6809: tons of improvements:

* stack variables
* large comparisons
* pointer optimizations
* fixed constant evaluation

docs/api/command-line.md

@@ -187,6 +187,14 @@ Allow using the IY register for other purposes.
 Compiling to 8086 is based on translating from a mix of 8085 and Z80 instructions to 8086.
 See [the 8086 support disclaimer](./../lang/x86disclaimer.md).
 
+#### 6809-related
+
+* `-fuse-u-for-stack`, `-fuse-y-for-stack`
+Which of Z80 index registers should be used as the base pointer for accessing stack variables, if any. 
+`.ini` equivalent: `u_stack` and `y_stack`. Default: none.  
+**Warning: Currently, picking one of those two options is required!**
+The compiler doesn't support accessing the stack variables via the S stack pointer register yet.
+
 ## Optimization options
 
 * `-O0` – Disable all optimizations except unused global symbol removal.

docs/api/custom-platform.md

@@ -114,6 +114,13 @@ This list cannot contain module template instantiations.
     
     * `iy_scratch` – allow using the IY register for other purposes, default is `false`
     
+    * `u_stack` – use the U register to access stack variables, default is `false`. Applicable only to 6809-based targets.
+    
+    * `y_stack` – use the Y register to access stack variables, default is `false`. Applicable only to 6809-based targets.  
+    **Warning: Currently, picking either `u_stack` or `y_stack` is required,
+    unless you want to always specify this option in the compiler's command line!**
+    The compiler doesn't support accessing the stack variables via the S stack pointer register yet.
+    
     * `software_stack` – use software stack for stack variables, default is `false`. Applicable only to 6502-based targets.
     
     * `output_intel_syntax` – use Intel syntax instead of Zilog syntax, default is `true` for Intel 8080/8085 and `false` otherwise