X-Git-Url: https://git.rrq.au/?a=blobdiff_plain;f=notes;h=69065509654742c56bc3c3f2a54ea5d7391169cd;hb=1da54a4e8c55400181194a2db2cce4b96e479d61;hp=d10de8057314223c1ebfea7266e1323716cd59d2;hpb=d6efaa23e200d7a061715f8a036221a03d9d8055;p=rrq%2Fjonasforth.git

diff --git a/notes b/notes
index d10de80..6906550 100644
--- a/notes
+++ b/notes
@@ -59,3 +59,41 @@ given function. In order to run such functions, we actually need two jumps when
 executing: In order to execute a word, we jump to the address at the location
 pointed to by the address in ESI.
 
+## Definitions
+
+What does the codeword of a Forth word contain? It needs to save the old value
+of ESI (so that we can resume execution of whatever outer definition we are
+executing at the time) and set the new version of ESI to point to the first word
+in the inner definition.
+
+The stack where the values of ESI are stored is called the "return stack". We
+will use EBP for the return stack.
+
+As mentioned, whenever we finish executing a Forth word, we will need to
+continue execution in the manner described in the previous section. When the
+word being executed is itself written in Forth, we need to pop the old value of
+ESI that we saved at the beginning of the definition before doing this.
+
+Thus, the actual data for a word in a dictionary will look something like this:
+
+      pointer to previous word
+       ^
+       |
+    +--|------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+
+    | LINK    | 6 | D | O | U | B | L | E | 0 | DOCOL      | DUP        | +          | EXIT       |
+    +---------+---+---+---+---+---+---+---+---+------------+--|---------+------------+------------+
+       ^       len                         pad  codeword      |
+       |                                                      V
+      LINK in next word                            points to codeword of DUP
+
+Here, DOCOL (the codeword) is address of the simple interpreter described above,
+while EXIT a word (implemented in assembly) that takes care of popping ESI and
+continuing execution. Note that DOCOL, DUP, + and EXIT are all stored as
+addresses which point to codewords.
+
+## Literals
+
+Literals are handled in a special way. There is a word in Forth, called LIT,
+implemented in assembly. When executed, this word looks at the next Forth
+instruction (i.e. the value of ESI), and places that on the stack as a literal,
+and then manipulates ESI to skip over the literal value.