Rename documentation files

docs/lang/interfacing.md

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+# Interfacing with external code 
+
+## Calling external functions at a static address
+
+To call an external function, you need to declare it as `asm extern`. For example:
+
+```
+asm void putchar(byte a) @$FFD2 extern
+```
+
+The function parameter will be passed via the accumulator,
+the function itself is located in ROM at $FFD2. A call like this:
+
+```
+putchar(13)
+```
+
+will be compiled to something like this:
+
+```
+LDA #13
+JSR $FFD2
+```
+
+For more details about how to pass parameters to `asm` functions,
+see [Using assembly within Millfork programs#Assembly functions](./assembly.md#assembly-functions).
+
+## Calling external functions at a dynamic address
+
+To call a function that has its address calculated dynamically, 
+you just need to do the same as what you would do in assembly:  
+
+```
+asm void call_function(byte a) {
+    JMP (function_address)
+}
+```
+
+where `function_address` is a variable that contains the address of the function to call.