> ** NULL assignment detected<p>That's without any hardware support (MMU) on DOS; compiler-generated code does the null checks.<p>Without it, you get unpredictable results for that bit of UB; the program can trample bytes around the null address.
<i>because it emulates an Ethernet packet driver but it is really just SLIP over a serial port</i><p>Today, the same is true of many other physical-layer protocols that developed later, such as WiFi and the GSMA mobile standards; they seem to have converged on the Ethernet frame format at the software interface, presenting the appearance of an Ethernet NIC to software, because that's the easiest way to make use of existing network stacks. There's also the weirdness of tunneling protocols like PPPoE which only exist to tunnel Ethernet through non-Ethernet systems.
I wonder if you could've tried "Fil-C" compiler to catch the bug<p>one of those things that "I heard about recently, would love a practical test, but have no current need to try on"