diff --git a/ueforth/arduino/arduino.template.ino b/ueforth/arduino/arduino.template.ino
index c6834ac..b0429d7 100644
--- a/ueforth/arduino/arduino.template.ino
+++ b/ueforth/arduino/arduino.template.ino
@@ -343,30 +343,24 @@ static cell_t ResizeFile(cell_t fd, cell_t size) {
 #ifdef ENABLE_WEBSERVER_SUPPORT
 static void InvokeWebServerOn(WebServer *ws, const char *url, cell_t xt) {
   ws->on(url, [xt]() {
-    cell_t *old_ip = g_sys.ip;
-    cell_t *old_rp = g_sys.rp;
-    cell_t *old_sp = g_sys.sp;
-    cell_t stack[16];
-    cell_t rstack[16];
-    g_sys.sp = stack + 1;
-    g_sys.rp = rstack;
     cell_t code[2];
     code[0] = xt;
     code[1] = g_sys.YIELD_XT;
-    g_sys.ip = code;
-    ueforth_run();
-    g_sys.ip = old_ip;
-    g_sys.rp = old_rp;
-    g_sys.sp = old_sp;
+    cell_t stack[16];
+    cell_t rstack[16];
+    cell_t *rp = rstack;
+    *++rp = (cell_t) (stack + 1);
+    *++rp = (cell_t) code;
+    ueforth_run(rp);
   });
 }
 #endif
 
 void setup() {
   cell_t *heap = (cell_t *) malloc(HEAP_SIZE);
-  ueforth(0, 0, heap, boot, sizeof(boot));
+  ueforth_init(0, 0, heap, boot, sizeof(boot));
 }
 
 void loop() {
-  ueforth_run();
+  g_sys.rp = ueforth_run(g_sys.rp);
 }
diff --git a/ueforth/common/core.h b/ueforth/common/core.h
index c2d3b09..4620076 100644
--- a/ueforth/common/core.h
+++ b/ueforth/common/core.h
@@ -14,7 +14,7 @@ static struct {
   cell_t *heap, *last, notfound;
   int argc;
   char **argv;
-  cell_t *ip, *sp, *rp;  // Parked alternates
+  cell_t *rp;  // spot to park main thread
   cell_t DOLIT_XT, DOEXIT_XT, YIELD_XT;
 } g_sys;
 
@@ -120,30 +120,29 @@ static cell_t *evaluate1(cell_t *sp) {
   return sp;
 }
 
-static void ueforth_run(void);
+static cell_t *ueforth_run(cell_t *initrp);
 
-static void ueforth(int argc, char *argv[], void *heap,
-                    const char *src, cell_t src_len) {
-  g_sys.ip = 0;
+static void ueforth_init(int argc, char *argv[], void *heap,
+                         const char *src, cell_t src_len) {
   g_sys.heap = (cell_t *) heap + 4;  // Leave a little room.
-  ueforth_run();
-  g_sys.sp = g_sys.heap + 1; g_sys.heap += STACK_SIZE;
-  g_sys.rp = g_sys.heap + 1; g_sys.heap += STACK_SIZE;
+  ueforth_run(0);
+  cell_t *sp = g_sys.heap + 1; g_sys.heap += STACK_SIZE;
+  cell_t *rp = g_sys.heap + 1; g_sys.heap += STACK_SIZE;
   g_sys.last[-1] = 1;  // Make ; IMMEDIATE
   g_sys.DOLIT_XT = FIND("DOLIT");
   g_sys.DOEXIT_XT = FIND("EXIT");
   g_sys.YIELD_XT = FIND("YIELD");
   g_sys.notfound = FIND("DROP");
-  g_sys.ip = g_sys.heap;
+  cell_t *start = g_sys.heap;
   *g_sys.heap++ = FIND("EVALUATE1");
   *g_sys.heap++ = FIND("BRANCH");
-  *g_sys.heap++ = (cell_t) g_sys.ip;
+  *g_sys.heap++ = (cell_t) start;
   g_sys.argc = argc;
   g_sys.argv = argv;
   g_sys.base = 10;
   g_sys.tib = src;
   g_sys.ntib = src_len;
-  for (;;) {
-    ueforth_run();
-  }
+  *++rp = (cell_t) sp;
+  *++rp = (cell_t) start;
+  g_sys.rp = rp;
 }
diff --git a/ueforth/common/interp.h b/ueforth/common/interp.h
index 4f9a933..76a7021 100644
--- a/ueforth/common/interp.h
+++ b/ueforth/common/interp.h
@@ -4,15 +4,16 @@
 #define ADDR_DOCREATE && OP_DOCREATE
 #define ADDR_DODOES && OP_DODOES
 
-static void ueforth_run(void) {
-  if (!g_sys.ip) {
+static cell_t *ueforth_run(cell_t *init_rp) {
+  if (!init_rp) {
 #define X(name, op, code) create(name, sizeof(name) - 1, name[0] == ';', && OP_ ## op);
     PLATFORM_OPCODE_LIST
     OPCODE_LIST
 #undef X
-    return;
+    return 0;
   }
-  register cell_t *ip = g_sys.ip, *rp = g_sys.rp, *sp = g_sys.sp, tos, w;
+  register cell_t *ip, *rp, *sp, tos, w;
+  rp = init_rp;  ip = (cell_t *) *rp--;  sp = (cell_t *) *rp--;
   DROP; NEXT;
 #define X(name, op, code) OP_ ## op: { code; } NEXT;
   PLATFORM_OPCODE_LIST
diff --git a/ueforth/common/opcodes.h b/ueforth/common/opcodes.h
index 72aa88c..5efa69b 100644
--- a/ueforth/common/opcodes.h
+++ b/ueforth/common/opcodes.h
@@ -11,7 +11,7 @@ typedef uintptr_t ucell_t;
 #define COMMA(n) *g_sys.heap++ = (n)
 #define IMMEDIATE() g_sys.last[-1] |= 1
 #define DOES(ip) *g_sys.last = (cell_t) ADDR_DODOES; g_sys.last[1] = (cell_t) ip
-#define PARK DUP; g_sys.ip = ip; g_sys.rp = rp; g_sys.sp = sp
+#define PARK DUP; *++rp = (cell_t) sp; *++rp = (cell_t) ip
 
 #ifndef SSMOD_FUNC
 # if __SIZEOF_POINTER__ == 8
@@ -71,7 +71,7 @@ typedef int64_t dcell_t;
   X("DOES>", DOES, DOES(ip); ip = (cell_t *) *rp; --rp) \
   X("IMMEDIATE", IMMEDIATE, IMMEDIATE()) \
   X("'SYS", SYS, DUP; tos = (cell_t) &g_sys) \
-  X("YIELD", YIELD, PARK; return) \
+  X("YIELD", YIELD, PARK; return rp) \
   X(":", COLON, DUP; DUP; tos = parse(32, sp); \
                 create((const char *) *sp, tos, 0, ADDR_DOCOLON); \
                 g_sys.state = -1; --sp; DROP) \
diff --git a/ueforth/posix/posix_main.c b/ueforth/posix/posix_main.c
index 674c5a0..9b9284c 100644
--- a/ueforth/posix/posix_main.c
+++ b/ueforth/posix/posix_main.c
@@ -18,6 +18,7 @@
 
 int main(int argc, char *argv[]) {
   void *heap = mmap(0, HEAP_SIZE, PROT_EXEC | PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
-  ueforth(argc, argv, heap, boot, sizeof(boot));
+  ueforth_init(argc, argv, heap, boot, sizeof(boot));
+  for (;;) { g_sys.rp = ueforth_run(g_sys.rp); }
   return 1;
 }
diff --git a/ueforth/windows/windows_interp.h b/ueforth/windows/windows_interp.h
index 562a1e8..e80733d 100644
--- a/ueforth/windows/windows_interp.h
+++ b/ueforth/windows/windows_interp.h
@@ -14,16 +14,17 @@ enum {
 #undef X
 };
 
-static void ueforth_run(void) {
-  if (!g_sys.ip) {
+static cell_t *ueforth_run(cell_t *init_rp) {
+  if (!init_rp) {
 #define X(name, op, code) \
     create(name, sizeof(name) - 1, name[0] == ';', (void *) OP_ ## op);
     PLATFORM_OPCODE_LIST
     OPCODE_LIST
 #undef X
-    return;
+    return 0;
   }
-  register cell_t *ip = g_sys.ip, *rp = g_sys.rp, *sp = g_sys.sp, tos, w;
+  register cell_t *ip, *rp, *sp, tos, w;
+  rp = init_rp;  ip = (cell_t *) *rp--;  sp = (cell_t *) *rp--;
   DROP;
   for (;;) {
 next:
diff --git a/ueforth/windows/windows_main.c b/ueforth/windows/windows_main.c
index 5293455..41e2f60 100644
--- a/ueforth/windows/windows_main.c
+++ b/ueforth/windows/windows_main.c
@@ -38,6 +38,7 @@ int WINAPI WinMain(HINSTANCE inst, HINSTANCE prev, LPSTR cmd, int show) {
 #endif
   void *heap = VirtualAlloc(
       NULL, HEAP_SIZE, MEM_RESERVE | MEM_COMMIT, PAGE_EXECUTE_READWRITE);
-  ueforth(0, 0, heap, boot, sizeof(boot));
+  ueforth_init(0, 0, heap, boot, sizeof(boot));
+  for (;;) { g_sys.rp = ueforth_run(g_sys.rp); }
 }