Enabling all 4 serial ports results in conflicts due to shared IRQ values

[T4nkcommander]

By default, COM ports 1 and 3 share an IRQ value of 4, while COM ports 2 and 4 share a value of 3. This means in a Windows guest that enabling these pairs results in conflicts, making them unusable.

This can be worked around via "User-defined" settings, but it seems strange that the default settings would allow for conflict in this way.

[T4nkcommander]

I provided a screenshot of what the conflict looks like in this post: ​https://forums.virtualbox.org/viewtopic.php?f=1&t=107954&p=529287#p528545

[aeichner]

This seems to be specific to Windows guests where the serial port driver doesn't seem to allow IRQ line sharing. All the documentation for the default hardware resources for COM ports indicate that we do the right thing here, see [1], [2] and [3]. From the hardware side it should be possible that COM ports can share the IRQ line because they contain a register to identify where the interrupt originated from originally so you could also put all four serial ports on the same IRQ line. Other guest operating systems like Linux shouldn't have a problem with and IRQ lines are precious resources so we can't just dedicate 4 of them to the serial ports and the assignment wouldn't be compatible with the standard IBM PC values.

[1] ​https://www.lammertbies.nl/comm/info/rs-232-io
[2] ​https://en.wikipedia.org/wiki/COM_(hardware_interface)
[3] ​https://www.oreilly.com/library/view/pc-hardware-in/059600513X/ch22s02s03.html