diff --git a/aux/build.sh b/aux/build.sh
index b8af9d59..56c7c104 100755
--- a/aux/build.sh
+++ b/aux/build.sh
@@ -7,4 +7,4 @@ set -xe
 cd $LEGO/lmssrc/adk/lmsasm
 java -jar assembler.jar $F
 cd "$P"
-node -p 'require("fs").readFileSync("pxt.rbf").toString("hex")'
+node -p 'require("fs").readFileSync("pxt.rbf").toString("hex").replace(/5858585858(58)+/, "XX")'
diff --git a/editor/extension.ts b/editor/extension.ts
index f9a7a141..366b3507 100644
--- a/editor/extension.ts
+++ b/editor/extension.ts
@@ -8,8 +8,7 @@ eval("if (typeof process === 'object' && process + '' === '[object process]') px
 
 namespace pxt.editor {
     // this comes from aux/pxt.lms
-    const rbfTemplate = "4c45474f5d0000006d000100000000001c0000000000000008000000821b028405018130813e805374617274696e672e2e2e00840060805858585858585858580044830383010640414082f5ff8405018130813e80427965210084000a";
-
+    const rbfTemplate = "4c45474f5d0000006d000100000000001c0000000000000008000000821b028405018130813e805374617274696e672e2e2e0084006080XX0044830383010640414082f5ff8405018130813e80427965210084000a";
 
     function hf2Async() {
         return pxt.HF2.mkPacketIOAsync()
@@ -38,14 +37,19 @@ namespace pxt.editor {
         return initAsync()
             .then(w_ => {
                 w = w_
+                // run LS to make sure we can talk to device first
+                // otherwise flashing a file might lock it
+                return w.lsAsync("/")
+            }).then(() => {
                 let f = U.stringToUint8Array(atob(resp.outfiles[pxt.outputName()]))
                 return w.flashAsync(elfPath, f)
-            })
-            .then(() => {
-                let rbfHex = rbfTemplate.replace(/58585858(58)+/, U.toHex(U.stringToUint8Array(elfPath)))
+            }).then(() => {
+                let rbfHex = rbfTemplate.replace("XX", U.toHex(U.stringToUint8Array(elfPath)))
                 let rbf = U.fromHex(rbfHex)
                 HF2.write16(rbf, 4, rbf.length)
                 return w.flashAsync(rbfPath, rbf)
+            }).then(() => {
+                return w.runAsync(rbfPath)
             }).then(() => {
                 if (isCli)
                     return w.disconnectAsync()
@@ -61,4 +65,4 @@ namespace pxt.editor {
         };
         return Promise.resolve<pxt.editor.ExtensionResult>(res);
     }
-}
\ No newline at end of file
+}
diff --git a/editor/wrap.ts b/editor/wrap.ts
index 2e4ac5d9..f37105bb 100644
--- a/editor/wrap.ts
+++ b/editor/wrap.ts
@@ -12,6 +12,8 @@ namespace pxt.editor {
         size?: number;
     }
 
+    const runTemplate = "C00882010084XX0060640301606400"
+
     export class Ev3Wrapper {
         msgs = new U.PromiseBuffer<Uint8Array>()
         private cmdSeq = U.randomUint32() & 0xffff;
@@ -24,33 +26,52 @@ namespace pxt.editor {
             }
         }
 
-        private alloc(addSize: number, cmd: number, replyType = 1) {
-            let len = 6 + addSize
+        private allocCore(addSize: number, replyType: number) {
+            let len = 5 + addSize
             let buf = new Uint8Array(len)
             HF2.write16(buf, 0, len - 2)  // pktLen
             HF2.write16(buf, 2, this.cmdSeq++) // msgCount
             buf[4] = replyType
+            return buf
+        }
+
+        private allocSystem(addSize: number, cmd: number, replyType = 1) {
+            let buf = this.allocCore(addSize + 1, replyType)
             buf[5] = cmd
             return buf
         }
 
+        runAsync(path: string) {
+            let codeHex = runTemplate.replace("XX", U.toHex(U.stringToUint8Array(path)))
+            let code = U.fromHex(codeHex)
+            let pkt = this.allocCore(2 + code.length, 0)
+            HF2.write16(pkt, 5, 0x0800)
+            U.memcpy(pkt, 7, code)
+            log(`run ${path}`)
+            return this.talkAsync(pkt)
+                .then(buf => {
+                })
+        }
+
         talkAsync(buf: Uint8Array) {
             return this.io.sendPacketAsync(buf)
                 .then(() => this.msgs.shiftAsync(1000))
                 .then(resp => {
                     if (resp[2] != buf[2] || resp[3] != buf[3])
                         U.userError("msg count de-sync")
-                    if (resp[5] != buf[5])
-                        U.userError("cmd de-sync")
-                    if (resp[6] != 0 && resp[6] != 8 /* EOF */)
-                        U.userError("cmd error: " + resp[6])
+                    if (buf[4] == 1) {
+                        if (resp[5] != buf[5])
+                            U.userError("cmd de-sync")
+                        if (resp[6] != 0 && resp[6] != 1 /* LS? */ && resp[6] != 8 /* EOF */)
+                            U.userError("cmd error: " + resp[6])
+                    }
                     return resp
                 })
         }
 
         flashAsync(path: string, file: Uint8Array) {
             log(`write ${file.length} to ${path}`)
-            let begin = this.alloc(4 + path.length + 1, 0x92)
+            let begin = this.allocSystem(4 + path.length + 1, 0x92)
             HF2.write32(begin, 6, file.length) // fileSize
             U.memcpy(begin, 10, U.stringToUint8Array(path))
 
@@ -58,7 +79,7 @@ namespace pxt.editor {
                 if (pos >= file.length) return Promise.resolve()
                 let size = file.length - pos
                 if (size > 1000) size = 1000
-                let upl = this.alloc(1 + size, 0x93, 0x1)
+                let upl = this.allocSystem(1 + size, 0x93, 0x1)
                 upl[6] = handle
                 U.memcpy(upl, 6 + 1, file, pos, size)
                 return this.talkAsync(upl)
@@ -74,7 +95,7 @@ namespace pxt.editor {
         }
 
         lsAsync(path: string): Promise<DirEntry[]> {
-            let lsReq = this.alloc(2 + path.length + 1, 0x99)
+            let lsReq = this.allocSystem(2 + path.length + 1, 0x99)
             HF2.write16(lsReq, 6, 1024) // maxRead
             U.memcpy(lsReq, 8, U.stringToUint8Array(path))
 
diff --git a/libs/core/test.ts b/libs/core/test.ts
index a57a41e3..a96989d8 100644
--- a/libs/core/test.ts
+++ b/libs/core/test.ts
@@ -1,6 +1,6 @@
 screen.clear()
 screen.setFont(ScreenFont.Large)
-screen.drawText(10, 30, "Hello PXT!")
+screen.drawText(10, 30, "Welcome PXT!")
 
 screen.drawEllipse(40, 40, 20, 10, Draw.Fill)
 output.setLights(LightsPattern.Orange)