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The ticket is suggesting that for building the BIOS that runs inside the VirtualBox image, they switch from OpenWatcom (another C compiler for DOS) to Digital Mars. VirtualBox emulates an IBM-compatible PC, and the traditional boot protocol for IBM-compatible PCs involves a BIOS that sets up the machine and provides a rudimentary runtime library for things like screen output or disk access. This BIOS is written for 16-bit x86, same as / binary-compatible with DOS, so any good compiler that can produce DOS object files can be made to work here too. (Most bootloaders / kernels will quickly switch to 32- or 64-bit mode, but they expect to be started in 16-bit mode and expect to make calls to the BIOS to load their own code.)

This ticket is not about changing the compiler that builds VirtualBox itself.

I don't know anything about VirtualBox. But DMC can generate 16 bit x86 code.

It would be for things like the BIOS you use to boot your VirtualBox VMs which has to be 16-bit real mode code. Hopefully the compiler itself runs on Linux already (though this is only needed for building the code).