc...) Video memory Video memory would be overwritten when the new executable runs But, the pre-execution menu should be restored after the executable exits 24
  • 26. The Wrong Solution ! To save state before the pre-execution menu UI is drawn ! But, the stack in the restored state is different... Pre-execution Menu Please choose how to open the ELF file. ^% *^() )*# An app runs exits 25
  • 27. The Correct Solution ! No need to save stack or video memory! ! ¡°done¡± is global variable not in the stack 26
  • 28. Debugging with the IDA Debugger 27
  • 29. Outline ! Introduction ! Design ! Implementation ! Discussion and Conclusion 28
  • 30. Discussion ! 3bOS¡¯s Important points Simplicity and Flexibility Just around 800 lines total (about one-eighth the size of FreeRTOS*) Users can easily understand how 3bOS runs on the MieruEMB system Users can easily implement their desired features Low Memory Usage 3bOS leaves around 400KB of usable space for running programs (80% of the total size of the main memory) ! It is suitable to use 3bOS as an educational material *FreeRTOS: http://www.freertos.org/ 29
  • 31. Conclusion ! 3bOS A simple and customizable embedded OS operated using only 3 buttons Running on the MieruEMB system ! 3bOS is suitable for educational purposes A Few # lines of code Users who want to learn OS programming can easily understand it Its flexibility makes it simple for users to modify, add, or remove features 30