[Research] Find out why we can't decompile TH05's hud_bar_put()

Part of P0033, funded by zorg.

Research/Borland C++ decompilation.md

@@ -0,0 +1,17 @@
+## Limits of decompilability
+
+### `MOV BX, SP`-style functions, no stack frame
+
+These almost certainly weren't compiled from C. By disabling stack frames
+using `#pragma option -k-`, it *might* be possible to still get the exact same
+code out of Turbo C++ – even though it will most certainly look horrible, and
+barely more readable than assembly (or even less so), with tons of inline ASM
+and register pseudovariables. However, it's futile to even try if the function
+contains one of the following:
+
+* A reference to the `DI` register. In that case, Turbo C++ always inserts a
+  `PUSH DI` at the beginning (before the `MOV BX, SP`), and a `POP DI` before
+  returning.
+
+  **Certainty:** Confirmed through reverse-engineering `TCC.EXE`, no way
+  around it.