user34350: I am wondering if anyone here knows where the source code for the Arduino "firmware" is available for reading? I think it must be more than the 'boot loader' but I am not sure. Thanks!
p.s. I assume this kind of question is ok here, even though it's not ok on StackOverflow?

user6301: If you are looking for the Flash Firmware, I think the github project documentation is pretty well developed, here:
https://github.com/arduino/Arduino
This is a good introduction to build and flash your Arduino UNO:
http://angryelectron.com/arduino-firmware/
And here a good start to build you own Arduino on a breadboard:
https://www.arduino.cc/en/main/standalone

user7508: The “firmware” is whatever program a microcontroller runs. On an
Arduino, it is typically the program you write and upload into the
microcontroller's flash memory.
If by “firmware” you mean the core library (the source code of
pinMode(), digitalRead() and so on), it is here:
https://github.com/arduino/ArduinoCore-avr
If by “firmware” you mean the bootloader (the small program that runs at
reset and will accept the upload of your firmware), is is here:
https://github.com/arduino/Arduino/tree/master/hardware/arduino/avr/bootloaders/optiboot
Note that this is valid for the AVR-based Arduinos.
The cores and bootloaders used in other architectures are stored is
separate repositories, e.g.
https://github.com/arduino/ArduinoCore-sam

34350: <p>I am wondering if anyone here knows where the source code for the Arduino "firmware" is available for reading? I think it must be more than the 'boot loader' but I am not sure. Thanks!</p>

<p>p.s. I assume this kind of question is ok here, even though it's not ok on StackOverflow?</p>
 6301: <p>If you are looking for the Flash Firmware, I think the github project documentation is pretty well developed, here:</p>

<p><a href="https://github.com/arduino/Arduino" rel="nofollow noreferrer">https://github.com/arduino/Arduino</a></p>

<p>This is a good introduction to build and flash your Arduino UNO:</p>

<p><a href="http://angryelectron.com/arduino-firmware/" rel="nofollow noreferrer">http://angryelectron.com/arduino-firmware/</a></p>

<p>And here a good start to build you own Arduino on a breadboard:</p>

<p><a href="https://www.arduino.cc/en/main/standalone" rel="nofollow noreferrer">https://www.arduino.cc/en/main/standalone</a></p>
 7508: <p>The “firmware” is whatever program a microcontroller runs. On an
Arduino, it is typically the program <strong>you</strong> write and upload into the
microcontroller's flash memory.</p>

<p>If by “firmware” you mean the core library (the source code of
<code>pinMode()</code>, <code>digitalRead()</code> and so on), it is here:</p>

<p><a href="https://github.com/arduino/ArduinoCore-avr" rel="nofollow noreferrer">https://github.com/arduino/ArduinoCore-avr</a></p>

<p>If by “firmware” you mean the bootloader (the small program that runs at
reset and will accept the upload of your firmware), is is here:</p>

<p><a href="https://github.com/arduino/Arduino/tree/master/hardware/arduino/avr/bootloaders/optiboot" rel="nofollow noreferrer">https://github.com/arduino/Arduino/tree/master/hardware/arduino/avr/bootloaders/optiboot</a></p>

<p>Note that this is valid for the AVR-based Arduinos.
The cores and bootloaders used in other architectures are stored is
separate repositories, e.g.</p>

<p><a href="https://github.com/arduino/ArduinoCore-sam" rel="nofollow noreferrer">https://github.com/arduino/ArduinoCore-sam</a></p>
