cvm: implement stack overflow error condition

blk/091

@@ -5,12 +5,12 @@ sets that don't change (well, not without some binary manipu-
 lation). Here's the complete list of these references:
 
 04 BOOT addr         06 (uflw) addr      08 LATEST
-2b (s) wordref       33 2>R wordref      42 EXIT wordref
-53 (br) wordref      67 (?br) wordref    80 (loop) wordref
-bf (n) wordref
+13 (oflw) addr       2b (s) wordref      33 2>R wordref
+42 EXIT wordref      53 (br) wordref     67 (?br) wordref
+80 (loop) wordref    bf (n) wordref
 
-BOOT and (uflw) exist because they are referred to before those
-words are defined (in core words). LATEST is a critical part
-of the initialization sequence.
+BOOT, (uflw) and (oflw) exist because they are referred to
+before those words are defined (in core words). LATEST is a
+critical part of the initialization sequence.
 
                                                         (cont.)

blk/381

@@ -7,4 +7,6 @@
 : NL 0x0a RAM+ @ ( NLPTR ) ?DUP IF EXECUTE ELSE CRLF THEN ;
 : (uflw) LIT" stack underflow" ERR ;
 XCURRENT @ _xapply ORG @ 0x06 ( stable ABI uflw ) + !
+: (oflw) LIT" stack overflow" ERR ;
+XCURRENT @ _xapply ORG @ 0x13 ( stable ABI oflw ) + !
 : (wnf) (print) SPC LIT" word not found" ERR ;

cvm/vm.c

@@ -76,23 +76,33 @@ static void sw(word addr, word val) {
 // pop word from SP
 static word pop() {
     if (vm.uflw) return 0;
-    if (vm.SP >= SP_ADDR) { vm.uflw = true; }
+    if (vm.SP >= SP_ADDR) { vm.uflw = true; return 0; }
     return vm.mem[vm.SP++] | vm.mem[vm.SP++] << 8;
 }
 // push word to SP
 static void push(word x) {
-    vm.SP -= 2; sw(vm.SP, x);
+    vm.SP -= 2;
+    if (vm.SP <= vm.RS) {
+        vm.oflw = true; vm.SP = SP_ADDR; vm.RS = RS_ADDR;
+        return;
+    }
+    sw(vm.SP, x);
     if (vm.SP < vm.minSP) { vm.minSP = vm.SP; }
 }
 // pop word from RS
 static word popRS() {
     if (vm.uflw) return 0;
-    if (vm.RS <= RS_ADDR) { vm.uflw = true; }
+    if (vm.RS <= RS_ADDR) { vm.uflw = true; return 0; }
     word x = gw(vm.RS); vm.RS -= 2; return x;
 }
 // push word to RS
 static void pushRS(word val) {
-    vm.RS += 2; sw(vm.RS, val);
+    vm.RS += 2;
+    if (vm.SP <= vm.RS) {
+        vm.oflw = true; vm.SP = SP_ADDR; vm.RS = RS_ADDR;
+        return;
+    }
+    sw(vm.RS, val);
     if (vm.RS > vm.maxRS) { vm.maxRS = vm.RS; }
 }
 
@@ -358,6 +368,7 @@ VM* VM_init() {
     sw(SYSVARS+0x02, gw(0x08)); // CURRENT
     sw(SYSVARS+0x04, gw(0x08)); // HERE
     vm.uflw = false;
+    vm.oflw = false;
     vm.running = true;
     return &vm;
 }
@@ -380,6 +391,10 @@ bool VM_steps(int n) {
             vm.uflw = false;
             execute(gw(0x06)); /* uflw */
         }
+        if (vm.oflw) {
+            vm.oflw = false;
+            execute(gw(0x13)); /* oflw */
+        }
         n--;
     }
     return vm.running;

cvm/vm.h

@@ -39,6 +39,11 @@ typedef struct {
     // execute cycle. The goal is to avoid over-popping in native words that
     // pop more than once and thus corrupt memory.
     bool uflw;
+    // Same as uflw, but for stack overflow. However, we behave differently with
+    // oflw than with uflw. We can't prevent push() and pushRS() because it
+    // would prevent us from calling (oflw). Instead, we clear both stacks on
+    // oflw conditions, which gives us the room to maneuver.
+    bool oflw;
 } VM;
 
 VM* VM_init();