Add word "ROLL"

blk/046

@@ -11,6 +11,6 @@ SWAP        a b -- b a
 2SWAP       a b c d -- c d a b
 PICK        Pick nth item from stack. "0 PICK" = DUP,
             "1 PICK" = OVER.
-
+ROLL        Rotate PSP over n items. "1 ROLL" = SWAP,
-
+            "2 ROLL" = ROT. 0 is noop.
 

forth/boot.z80

@@ -403,6 +403,27 @@ CODE PICK
     BC PUSHqq,
 ;CODE
 
+( this is only a part of ROLL, the other part is performed in
+  high level Forth. This receives from PSP the number of bytes
+  to copy and then performs A move-by-2 operation from SP.
+  This copies SP's TOS and overwrites the last item involved.
+
+  For example, if stack is "1 2 3 4", calling with "4" would
+  result in the stack "1 3 4 4". Never call with 0, there is
+  no sanity check.
+)
+CODE (roll)
+    HL POPqq,
+    B H LDrr,
+    C L LDrr,
+    SP ADDHLss,
+    D H LDrr,
+    E L LDrr,
+    HL DECss,
+    HL DECss,
+    LDDR,
+;CODE
+
 ( a b -- a b a b )
 CODE 2DUP
     HL POPqq,       ( B )

forth/core.fs

@@ -55,6 +55,13 @@
     H@ 2-          ( push a. -2 for allot offset )
 ; IMMEDIATE
 
+: ROLL
+    DUP NOT IF EXIT THEN
+    1+ DUP PICK          ( n val )
+    SWAP 2 * (roll)      ( val )
+    SWAP DROP
+;
+
 ( During a CASE, the stack grows by 1 at each ENDOF so that
   we can fill all those ENDOF branching addrs. So that we
   know when to stop, we put a 0 on PSP. That's our stopgap. )