diff --git a/README.md b/README.md
index 3a75310..5641f71 100644
--- a/README.md
+++ b/README.md
@@ -203,6 +203,46 @@ The following functions are used to access syscall parameters safely:
     void uvm32_setval(uvm32_state_t *vmst, uvm32_evt_t *evt, uvm32_arg_t, uint32_t val);
     uvm32_evt_syscall_buf_t uvm32_getbuf(uvm32_state_t *vmst, uvm32_evt_t *evt, uvm32_arg_t argPtr, uvm32_arg_t argLen);
 
+## Event driven operation
+
+A useful pattern for code running in the VM is to be event-driven. In this setup the program requests blocks until woken up with a reason. This requires some support in the host, but can be implemented as follows.
+
+In the VM code:
+
+```c
+while(1) {
+    // ask host to suspend running until one of the following events
+    uint32_t wakeReason = yield(KEYPRESS_EVENT_MASK | MOUSE_EVENT_MASK | NETWORK_EVENT_MASK);
+    switch(wakeReason) {
+        ...
+    }
+}
+```
+
+In the host code:
+
+```c
+uvm32_run(&vmst, &evt, 1000);
+switch(evt.typ) {
+    case UVM32_EVT_SYSCALL:
+        switch(evt.data.syscall.code) {
+            case UVM32_SYSCALL_YIELD:
+                uint32_t events = uvm32_getval(&vmst, &evt, ARG0);
+                // do not call uvm32_run() again until something in the events set is triggered
+            break;
+            ...
+        }
+    break;
+    ...
+}
+```
+
+Then, to wake the VM once an (eg. key) event has occurred
+
+```c
+uvm32_setval(&vmst, &evt, RET, KEYPRESS_EVENT_MASK);
+uvm32_run(&vmst, &evt, 1000);
+```
 
 ## Configuration
 
diff --git a/apps/lissajous/lissajous.c b/apps/lissajous/lissajous.c
index 8aa19bb..e957a96 100644
--- a/apps/lissajous/lissajous.c
+++ b/apps/lissajous/lissajous.c
@@ -13,13 +13,6 @@ void movecursor(int x, int y) {
     print("f");
 }
 
-void sleep(uint32_t ms) {
-    uint32_t start = millis();
-    while (millis() < start + ms) {
-        yield();
-    }
-}
-
 // Absolute value function for doubles
 double abs_c(double x) {
     if (x < 0) {
@@ -116,7 +109,7 @@ void main(void) {
 
         // wait for next frame
         while (millis() < framestart + (1000 / 30)) {
-            yield();
+            yield(0);
         }
         beta += 0.05;
         freq1 += 0.01;
diff --git a/apps/self/self.c b/apps/self/self.c
index 5821e1a..fa98ab9 100644
--- a/apps/self/self.c
+++ b/apps/self/self.c
@@ -45,6 +45,8 @@ void main(void) {
             break;
             case UVM32_EVT_SYSCALL:    // vm has paused to handle UVM32_SYSCALL
                 switch(evt.data.syscall.code) {
+                    case UVM32_SYSCALL_YIELD:
+                    break;
                     case UVM32_SYSCALL_PUTC:
                         putc(uvm32_getval(&vmst, &evt, ARG0));
                     break;
diff --git a/apps/sketch/sketch.c b/apps/sketch/sketch.c
index 27c6a59..67d23fd 100644
--- a/apps/sketch/sketch.c
+++ b/apps/sketch/sketch.c
@@ -25,7 +25,7 @@ void setup(void) {
 void main(void) {
     setup();
     while(loop()) {
-        yield();
+        yield(0);
     }
 }
 
diff --git a/common/uvm32_target.h b/common/uvm32_target.h
index 4439804..15144a1 100644
--- a/common/uvm32_target.h
+++ b/common/uvm32_target.h
@@ -56,7 +56,7 @@ static uint32_t syscall(uint32_t id, uint32_t param1, uint32_t param2) {
 #define millis()        syscall_cast(UVM32_SYSCALL_MILLIS, 0, 0)
 #define putc(x)         syscall_cast(UVM32_SYSCALL_PUTC, x, 0)
 #define getc()          syscall_cast(UVM32_SYSCALL_GETC, 0, 0)
-#define yield()         syscall_cast(UVM32_SYSCALL_YIELD, 0, 0)
+#define yield(x)        syscall_cast(UVM32_SYSCALL_YIELD, x, 0)
 #define printbuf(x, y)  syscall_cast(UVM32_SYSCALL_PRINTBUF, x, y)
 
 extern char _estack;
diff --git a/host-arduino/host-arduino.ino b/host-arduino/host-arduino.ino
index 6487c1f..9abc23c 100644
--- a/host-arduino/host-arduino.ino
+++ b/host-arduino/host-arduino.ino
@@ -57,6 +57,8 @@ void loop(void) {
             break;
             case UVM32_EVT_SYSCALL:    // vm has paused to handle UVM32_SYSCALL
                 switch(evt.data.syscall.code) {
+                    case UVM32_SYSCALL_YIELD:
+                    break;
                     case UVM32_SYSCALL_PUTC:
                         Serial.print((char)uvm32_getval(&vmst, &evt, ARG0));
                     break;
diff --git a/host-arduino/uvm32.cpp b/host-arduino/uvm32.cpp
index dd9097c..f93ab92 100644
--- a/host-arduino/uvm32.cpp
+++ b/host-arduino/uvm32.cpp
@@ -1,4 +1,5 @@
 #include "config.h"
+
 #define MINIRV32_IMPLEMENTATION
 #include "uvm32.h"
 
@@ -11,6 +12,9 @@
 // On an invalid operation, an error is set in uvm32_state_t, but a valid pointer still needs to be temporarily used
 static uint32_t garbage;
 
+// magic value for stack canary
+#define STACK_CANARY_VALUE 0x42
+
 #ifndef UVM32_MEMCPY
 #define UVM32_MEMCPY uvm32_memcpy
 void uvm32_memcpy(void *dst, const void *src, int len) {
@@ -85,6 +89,8 @@ bool uvm32_load(uvm32_state_t *vmst, const uint8_t *rom, int len) {
     }
 
     UVM32_MEMCPY(vmst->memory, rom, len);
+    vmst->stack_canary = (uint8_t *)UVM32_NULL;
+    
     return true;
 }
 
@@ -136,6 +142,12 @@ void uvm32_clearError(uvm32_state_t *vmst) {
 uint32_t uvm32_run(uvm32_state_t *vmst, uvm32_evt_t *evt, uint32_t instr_meter) {
     uint32_t num_instr = 0;
 
+    if (vmst->stack_canary != UVM32_NULL && *vmst->stack_canary != STACK_CANARY_VALUE) {
+        setStatusErr(vmst, UVM32_ERR_STACKOVERFLOW);
+        setup_err_evt(vmst, evt);
+        return num_instr;
+    }
+
     if (vmst->status != UVM32_STATUS_PAUSED) {
         setStatusErr(vmst, UVM32_ERR_NOTREADY);
         setup_err_evt(vmst, evt);
@@ -160,10 +172,25 @@ uint32_t uvm32_run(uvm32_state_t *vmst, uvm32_evt_t *evt, uint32_t instr_meter)
                  case UVM32_SYSCALL_HALT:
                     setStatus(vmst, UVM32_STATUS_ENDED);
                 break;
-                case UVM32_SYSCALL_YIELD:
-                    vmst->ioevt.typ = UVM32_EVT_YIELD;
-                    setStatus(vmst, UVM32_STATUS_PAUSED);
-                break;
+                case UVM32_SYSCALL_STACKPROTECT: {
+                    // don't allow errant code to change it once set
+                    if (vmst->stack_canary == (uint8_t *)UVM32_NULL) {
+                        uint32_t param0 = vmst->core.regs[10]; // a0
+                        uint32_t mem_offset = param0 - MINIRV32_RAM_IMAGE_OFFSET;
+
+                        // check data fits in ram
+                        if (mem_offset > UVM32_MEMORY_SIZE) {
+                            setStatusErr(vmst, UVM32_ERR_STACKOVERFLOW);
+                            setup_err_evt(vmst, evt);
+                        }
+                        // check canary is inside valid memory
+                        if (mem_offset < UVM32_MEMORY_SIZE) {
+                            // set canary
+                            vmst->stack_canary = &vmst->memory[mem_offset];
+                            *vmst->stack_canary = STACK_CANARY_VALUE;
+                        }
+                    }
+                } break;
                 default:
                     // user defined syscalls
                     vmst->ioevt.typ = UVM32_EVT_SYSCALL;
diff --git a/host-mini/host-mini.c b/host-mini/host-mini.c
index e32ad0d..a9eb539 100644
--- a/host-mini/host-mini.c
+++ b/host-mini/host-mini.c
@@ -47,6 +47,8 @@ int main(int argc, char *argv[]) {
             break;
             case UVM32_EVT_SYSCALL:    // vm has paused to handle UVM32_SYSCALL
                 switch(evt.data.syscall.code) {
+                    case UVM32_SYSCALL_YIELD:
+                    break;
                     case UVM32_SYSCALL_PUTC:
                         printf("%c", uvm32_getval(&vmst, &evt, ARG0));
                     break;
diff --git a/host-parallel/host-parallel.c b/host-parallel/host-parallel.c
index 13cd69b..d2d3fd7 100644
--- a/host-parallel/host-parallel.c
+++ b/host-parallel/host-parallel.c
@@ -81,6 +81,8 @@ int main(int argc, char *argv[]) {
             break;
             case UVM32_EVT_SYSCALL:    // vm has paused to handle UVM32_SYSCALL
                 switch(evt.data.syscall.code) {
+                    case UVM32_SYSCALL_YIELD:
+                    break;
                     case UVM32_SYSCALL_PRINTLN: {
                         const char *str = uvm32_getcstr(&vmst[scheduler_index], &evt, ARG0);
                         if (str[0] != 0) {
diff --git a/host/host.c b/host/host.c
index 2762881..27860bc 100644
--- a/host/host.c
+++ b/host/host.c
@@ -167,10 +167,6 @@ int main(int argc, char *argv[]) {
                 printf("UVM32_EVT_END\n");
                 isrunning = false;
             break;
-            case UVM32_EVT_YIELD:
-                //printf("UVM32_EVT_YIELD\n");
-                // program has paused, but no syscall
-            break;
             case UVM32_EVT_ERR:
                 printf("UVM32_EVT_ERR '%s' (%d)\n", evt.data.err.errstr, (int)evt.data.err.errcode);
                 if (evt.data.err.errcode == UVM32_ERR_HUNG) {
@@ -187,6 +183,11 @@ int main(int argc, char *argv[]) {
                             printf("%02x", *buf.ptr++);
                         }
                     } break;
+                    case UVM32_SYSCALL_YIELD: {
+                        // uint32_t yield_typ = uvm32_getval(&vmst, &evt, ARG0);
+                        // printf("YIELD type=%d\n", yield_typ);
+                        // uvm32_setval(&vmst, &evt, RET, 123);
+                    } break;
                     case UVM32_SYSCALL_PRINT: {
                         const char *str = uvm32_getcstr(&vmst, &evt, ARG0);
                         printf("%s", str);
diff --git a/precompiled/fib.bin b/precompiled/fib.bin
index 9d95c6d..a92bdfd 100755
Binary files a/precompiled/fib.bin and b/precompiled/fib.bin differ
diff --git a/precompiled/self.bin b/precompiled/self.bin
index 15a686a..87d3517 100755
Binary files a/precompiled/self.bin and b/precompiled/self.bin differ
diff --git a/uvm32/uvm32.c b/uvm32/uvm32.c
index 6215f09..2cb61db 100644
--- a/uvm32/uvm32.c
+++ b/uvm32/uvm32.c
@@ -170,14 +170,10 @@ uint32_t uvm32_run(uvm32_state_t *vmst, uvm32_evt_t *evt, uint32_t instr_meter)
                  case UVM32_SYSCALL_HALT:
                     setStatus(vmst, UVM32_STATUS_ENDED);
                 break;
-                case UVM32_SYSCALL_YIELD:
-                    vmst->ioevt.typ = UVM32_EVT_YIELD;
-                    setStatus(vmst, UVM32_STATUS_PAUSED);
-                break;
                 case UVM32_SYSCALL_STACKPROTECT: {
                     // don't allow errant code to change it once set
                     if (vmst->stack_canary == (uint8_t *)UVM32_NULL) {
-                        uint32_t param0 = vmst->core.regs[10];
+                        uint32_t param0 = vmst->core.regs[10]; // a0
                         uint32_t mem_offset = param0 - MINIRV32_RAM_IMAGE_OFFSET;
 
                         // check data fits in ram
diff --git a/uvm32/uvm32.h b/uvm32/uvm32.h
index 588e761..6fd44c5 100644
--- a/uvm32/uvm32.h
+++ b/uvm32/uvm32.h
@@ -26,7 +26,6 @@ typedef enum {
 typedef enum {
     UVM32_EVT_ERR,
     UVM32_EVT_SYSCALL,
-    UVM32_EVT_YIELD,
     UVM32_EVT_END,
 } uvm32_evt_typ_t;