Add constants for segment bounds

docs/doc_index.md

@@ -22,6 +22,8 @@
 
 * [Literals](lang/literals.md)
 
+* [Predefined constants](lang/predefined_constants.md)
+
 * [List of text encodings and escape sequences](lang/text.md)
 
 * [Operators reference](lang/operators.md)

docs/lang/predefined_constants.md

@@ -0,0 +1,19 @@
+[< back to index](../doc_index.md)
+
+# Predefined constants
+
+* `byte nullchar` – the null terminator for strings in the default encoding, equivalent to `""z[0]`
+
+* `null$ nullptr` – the invalid pointer value; the value of the `NULLPTR` feature
+
+* `bool true`, `bool false` – boolean constants
+
+* `pointer segment.N.start` – the value of `segment_N_start` from the platform definition
+
+* `pointer segment.N.end` – the value of `segment_N_end` from the platform definition
+
+* `pointer segment.N.heapstart` – the address of the first byte in the `N` segment that was not automatically allocated
+
+* `word segment.N.length` – the number of byte locations between `segment_N_start` and `segment_N_end`, inclusive
+
+* `byte segment.N.bank` – the value of `segment_N_bank` from the platform definition