[Tested successfully!] Atmega328p port of mini-rv32ima. Let's run Linux on the world's worst one of the world's worst Linux PC (and beat Dmitry Grinberg)
Note: The code is in pure AVR C. Arduino IDE is just used as serial terminal.
UPDATE: I found a worse Linux PC: C64!
This is a port of mini-rv32ima (a minimum RISC-V emulator, capable of booting Linux) on atmega328p (the core of Arduino UNO, a 8-bit AVR microcontroller). So basically, this code is for booting Linux on Arduino UNO.
Yes you are reading it correctly, Arduino UNO can (theorically, but not practically) boot Linux. And it definitely beats Dmitry Grinberg's (once) world's worst Linux PC.
The idea is really simple: you have an Arduino UNO (or atmega328p) to run the emulator's logic, and emulator's RAM is accessed via swapping with an SD card (which is communicated through SPI interface, see more below). The emulator also has 3 512-bytes cache (1 icache and 2 dcache interchangable) and lazy/delayed cache write system.
The code is written in pure C (and not Arduino) to reduce Arduino overhead (if any). It initializes UART, SPI, SD card, and a digital input-pullup pin for triggering emulator state dump. Finally, it initialize cache, then mini-rv32ima and let the emulator does its works.
About 175Hz - 205Hz 426 - 600Hz most of the time 700 Hz 1500Hz, peak 1500Hz 2000Hz, lowest 32Hz with -O3 code on an Arduino UNO based on atmega328p, clocked at 16MHz, with a class 4 SDHC card connected via 1-bit SPI interface. Complete boot time (from start to shell) is about 15 hours and 44 minutes 8 hours and 18 minutes.
Update 24/9/2023: The speed is double/tripled by implementing icache
Update 26/9/2023: The speed is x1.5 by implementing 3 cache + lazy write system
Update 25/10/2023: SPI speed is now 4MHz, thanks to @kittennbfive and @mjungwirth's suggestions
Why it's that slow? Read Current issues and drawbacks section below.
The pinout is really simple. On Arduino UNO, it should be:
| Arduino UNO pin | Connect to | Description |
|---|---|---|
| 9 | ground-connected button | When connected to GND, emulator state and effective emulated speed will be dump via UART |
| 10 | CS pin on SD card |
SD's chip select/CS/EN/SS pin |
| 11 | MOSI pin on SD card |
SD's DI/CMD pin |
| 12 | MISO pin on SD card |
SD's DO pin |
| 13 | SCLK pin on SD card |
SD's clock/SCK/CLK pin |
Warning
Since Arduino UNO (atmega328p) uses 5V logic level, while SD card use 3.3V (or lower) logic level, you will need a level shifter. You can build your own using MOSFET, but the simplest way is to buy an SD card adapter.
Note
You can change the CS and the pin used to dump state to any pins you want. Just modify the code (see below).
- First, you will need an SD card. Any type larger than 16MB.
Warning
You should backup all your files, since doing this SD card preparation will destroy your SD card's filesystem.
- Then you need some tool to directly write a file to SD card. On Windows, you can use HDD Raw Copy Tool, on *nix you can use
dd if=file.bin of=/dev/sdX conv=notrunc. - Download the file
arv32.binfile. FYI, this file is generated by dumpingmini-rv32imaemulator's RAM array to a file when it just finished loading the kernel and stuffs (but has not boot). You can generate your own version for newer updates of Linux kernel or Buildroot. - Write it directly to the SD card using the tools in step 2.
- Now you are done preparing the card.
Important
The Preparing the SD card section must be repeated every time you boot your emulator. Otherwise it might not boot (if Linux has initialized enough and start cleaning the memory). This might be fixed in the future.
You can just download the latest HEX file in Release. But if you want to build on your own, here are the steps:
- First, clone the repo:
git clone https://github.com/raspiduino/arv32-opt. Thencd arv32-opt - Then use build the code using the following commands:
avr-gcc -c -mmcu=atmega328p -I. -DF_CPU=16000000UL -O3 -funsigned-char -funsigned-bitfields -fpack-struct -fshort-enums -Wall -Wstrict-prototypes -Wa,-adhlns=main.lst -std=gnu99 main.c -o main.o
avr-gcc -c -mmcu=atmega328p -I. -DF_CPU=16000000UL -O3 -funsigned-char -funsigned-bitfields -fpack-struct -fshort-enums -Wall -Wstrict-prototypes -Wa,-adhlns=sdcard.lst -std=gnu99 sdcard.c -o sdcard.o
avr-gcc -c -mmcu=atmega328p -I. -DF_CPU=16000000UL -O3 -funsigned-char -funsigned-bitfields -fpack-struct -fshort-enums -Wall -Wstrict-prototypes -Wa,-adhlns=sdprint.lst -std=gnu99 sdprint.c -o sdprint.o
avr-gcc -c -mmcu=atmega328p -I. -DF_CPU=16000000UL -O3 -funsigned-char -funsigned-bitfields -fpack-struct -fshort-enums -Wall -Wstrict-prototypes -Wa,-adhlns=spi.lst -std=gnu99 spi.c -o spi.o
avr-gcc -c -mmcu=atmega328p -I. -DF_CPU=16000000UL -O3 -funsigned-char -funsigned-bitfields -fpack-struct -fshort-enums -Wall -Wstrict-prototypes -Wa,-adhlns=uart.lst -std=gnu99 uart.c -o uart.o
avr-gcc -mmcu=atmega328p -I. -DF_CPU=16000000UL -O3 -funsigned-char -funsigned-bitfields -fpack-struct -fshort-enums -Wall -Wstrict-prototypes -std=gnu99 main.o sdcard.o sdprint.o spi.o uart.o --output main.elf -Wl,-Map=main.map,--cref
avr-objcopy -O ihex -R .eeprom main.elf main.hex
avr-size -A -d main.elf- Result HEX file will be
main.hex - You can then flash the code to the board using
avrdude -v -V -patmega328p -carduino "-PCOM10" -b115200 -D "-Uflash:w:main.hex:i". ReplaceCOM10with yourCOMport (on Windows), or/dev/ttyUSB*port on Linux. (Idk about Mac, but it should be the same)
Important
If you build for another AVR microcontroller, or if you use different clock speed, please change the -mmcu=atmega328p and -DF_CPU=16000000UL options to match your situation.
Note
Windows users can refer to the script build.bat in this repo. Just change the path to your correct path and it will work.
After completing 2 sections above, you are now ready to boot Linux on Arduino UNO. Just connect the SD card to Arduino UNO, insert the card in, then power the Arduino on. Open a serial connection at baudrate 9600 bps to connect.
You should see the following output (immidiately):
arv32-opt: mini-rv32ima on Arduino UNO
SD card initialized successfully!
Important
The Preparing the SD card section must be repeated every time you boot your emulator. Otherwise it might not boot (if Linux has initialized enough and start cleaning the memory). This might be fixed in the future.
Important
If you get Error initializaing SD card when you just plugged your board to your computer, just press the reset button. It should work.
You can also dump the state of the emulator while it's running. Just connect a button to GND and then connect pin 9 to the button. When you click the button, state will be dump, and you should see something like this:
Effective emulated speed: 1442 Hz, dtime=233596ms, dcycle=336896
Current AVR free memory: 155 bytes
icache hit/miss: 17931786/983542; dcache hit/miss: 4078057/1731330
==============================================================================
Dumping emulator state:
Registers x0 - x31:
0x00000000 0x800E6B34 0x8041F7A0 0x80332FC8
0x80420000 0x9B779A7E 0xDFB0F4A6 0x29EEBE42
0x8041F890 0xDEC0D0A6 0x9EE3B78E 0xA125AEE8
0x0A29015A 0x969C9575 0x31D9B5BA 0x1A086279
0xEFA9A208 0xAF715DDD 0x10C7F938 0x42557158
0x9A2FA369 0x6AA2616B 0x8041F8B4 0x00000000
0x00000000 0x36AAA20D 0x4BD078AB 0x000AF715
0x68A94F06 0x53A31796 0xF822BFDD 0x1A73C5BB
pc: 0x800E7144
mstatus: 0x00001880
cyclel: 0x0120A000
cycleh: 0x00000000
timerl: 0x00904E00
timerh: 0x00000000
timermatchl: 0x009050BF
timermatchh: 0x00000000
mscratch: 0x00000000
mtvec: 0x80001CBC
mie: 0x00000088
mip: 0x00000000
mepc: 0x800E6640
mtval: 0x00000000
mcause: 0x80000007
extraflags: 0x019446E3
Effective emulated speed is the number of instructions the emulator can execute in 1 second at that time. dtime is the time difference between current time and the last time you dump the status. dcycle is the number of cycle (instructions) excuted from the last time you dump the status until now.
Registers x0 - x31 sections show registers from x0 to x31, listed in order from left to right then from top down.
Note
The emulator will resume when you release the button (you should see something like B1 is set to HIGH, emulator resume). As long as you keep holding the button, the emulator will pause. You can also use a wire connected to GND instead of a button (just like me).
- UART baudrate @
main.c - SPI bus speed @
main.c - Dump state trigger button @
main.c - SD's CS pin @
spi.h
- The effective emulated speed is super slow. This is mostly due to the high overhead of 1-bit SPI connection with SD card, when every instruction that access memory must also access SD card.
- Max tested SPI bus speed is FCLK/4, which is 4MHz on 16MHz atmega328p. If you set the speed higher than this, SD card will not be stable. We don't know why, so please give us some light if you know the reason.
- Automatically replacing new state is not implemented yet. So every time you run the emulator, you must repeat the
Preparing the SD cardsection.
- cnlohr for writing mini-rv32ima.
- ryanj1234 for writing SD_TUTORIAL_PART4.
- adnbr for writing 1 ms counter
- me (gvl610/raspiduino) for bringing all this stuff together.
Hackaday, Hackster, Habr, Maker News, internetua, futuranet.it, zhihu.com, tuxmachines.org, arduino.cc forum, sohu.com
If you can run this, you probably are running one of the world's worst Linux PC. Enjoy!