Why DOS programs fail if loaded too low and A20 gate is enabled

[joncampbell123]

Older DOS code will sometimes use the 8086 segment registers in a way that assumes a 1MB wraparound to access memory.

If the A20 gate is enabled, this wraparound doesn't happen and the DOS program malfunctions.

Here's an example of just such a malfunction right there in the startup code of an old C compiler from the late 1980s. To nobody's surprise, this program crashes and fails unless you LOADFIX -a first.

[joncampbell123]

Just so you know, in case you think I'm just being lazy by asking you to try "LOADFIX -a" first. For anything earlier than perhaps up to the mid 1990s, this is probably the first thing you should check if it immediately crashes when you run it.

[joncampbell123]

This particular startup code appears in anything I compile using an old C compiler from the late 1980s called the Mix C compiler. Before I had better compilers (yes, Microsoft C/C++ in 1993 was better than this), this was a DOS compiler I used a lot back in the day.