diff --git a/CHANGELOG.md b/CHANGELOG.md
index 971f6d4ae..a736bb65d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -13,9 +13,16 @@
   correction of the `hub.imu.heading()` value ([support#1678]).
 - Added `update_heading_correction` to interactively set the heading
   correction value ([support#1678]).
+- Added command to start downloaded or builtin user program with a one byte
+  program identifier. The existing START_PROGRAM and START_REPL are special
+  cases of this new more generic command, but will remain available for
+  backwards compatibility. For now, this is added to start various builtin
+  programs, but it prepares for the ability to start different downloaded
+  programs too  ([pybricks-micropython#254]).
 
 ### Changed
 
+- Changed protocol to Pybricks Profile v1.4.0.
 - When upgrading the firmware to a new version, the user program will now
   be erased. This avoids issues with incompatible program files ([support#1622]).
 - The `angular_velocity_threshold`, and `acceleration_threshold` settings
@@ -40,6 +47,7 @@
 
 [pybricks-micropython#250]: https://github.com/pybricks/pybricks-micropython/pull/250
 [pybricks-micropython#253]: https://github.com/pybricks/pybricks-micropython/pull/253
+[pybricks-micropython#254]: https://github.com/pybricks/pybricks-micropython/pull/254
 [support#1429]: https://github.com/pybricks/support/issues/1429
 [support#1460]: https://github.com/pybricks/support/issues/1460
 [support#1615]: https://github.com/pybricks/support/issues/1615
diff --git a/bricks/_common/micropython.c b/bricks/_common/micropython.c
index ef2ca1fd9..6ddc1084d 100644
--- a/bricks/_common/micropython.c
+++ b/bricks/_common/micropython.c
@@ -117,7 +117,7 @@ static void print_final_exception(mp_obj_t exc) {
     mp_obj_print_exception(&mp_plat_print, exc);
 }
 
-#if PYBRICKS_OPT_COMPILER
+#if PBSYS_CONFIG_FEATURE_BUILTIN_USER_PROGRAM_REPL
 static void run_repl(void) {
     readline_init0();
     pyexec_system_exit = 0;
@@ -145,7 +145,7 @@ static void run_repl(void) {
 
     nlr_set_abort(NULL);
 }
-#endif
+#endif // PBSYS_CONFIG_FEATURE_BUILTIN_USER_PROGRAM_REPL
 
 // From micropython/py/builtinimport.c, but copied because it is static.
 static void do_execute_raw_code(mp_module_context_t *context, const mp_raw_code_t *rc, const mp_module_context_t *mc) {
@@ -277,7 +277,7 @@ static void run_user_program(void) {
 
         print_final_exception(MP_OBJ_FROM_PTR(nlr.ret_val));
 
-        #if PYBRICKS_OPT_COMPILER
+        #if PBSYS_CONFIG_FEATURE_BUILTIN_USER_PROGRAM_REPL
         // On KeyboardInterrupt, drop to REPL for debugging.
         if (mp_obj_exception_match(MP_OBJ_FROM_PTR(nlr.ret_val), MP_OBJ_FROM_PTR(&mp_type_KeyboardInterrupt))) {
 
@@ -289,47 +289,39 @@ static void run_user_program(void) {
             // Enter REPL.
             run_repl();
         }
-        #endif
+        #endif // PBSYS_CONFIG_FEATURE_BUILTIN_USER_PROGRAM_REPL
     }
 
     nlr_set_abort(NULL);
 }
 
-/**
- * Builtin Pybricks MicroPython program identifiers.
- */
-typedef enum {
-    /**
-     * The MicroPython REPL.
-     */
-    PYBRICKS_MICROPYTHON_BUILTIN_USER_PROGRAM_ID_REPL = 0,
-    /**
-     * Program that detects attached devices, displays sensor values, and
-     * relays sensor data to host if connected.
-     */
-    PYBRICKS_MICROPYTHON_BUILTIN_USER_PROGRAM_ID_PORT_VIEW = 1,
-    /**
-     * The number of builtin user programs.
-     */
-    PYBRICKS_MICROPYTHON_BUILTIN_USER_PROGRAM_NUMBER_OF_PROGRAMS,
-} pybricks_micropython_builtin_user_program_id_t;
-
 pbio_error_t pbsys_main_program_validate(pbsys_main_program_t *program) {
 
-    // Validate builtin user programs for existence.
-    if (program->type == PBSYS_MAIN_PROGRAM_TYPE_BUILTIN) {
-        #if PBSYS_CONFIG_APP_BUILTIN_USER_PROGRAMS
-        if (program->id < PYBRICKS_MICROPYTHON_BUILTIN_USER_PROGRAM_NUMBER_OF_PROGRAMS) {
-            return PBIO_SUCCESS;
-        }
-        #endif
+    // For builtin programs, check requested ID against feature flags.
+    #if PBSYS_CONFIG_FEATURE_BUILTIN_USER_PROGRAM_REPL
+    if (program->id == PBIO_PYBRICKS_USER_PROGRAM_ID_REPL) {
+        return PBIO_SUCCESS;
+    }
+    #endif
+    #if PBSYS_CONFIG_FEATURE_BUILTIN_USER_PROGRAM_PORT_VIEW
+    if (program->id == PBIO_PYBRICKS_USER_PROGRAM_ID_PORT_VIEW) {
+        return PBIO_SUCCESS;
+    }
+    #endif
+    #if PBSYS_CONFIG_FEATURE_BUILTIN_USER_PROGRAM_IMU_CALIBRATION
+    if (program->id == PBIO_PYBRICKS_USER_PROGRAM_ID_IMU_CALIBRATION) {
+        return PBIO_SUCCESS;
+    }
+    #endif
+
+    // Only user program 0 is supported for now.
+    if (program->id != PBIO_PYBRICKS_USER_PROGRAM_ID_FIRST_SLOT) {
         return PBIO_ERROR_NOT_SUPPORTED;
     }
 
     // If requesting a user program, ensure that it exists and is valid.
-    // Currently, only programs on slot 0 are supported.
     uint32_t program_size = program->code_end - program->code_start;
-    if (program->id != 0 || program_size == 0 || program_size > PBSYS_STORAGE_MAX_PROGRAM_SIZE) {
+    if (program_size == 0 || program_size > PBSYS_STORAGE_MAX_PROGRAM_SIZE) {
         return PBIO_ERROR_NOT_SUPPORTED;
     }
 
@@ -361,31 +353,37 @@ void pbsys_main_run_program(pbsys_main_program_t *program) {
     // Initialize MicroPython.
     mp_init();
 
-    // Check for run type.
-    if (program->type == PBSYS_MAIN_PROGRAM_TYPE_USER) {
-        // Init Pybricks package without auto-import.
-        pb_package_pybricks_init(false);
-        // Run loaded user program.
-        run_user_program();
-    }
-    #if PBSYS_CONFIG_APP_BUILTIN_USER_PROGRAMS
-    else {
-        switch (program->id) {
-            case PYBRICKS_MICROPYTHON_BUILTIN_USER_PROGRAM_ID_REPL:
-                // Run REPL with everything auto-imported.
-                pb_package_pybricks_init(true);
-                run_repl();
-                break;
-            case PYBRICKS_MICROPYTHON_BUILTIN_USER_PROGRAM_ID_PORT_VIEW:
-                pyexec_frozen_module("_builtin_port_view.py", false);
-                break;
-            default:
-                // Existence was already validated above, so just quietly exit
-                // since we can't get here.
-                break;
-        }
+    // Runs the requested downloaded or builtin user program.
+    switch (program->id) {
+
+        #if PBSYS_CONFIG_FEATURE_BUILTIN_USER_PROGRAM_REPL
+        case PBIO_PYBRICKS_USER_PROGRAM_ID_REPL:
+            // Run REPL with everything auto-imported.
+            pb_package_pybricks_init(true);
+            run_repl();
+            break;
+        #endif
+
+        #if PBSYS_CONFIG_FEATURE_BUILTIN_USER_PROGRAM_PORT_VIEW
+        case PBIO_PYBRICKS_USER_PROGRAM_ID_PORT_VIEW:
+            pb_package_pybricks_init(false);
+            pyexec_frozen_module("_builtin_port_view.py", false);
+            break;
+        #endif
+
+        #if PBSYS_CONFIG_FEATURE_BUILTIN_USER_PROGRAM_IMU_CALIBRATION
+        case PBIO_PYBRICKS_USER_PROGRAM_ID_IMU_CALIBRATION:
+            // Todo
+            break;
+        #endif
+
+        default:
+            // Init Pybricks package without auto-import.
+            pb_package_pybricks_init(false);
+            // Run loaded user program (just slot 0 for now).
+            run_user_program();
+            break;
     }
-    #endif // PYBRICKS_OPT_COMPILER
 
     // De-init bluetooth resources (including flushing stdout) that may use
     // memory allocated by MicroPython before we wipe it.
diff --git a/bricks/ev3rt/app.c b/bricks/ev3rt/app.c
index 93b5ee486..f559aac67 100644
--- a/bricks/ev3rt/app.c
+++ b/bricks/ev3rt/app.c
@@ -26,7 +26,7 @@ void main_task(intptr_t unused) {
 
     while (true) {
         pbsys_main_program_t program = {
-            .type = PBSYS_MAIN_PROGRAM_TYPE_BUILTIN,
+            .id = PBIO_PYBRICKS_USER_PROGRAM_ID_REPL,
             .code_end = heap,
             .data_end = heap + sizeof(heap),
         };
diff --git a/lib/pbio/include/pbio/protocol.h b/lib/pbio/include/pbio/protocol.h
index 6a2b9826d..bae7dbaa6 100644
--- a/lib/pbio/include/pbio/protocol.h
+++ b/lib/pbio/include/pbio/protocol.h
@@ -39,6 +39,38 @@
     PBIO_XSTR(PBIO_PROTOCOL_VERSION_MINOR) "." \
     PBIO_XSTR(PBIO_PROTOCOL_VERSION_PATCH)
 
+
+/**
+ * User program identifiers.
+ *
+ *   0--127: Downloadabled user programs.
+ * 128--255: Builtin user programs.
+ */
+typedef enum {
+    /**
+     * First possible downloadable user program.
+     */
+    PBIO_PYBRICKS_USER_PROGRAM_ID_FIRST_SLOT = 0,
+    /**
+     * Last possible downloadable user program.
+     */
+    PBIO_PYBRICKS_USER_PROGRAM_ID_LAST_SLOT = 127,
+    /**
+     * Read-eval-print loop (REPL) interface.
+     */
+    PBIO_PYBRICKS_USER_PROGRAM_ID_REPL = 128,
+    /**
+     * Program that detects attached devices, displays sensor values, and
+     * relays sensor data to host if connected.
+     */
+    PBIO_PYBRICKS_USER_PROGRAM_ID_PORT_VIEW = 129,
+    /**
+     * Program that calibrates the internal inertial measurement unit and saves
+     * data persistently on the hub.
+     */
+    PBIO_PYBRICKS_USER_PROGRAM_ID_IMU_CALIBRATION = 130,
+} pbio_pybricks_user_program_id_t;
+
 /**
  * Pybricks command types.
  *
@@ -53,36 +85,28 @@ typedef enum {
     PBIO_PYBRICKS_COMMAND_STOP_USER_PROGRAM = 0,
 
     /**
-     * Requests that a specified user program should be started.
-     *
-     * Parameters:
-     * - id    (optional): The identifier of the user program (32-bit little-endian unsigned integer).
-     *                     Defaults to 0 if no identifier is provided.
-     *                     This optional parameter was introduced in Pybricks Profile v1.4.0
+     * Requests that the user program should be started.
      *
      * Errors:
-     * - ::PBIO_PYBRICKS_ERROR_BUSY if a program is already running.
-     * - ::PBIO_PYBRICKS_ERROR_INVALID_COMMAND if the builtin user program is not available (Since Pybricks Profile v1.4.0).
+     * - ::PBIO_PYBRICKS_ERROR_BUSY if another program is already running.
      *
      * @since Pybricks Profile v1.2.0
+     * @deprecated in Pybricks Profile v1.4.0.
+     *   Use ::PBIO_PYBRICKS_COMMAND_START_USER_OR_BUILTIN_PROGRAM instead.
      */
     PBIO_PYBRICKS_COMMAND_START_USER_PROGRAM = 1,
 
     /**
-     * Requests that a specified builtin user program should be started.
-     *
-     * Parameters:
-     * - id    (optional): The identifier of the builtin user program (32-bit little-endian unsigned integer).
-     *                     Defaults to 0 if no identifier is provided.
-     *                     This optional parameter was introduced in Pybricks Profile v1.4.0
+     * Requests that the REPL should be started.
      *
      * Errors:
-     * - ::PBIO_PYBRICKS_ERROR_BUSY if a program is already running.
-     * - ::PBIO_PYBRICKS_ERROR_INVALID_COMMAND if the builtin user program is not available (Since Pybricks Profile v1.4.0).
+     * - ::PBIO_PYBRICKS_ERROR_BUSY if another program is already running.
      *
      * @since Pybricks Profile v1.2.0
+     * @deprecated in Pybricks Profile v1.4.0.
+     *   Use ::PBIO_PYBRICKS_COMMAND_START_USER_OR_BUILTIN_PROGRAM instead.
      */
-    PBIO_PYBRICKS_COMMAND_START_BUILTIN_USER_PROGRAM = 2,
+    PBIO_PYBRICKS_COMMAND_START_REPL = 2,
 
     /**
      * Requests to write user program metadata.
@@ -159,8 +183,24 @@ typedef enum {
      * @since Pybricks Profile v1.4.0
      */
     PBIO_PYBRICKS_COMMAND_WRITE_APP_DATA = 7,
-} pbio_pybricks_command_t;
 
+    /**
+     * Requests that a previously downloaded user program or builtin user
+     * program is started.
+     *
+     * Parameters:
+     * - payload: Program identifier (one byte). Slots 0--127 are reserved for
+     *            downloaded user programs. Slots 128--255 are for builtin user
+     *            programs.
+     *
+     * Errors:
+     * - ::PBIO_PYBRICKS_ERROR_BUSY if another program is already running.
+     * - ::PBIO_PYBRICKS_ERROR_INVALID_COMMAND if the requested program is not available.
+     *
+     * @since Pybricks Profile v1.4.0.
+     */
+    PBIO_PYBRICKS_COMMAND_START_USER_OR_BUILTIN_PROGRAM = 8,
+} pbio_pybricks_command_t;
 /**
  * Application-specific error codes that are used in ATT_ERROR_RSP.
  */
@@ -310,14 +350,11 @@ typedef enum {
     // NB: the values are part of the protocol, so don't change the values!
 
     /**
-     * Hub supports builtin user programs, such as an interactive REPL or Port View.
-     *
-     * Prior to version 1.4.0 this flag was exclusively used to indicate REPL
-     * support since there were no other builtin user programs.
+     * Hub supports interactive REPL.
      *
      * @since Pybricks Profile v1.2.0.
      */
-    PBIO_PYBRICKS_FEATURE_BUILTIN_USER_PROGRAMS = 1 << 0,
+    PBIO_PYBRICKS_FEATURE_FLAG_BUILTIN_USER_PROGRAM_REPL = 1 << 0,
     /**
      * Hub supports user program with multiple MicroPython .mpy files ABI v6
      *
@@ -326,14 +363,26 @@ typedef enum {
      *
      * @since Pybricks Profile v1.2.0.
      */
-    PBIO_PYBRICKS_FEATURE_USER_PROG_FORMAT_MULTI_MPY_V6 = 1 << 1,
+    PBIO_PYBRICKS_FEATURE_FLAG_USER_PROG_FORMAT_MULTI_MPY_V6 = 1 << 1,
     /**
      * Hub supports user program with multiple MicroPython .mpy files ABI v6.1
      * including native module support.
      *
      * @since Pybricks Profile v1.3.0.
      */
-    PBIO_PYBRICKS_FEATURE_USER_PROG_FORMAT_MULTI_MPY_V6_1_NATIVE = 1 << 2,
+    PBIO_PYBRICKS_FEATURE_FLAG_USER_PROG_FORMAT_MULTI_MPY_V6_1_NATIVE = 1 << 2,
+    /**
+     * Hub supports builtin sensor port view monitoring program.
+     *
+     * @since Pybricks Profile v1.4.0.
+     */
+    PBIO_PYBRICKS_FEATURE_FLAG_BUILTIN_USER_PROGRAM_PORT_VIEW = 1 << 3,
+    /**
+     * Hub supports builtin IMU calibration program.
+     *
+     * @since Pybricks Profile v1.4.0.
+     */
+    PBIO_PYBRICKS_FEATURE_FLAG_BUILTIN_USER_PROGRAM_IMU_CALIBRATION = 1 << 4,
 } pbio_pybricks_feature_flags_t;
 
 void pbio_pybricks_hub_capabilities(uint8_t *buf,
diff --git a/lib/pbio/include/pbsys/config.h b/lib/pbio/include/pbsys/config.h
index ba27b3dab..5c10a2730 100644
--- a/lib/pbio/include/pbsys/config.h
+++ b/lib/pbio/include/pbsys/config.h
@@ -11,9 +11,11 @@
 #include <pbio/protocol.h>
 
 #define PBSYS_CONFIG_APP_FEATURE_FLAGS (0 \
-    + PBSYS_CONFIG_APP_BUILTIN_USER_PROGRAMS * PBIO_PYBRICKS_FEATURE_BUILTIN_USER_PROGRAMS \
-    + PBSYS_CONFIG_APP_USER_PROG_FORMAT_MULTI_MPY_V6 * PBIO_PYBRICKS_FEATURE_USER_PROG_FORMAT_MULTI_MPY_V6 \
-    + PBSYS_CONFIG_APP_USER_PROG_FORMAT_MULTI_MPY_V6_1_NATIVE * PBIO_PYBRICKS_FEATURE_USER_PROG_FORMAT_MULTI_MPY_V6_1_NATIVE \
+    + PBSYS_CONFIG_FEATURE_BUILTIN_USER_PROGRAM_REPL * PBIO_PYBRICKS_FEATURE_FLAG_BUILTIN_USER_PROGRAM_REPL \
+    + PBSYS_CONFIG_FEATURE_BUILTIN_USER_PROGRAM_PORT_VIEW * PBIO_PYBRICKS_FEATURE_FLAG_BUILTIN_USER_PROGRAM_PORT_VIEW \
+    + PBSYS_CONFIG_FEATURE_BUILTIN_USER_PROGRAM_IMU_CALIBRATION * PBIO_PYBRICKS_FEATURE_FLAG_BUILTIN_USER_PROGRAM_IMU_CALIBRATION \
+    + PBSYS_CONFIG_FEATURE_PROGRAM_FORMAT_MULTI_MPY_V6 * PBIO_PYBRICKS_FEATURE_FLAG_USER_PROG_FORMAT_MULTI_MPY_V6 \
+    + PBSYS_CONFIG_FEATURE_PROGRAM_FORMAT_MULTI_MPY_V6_1_NATIVE * PBIO_PYBRICKS_FEATURE_FLAG_USER_PROG_FORMAT_MULTI_MPY_V6_1_NATIVE \
     )
 
 // When set to (1) PBSYS_CONFIG_STATUS_LIGHT indicates that a hub has a hub status light
diff --git a/lib/pbio/include/pbsys/main.h b/lib/pbio/include/pbsys/main.h
index e22cc1597..97fe4ebcd 100644
--- a/lib/pbio/include/pbsys/main.h
+++ b/lib/pbio/include/pbsys/main.h
@@ -16,14 +16,6 @@
 #include <stdbool.h>
 #include <stdint.h>
 
-/**
- * Main program types.
- */
-typedef enum {
-    PBSYS_MAIN_PROGRAM_TYPE_USER,    // User-defined program
-    PBSYS_MAIN_PROGRAM_TYPE_BUILTIN  // Built-in program
-} pbsys_main_program_type_t;
-
 /**
  * Main application program data information.
  */
@@ -41,14 +33,9 @@ typedef struct _pbsys_main_program_t {
      */
     void *data_end;
     /**
-     * Program identifier (selects one user program or one of the builtins).
-     */
-    uint32_t id;
-    /**
-     * Whether to run an application-specific builtin user program instead of the
-     * program given by the data. The builtin user program may still use the data.
+     * User program identifier.
      */
-    pbsys_main_program_type_t type;
+    pbio_pybricks_user_program_id_t id;
     /**
      * Whether a request was made to start the program.
      */
@@ -57,7 +44,7 @@ typedef struct _pbsys_main_program_t {
 
 #if PBSYS_CONFIG_MAIN
 
-pbio_error_t pbsys_main_program_request_start(pbsys_main_program_type_t type, uint32_t id);
+pbio_error_t pbsys_main_program_request_start(pbio_pybricks_user_program_id_t id);
 
 /**
  * Validates the program that is being requested to start.
@@ -101,7 +88,7 @@ bool pbsys_main_stdin_event(uint8_t c);
 
 #else // PBSYS_CONFIG_MAIN
 
-static inline pbio_error_t pbsys_main_program_request_start(pbsys_main_program_type_t type, uint32_t id) {
+static inline pbio_error_t pbsys_main_program_request_start(pbio_pybricks_user_program_id_t id) {
     return PBIO_ERROR_NOT_SUPPORTED;
 }
 
diff --git a/lib/pbio/platform/city_hub/pbsysconfig.h b/lib/pbio/platform/city_hub/pbsysconfig.h
index aa7459ccb..72ba157de 100644
--- a/lib/pbio/platform/city_hub/pbsysconfig.h
+++ b/lib/pbio/platform/city_hub/pbsysconfig.h
@@ -3,9 +3,11 @@
 
 #include "pbdrvconfig.h"
 
-#define PBSYS_CONFIG_APP_BUILTIN_USER_PROGRAMS                        (1)
-#define PBSYS_CONFIG_APP_USER_PROG_FORMAT_MULTI_MPY_V6           (1)
-#define PBSYS_CONFIG_APP_USER_PROG_FORMAT_MULTI_MPY_V6_1_NATIVE  (0)
+#define PBSYS_CONFIG_FEATURE_BUILTIN_USER_PROGRAM_REPL             (1)
+#define PBSYS_CONFIG_FEATURE_BUILTIN_USER_PROGRAM_PORT_VIEW        (1)
+#define PBSYS_CONFIG_FEATURE_BUILTIN_USER_PROGRAM_IMU_CALIBRATION  (0)
+#define PBSYS_CONFIG_FEATURE_PROGRAM_FORMAT_MULTI_MPY_V6           (1)
+#define PBSYS_CONFIG_FEATURE_PROGRAM_FORMAT_MULTI_MPY_V6_1_NATIVE  (0)
 #define PBSYS_CONFIG_BATTERY_CHARGER                (0)
 #define PBSYS_CONFIG_BLUETOOTH                      (1)
 #define PBSYS_CONFIG_HUB_LIGHT_MATRIX               (0)
diff --git a/lib/pbio/platform/essential_hub/pbsysconfig.h b/lib/pbio/platform/essential_hub/pbsysconfig.h
index 9a2ecd785..7726dd886 100644
--- a/lib/pbio/platform/essential_hub/pbsysconfig.h
+++ b/lib/pbio/platform/essential_hub/pbsysconfig.h
@@ -1,9 +1,11 @@
 // SPDX-License-Identifier: MIT
 // Copyright (c) 2021-2023 The Pybricks Authors
 
-#define PBSYS_CONFIG_APP_BUILTIN_USER_PROGRAMS                        (1)
-#define PBSYS_CONFIG_APP_USER_PROG_FORMAT_MULTI_MPY_V6           (1)
-#define PBSYS_CONFIG_APP_USER_PROG_FORMAT_MULTI_MPY_V6_1_NATIVE  (1)
+#define PBSYS_CONFIG_FEATURE_BUILTIN_USER_PROGRAM_REPL             (1)
+#define PBSYS_CONFIG_FEATURE_BUILTIN_USER_PROGRAM_PORT_VIEW        (1)
+#define PBSYS_CONFIG_FEATURE_BUILTIN_USER_PROGRAM_IMU_CALIBRATION  (0)
+#define PBSYS_CONFIG_FEATURE_PROGRAM_FORMAT_MULTI_MPY_V6           (1)
+#define PBSYS_CONFIG_FEATURE_PROGRAM_FORMAT_MULTI_MPY_V6_1_NATIVE  (1)
 #define PBSYS_CONFIG_BATTERY_CHARGER                (1)
 #define PBSYS_CONFIG_BLUETOOTH                      (1)
 #define PBSYS_CONFIG_HUB_LIGHT_MATRIX               (0)
diff --git a/lib/pbio/platform/ev3/pbsysconfig.h b/lib/pbio/platform/ev3/pbsysconfig.h
index 1a82f5535..90a8bba36 100644
--- a/lib/pbio/platform/ev3/pbsysconfig.h
+++ b/lib/pbio/platform/ev3/pbsysconfig.h
@@ -1,9 +1,11 @@
 // SPDX-License-Identifier: MIT
 // Copyright (c) 2020-2024 The Pybricks Authors
 
-#define PBSYS_CONFIG_APP_BUILTIN_USER_PROGRAMS                        (0)
-#define PBSYS_CONFIG_APP_USER_PROG_FORMAT_MULTI_MPY_V6           (1)
-#define PBSYS_CONFIG_APP_USER_PROG_FORMAT_MULTI_MPY_V6_1_NATIVE  (0)
+#define PBSYS_CONFIG_FEATURE_BUILTIN_USER_PROGRAM_REPL             (0)
+#define PBSYS_CONFIG_FEATURE_BUILTIN_USER_PROGRAM_PORT_VIEW        (0)
+#define PBSYS_CONFIG_FEATURE_BUILTIN_USER_PROGRAM_IMU_CALIBRATION  (0)
+#define PBSYS_CONFIG_FEATURE_PROGRAM_FORMAT_MULTI_MPY_V6           (1)
+#define PBSYS_CONFIG_FEATURE_PROGRAM_FORMAT_MULTI_MPY_V6_1_NATIVE  (0)
 #define PBSYS_CONFIG_MAIN                           (1)
 #define PBSYS_CONFIG_STORAGE                        (1)
 #define PBSYS_CONFIG_STORAGE_RAM_SIZE               (10 * 1024)
diff --git a/lib/pbio/platform/ev3rt/pbsysconfig.h b/lib/pbio/platform/ev3rt/pbsysconfig.h
index 0e9900a1a..78bb8d799 100644
--- a/lib/pbio/platform/ev3rt/pbsysconfig.h
+++ b/lib/pbio/platform/ev3rt/pbsysconfig.h
@@ -1,9 +1,11 @@
 // SPDX-License-Identifier: MIT
 // Copyright (c) 2020-2023 The Pybricks Authors
 
-#define PBSYS_CONFIG_APP_BUILTIN_USER_PROGRAMS                        (0)
-#define PBSYS_CONFIG_APP_USER_PROG_FORMAT_MULTI_MPY_V6           (1)
-#define PBSYS_CONFIG_APP_USER_PROG_FORMAT_MULTI_MPY_V6_1_NATIVE  (0)
+#define PBSYS_CONFIG_FEATURE_BUILTIN_USER_PROGRAM_REPL             (0)
+#define PBSYS_CONFIG_FEATURE_BUILTIN_USER_PROGRAM_PORT_VIEW        (0)
+#define PBSYS_CONFIG_FEATURE_BUILTIN_USER_PROGRAM_IMU_CALIBRATION  (0)
+#define PBSYS_CONFIG_FEATURE_PROGRAM_FORMAT_MULTI_MPY_V6           (1)
+#define PBSYS_CONFIG_FEATURE_PROGRAM_FORMAT_MULTI_MPY_V6_1_NATIVE  (0)
 #define PBSYS_CONFIG_BATTERY_CHARGER                (0)
 #define PBSYS_CONFIG_BLUETOOTH                      (0)
 #define PBSYS_CONFIG_HUB_LIGHT_MATRIX               (0)
diff --git a/lib/pbio/platform/move_hub/pbsysconfig.h b/lib/pbio/platform/move_hub/pbsysconfig.h
index 48ba8fb3f..c26beba8e 100644
--- a/lib/pbio/platform/move_hub/pbsysconfig.h
+++ b/lib/pbio/platform/move_hub/pbsysconfig.h
@@ -3,9 +3,11 @@
 
 #include "pbdrvconfig.h"
 
-#define PBSYS_CONFIG_APP_BUILTIN_USER_PROGRAMS                        (0)
-#define PBSYS_CONFIG_APP_USER_PROG_FORMAT_MULTI_MPY_V6           (1)
-#define PBSYS_CONFIG_APP_USER_PROG_FORMAT_MULTI_MPY_V6_1_NATIVE  (0)
+#define PBSYS_CONFIG_FEATURE_BUILTIN_USER_PROGRAM_REPL             (0)
+#define PBSYS_CONFIG_FEATURE_BUILTIN_USER_PROGRAM_PORT_VIEW        (0)
+#define PBSYS_CONFIG_FEATURE_BUILTIN_USER_PROGRAM_IMU_CALIBRATION  (0)
+#define PBSYS_CONFIG_FEATURE_PROGRAM_FORMAT_MULTI_MPY_V6           (1)
+#define PBSYS_CONFIG_FEATURE_PROGRAM_FORMAT_MULTI_MPY_V6_1_NATIVE  (0)
 #define PBSYS_CONFIG_BATTERY_CHARGER                (0)
 #define PBSYS_CONFIG_BLUETOOTH                      (1)
 #define PBSYS_CONFIG_HUB_LIGHT_MATRIX               (0)
diff --git a/lib/pbio/platform/nxt/pbsysconfig.h b/lib/pbio/platform/nxt/pbsysconfig.h
index 381dc993e..b5a11f5e3 100644
--- a/lib/pbio/platform/nxt/pbsysconfig.h
+++ b/lib/pbio/platform/nxt/pbsysconfig.h
@@ -1,6 +1,11 @@
 // SPDX-License-Identifier: MIT
 // Copyright (c) 2020-2023 The Pybricks Authors
 
+#define PBSYS_CONFIG_FEATURE_BUILTIN_USER_PROGRAM_REPL             (0)
+#define PBSYS_CONFIG_FEATURE_BUILTIN_USER_PROGRAM_PORT_VIEW        (0)
+#define PBSYS_CONFIG_FEATURE_BUILTIN_USER_PROGRAM_IMU_CALIBRATION  (0)
+#define PBSYS_CONFIG_FEATURE_PROGRAM_FORMAT_MULTI_MPY_V6           (1)
+#define PBSYS_CONFIG_FEATURE_PROGRAM_FORMAT_MULTI_MPY_V6_1_NATIVE  (0)
 #define PBSYS_CONFIG_MAIN                           (1)
 #define PBSYS_CONFIG_STORAGE                        (1)
 #define PBSYS_CONFIG_STORAGE_RAM_SIZE               (10 * 1024)
diff --git a/lib/pbio/platform/prime_hub/pbsysconfig.h b/lib/pbio/platform/prime_hub/pbsysconfig.h
index bd1684420..d5f744ab9 100644
--- a/lib/pbio/platform/prime_hub/pbsysconfig.h
+++ b/lib/pbio/platform/prime_hub/pbsysconfig.h
@@ -1,9 +1,11 @@
 // SPDX-License-Identifier: MIT
 // Copyright (c) 2020-2023 The Pybricks Authors
 
-#define PBSYS_CONFIG_APP_BUILTIN_USER_PROGRAMS                        (1)
-#define PBSYS_CONFIG_APP_USER_PROG_FORMAT_MULTI_MPY_V6           (1)
-#define PBSYS_CONFIG_APP_USER_PROG_FORMAT_MULTI_MPY_V6_1_NATIVE  (1)
+#define PBSYS_CONFIG_FEATURE_BUILTIN_USER_PROGRAM_REPL             (1)
+#define PBSYS_CONFIG_FEATURE_BUILTIN_USER_PROGRAM_PORT_VIEW        (1)
+#define PBSYS_CONFIG_FEATURE_BUILTIN_USER_PROGRAM_IMU_CALIBRATION  (0)
+#define PBSYS_CONFIG_FEATURE_PROGRAM_FORMAT_MULTI_MPY_V6           (1)
+#define PBSYS_CONFIG_FEATURE_PROGRAM_FORMAT_MULTI_MPY_V6_1_NATIVE  (1)
 #define PBSYS_CONFIG_BATTERY_CHARGER                (1)
 #define PBSYS_CONFIG_BLUETOOTH                      (1)
 #define PBSYS_CONFIG_BLUETOOTH_TOGGLE               (1)
diff --git a/lib/pbio/platform/technic_hub/pbsysconfig.h b/lib/pbio/platform/technic_hub/pbsysconfig.h
index b01ef1e8e..0b77fb356 100644
--- a/lib/pbio/platform/technic_hub/pbsysconfig.h
+++ b/lib/pbio/platform/technic_hub/pbsysconfig.h
@@ -3,9 +3,11 @@
 
 #include "pbdrvconfig.h"
 
-#define PBSYS_CONFIG_APP_BUILTIN_USER_PROGRAMS                        (1)
-#define PBSYS_CONFIG_APP_USER_PROG_FORMAT_MULTI_MPY_V6           (1)
-#define PBSYS_CONFIG_APP_USER_PROG_FORMAT_MULTI_MPY_V6_1_NATIVE  (0)
+#define PBSYS_CONFIG_FEATURE_BUILTIN_USER_PROGRAM_REPL             (1)
+#define PBSYS_CONFIG_FEATURE_BUILTIN_USER_PROGRAM_PORT_VIEW        (1)
+#define PBSYS_CONFIG_FEATURE_BUILTIN_USER_PROGRAM_IMU_CALIBRATION  (0)
+#define PBSYS_CONFIG_FEATURE_PROGRAM_FORMAT_MULTI_MPY_V6           (1)
+#define PBSYS_CONFIG_FEATURE_PROGRAM_FORMAT_MULTI_MPY_V6_1_NATIVE  (0)
 #define PBSYS_CONFIG_BATTERY_CHARGER                (0)
 #define PBSYS_CONFIG_BLUETOOTH                      (1)
 #define PBSYS_CONFIG_HUB_LIGHT_MATRIX               (0)
diff --git a/lib/pbio/platform/test/pbsysconfig.h b/lib/pbio/platform/test/pbsysconfig.h
index 3da4eba60..2f3523848 100644
--- a/lib/pbio/platform/test/pbsysconfig.h
+++ b/lib/pbio/platform/test/pbsysconfig.h
@@ -1,9 +1,11 @@
 // SPDX-License-Identifier: MIT
 // Copyright (c) 2020-2023 The Pybricks Authors
 
-#define PBSYS_CONFIG_APP_BUILTIN_USER_PROGRAMS                        (0)
-#define PBSYS_CONFIG_APP_USER_PROG_FORMAT_MULTI_MPY_V6           (0)
-#define PBSYS_CONFIG_APP_USER_PROG_FORMAT_MULTI_MPY_V6_1_NATIVE  (0)
+#define PBSYS_CONFIG_FEATURE_BUILTIN_USER_PROGRAM_REPL             (0)
+#define PBSYS_CONFIG_FEATURE_BUILTIN_USER_PROGRAM_PORT_VIEW        (0)
+#define PBSYS_CONFIG_FEATURE_BUILTIN_USER_PROGRAM_IMU_CALIBRATION  (0)
+#define PBSYS_CONFIG_FEATURE_PROGRAM_FORMAT_MULTI_MPY_V6           (0)
+#define PBSYS_CONFIG_FEATURE_PROGRAM_FORMAT_MULTI_MPY_V6_1_NATIVE  (0)
 #define PBSYS_CONFIG_BLUETOOTH                      (1)
 #define PBSYS_CONFIG_HUB_LIGHT_MATRIX               (1)
 #define PBSYS_CONFIG_MAIN                           (0)
diff --git a/lib/pbio/sys/command.c b/lib/pbio/sys/command.c
index 638efc9e5..c9f0a1bdf 100644
--- a/lib/pbio/sys/command.c
+++ b/lib/pbio/sys/command.c
@@ -37,36 +37,37 @@ pbio_pybricks_error_t pbsys_command(const uint8_t *data, uint32_t size) {
     pbio_pybricks_command_t cmd = data[0];
 
     switch (cmd) {
+
         case PBIO_PYBRICKS_COMMAND_STOP_USER_PROGRAM:
             pbsys_program_stop(false);
             return PBIO_PYBRICKS_ERROR_OK;
-        case PBIO_PYBRICKS_COMMAND_START_USER_PROGRAM: {
-            uint32_t id = 0;
-            if (size == (1 + sizeof(uint32_t))) {
-                id = pbio_get_uint32_le(&data[1]);
-            }
+
+        case PBIO_PYBRICKS_COMMAND_START_USER_PROGRAM:
+            // Deprecated. For backwards compatibility with Pybricks
+            // Profile < v1.4.0, assume we should start user program 0.
             return pbio_pybricks_error_from_pbio_error(
-                pbsys_main_program_request_start(PBSYS_MAIN_PROGRAM_TYPE_USER, id));
-        }
-        #if PBSYS_CONFIG_APP_BUILTIN_USER_PROGRAMS
-        case PBIO_PYBRICKS_COMMAND_START_BUILTIN_USER_PROGRAM: {
-            uint32_t id = 0;
-            if (size == (1 + sizeof(uint32_t))) {
-                id = pbio_get_uint32_le(&data[1]);
-            }
+                pbsys_main_program_request_start(PBIO_PYBRICKS_USER_PROGRAM_ID_FIRST_SLOT));
+
+        #if PBSYS_CONFIG_FEATURE_BUILTIN_USER_PROGRAM_REPL
+        case PBIO_PYBRICKS_COMMAND_START_REPL:
+            // Deprecated. For backwards compatibility with Pybricks
+            // Profile < v1.4.0, make it work anyway.
             return pbio_pybricks_error_from_pbio_error(
-                pbsys_main_program_request_start(PBSYS_MAIN_PROGRAM_TYPE_BUILTIN, id));
-        }
-        #endif // PBIO_PYBRICKS_FEATURE_TEST(PBIO_PYBRICKS_FEATURE_BUILTIN_USER_PROGRAMS)
+                pbsys_main_program_request_start(PBIO_PYBRICKS_USER_PROGRAM_ID_REPL));
+        #endif // PBSYS_CONFIG_FEATURE_BUILTIN_USER_PROGRAM_REPL
+
         case PBIO_PYBRICKS_COMMAND_WRITE_USER_PROGRAM_META:
             return pbio_pybricks_error_from_pbio_error(pbsys_storage_set_program_size(
                 pbio_get_uint32_le(&data[1])));
+
         case PBIO_PYBRICKS_COMMAND_WRITE_USER_RAM:
             return pbio_pybricks_error_from_pbio_error(pbsys_storage_set_program_data(
                 pbio_get_uint32_le(&data[1]), &data[5], size - 5));
+
         case PBIO_PYBRICKS_COMMAND_REBOOT_TO_UPDATE_MODE:
             pbdrv_reset(PBDRV_RESET_ACTION_RESET_IN_UPDATE_MODE);
             return PBIO_PYBRICKS_ERROR_OK;
+
         case PBIO_PYBRICKS_COMMAND_WRITE_STDIN:
             #if PBSYS_CONFIG_BLUETOOTH
             if (pbsys_bluetooth_rx_get_free() < size - 1) {
@@ -76,6 +77,7 @@ pbio_pybricks_error_t pbsys_command(const uint8_t *data, uint32_t size) {
             #endif
             // If no consumers are configured, goes to "/dev/null" without error
             return PBIO_PYBRICKS_ERROR_OK;
+
         case PBIO_PYBRICKS_COMMAND_WRITE_APP_DATA: {
             if (!write_app_data_callback) {
                 // No errors when no consumer is configured. This avoids errors
@@ -93,6 +95,15 @@ pbio_pybricks_error_t pbsys_command(const uint8_t *data, uint32_t size) {
             const uint8_t *data_to_write = &data[3];
             return pbio_pybricks_error_from_pbio_error(write_app_data_callback(offset, data_size, data_to_write));
         }
+
+        case PBIO_PYBRICKS_COMMAND_START_USER_OR_BUILTIN_PROGRAM:
+            // Identifier payload required
+            if (size != 2) {
+                return PBIO_PYBRICKS_ERROR_VALUE_NOT_ALLOWED;
+            }
+            return pbio_pybricks_error_from_pbio_error(
+                pbsys_main_program_request_start(data[1]));
+
         default:
             return PBIO_PYBRICKS_ERROR_INVALID_COMMAND;
     }
diff --git a/lib/pbio/sys/hmi.c b/lib/pbio/sys/hmi.c
index 9558e1d39..e26844ce8 100644
--- a/lib/pbio/sys/hmi.c
+++ b/lib/pbio/sys/hmi.c
@@ -50,9 +50,10 @@ static PT_THREAD(update_program_run_button_wait_state(bool button_pressed)) {
         PT_WAIT_UNTIL(pt, button_pressed);
         PT_WAIT_UNTIL(pt, !button_pressed);
 
-        // if we made it through a full press and release, without the user
-        // program running, then start the user program
-        pbsys_main_program_request_start(PBSYS_MAIN_PROGRAM_TYPE_USER, 0);
+        // If we made it through a full press and release, without the user
+        // program running, then start the user program. There is no UI for
+        // multiple programs yet, so start the one and only program.
+        pbsys_main_program_request_start(PBIO_PYBRICKS_USER_PROGRAM_ID_FIRST_SLOT);
     }
 
     PT_END(pt);
diff --git a/lib/pbio/sys/main.c b/lib/pbio/sys/main.c
index 6636dfcda..a668abbbf 100644
--- a/lib/pbio/sys/main.c
+++ b/lib/pbio/sys/main.c
@@ -22,8 +22,6 @@
 
 static pbsys_main_program_t program;
 
-#include <stdio.h>
-
 /**
  * Requests to start the main user application program.
  *
@@ -33,14 +31,13 @@ static pbsys_main_program_t program;
  *              ::PBIO_ERROR_NOT_SUPPORTED if the program is not available.
  *              Otherwise ::PBIO_SUCCESS.
  */
-pbio_error_t pbsys_main_program_request_start(pbsys_main_program_type_t type, uint32_t id) {
+pbio_error_t pbsys_main_program_request_start(pbio_pybricks_user_program_id_t id) {
 
     // Can't start new program if already running or new requested.
     if (pbsys_status_test(PBIO_PYBRICKS_STATUS_USER_PROGRAM_RUNNING) || program.start_requested) {
         return PBIO_ERROR_BUSY;
     }
 
-    program.type = type;
     program.id = id;
 
     // Builtin user programs are also allowed to access user program,