Update nest() to DTC. Add Idea.md

portable/Idea.md

@@ -0,0 +1,17 @@
+# A virtual C-implemented VM as core for a portable VolksForth
+
+Basic idea is a Forth VM with direct or indirect threaded code,
+operating on a 64kB byte array core, implemented in C.
+For compatibility with the other VolksForth flavours we would likely
+let data stack and return stack live inside the core, not in separate
+C data structures.
+
+Forth primitives like nest, unnest, dup, emit, + etc. are implemnted
+as C functions. Each primitive has a virtual bytecode opcode associated
+with it. These virtual opcodes usually sit in the CFA of Forth words.
+
+Yet TBD is whether it is worth enhancing the execution of these opcodes
+to a virtual bytecode CPU allowing CODE words in Forth code using
+these virtual opcodes, or whether each opcode is considered to have
+an implicit NEXT call at its end, i.e. only predefined CODE words of
+length 1 are supported.

portable/core.c

@@ -40,7 +40,8 @@ void jumptable[0x40]() = {
 
 void nest() {
   rs[rp++] = ip;
-  ip = w + 2;
+  ip = w + 1;  // + 1 assuming direct threaded code where at core[w]
+  // is sitting the 1 byte virtual opcode for "next".
 }
 
 void unnest() {