diff --git a/examples/stm32/nucleo-h563zi-make-baremetal-builtin/mongoose_custom.h b/examples/stm32/nucleo-h563zi-make-baremetal-builtin/mongoose_custom.h
index 0a2df18d..09860117 100644
--- a/examples/stm32/nucleo-h563zi-make-baremetal-builtin/mongoose_custom.h
+++ b/examples/stm32/nucleo-h563zi-make-baremetal-builtin/mongoose_custom.h
@@ -2,11 +2,11 @@
 
 #define MG_ARCH MG_ARCH_NEWLIB
 #define MG_OTA MG_OTA_FLASH
+#define MG_SYS MG_SYS_STM32H5
 
 #define MG_ENABLE_TCPIP 1
 #define MG_ENABLE_CUSTOM_MILLIS 1
 #define MG_ENABLE_CUSTOM_RANDOM 1
 #define MG_ENABLE_PACKED_FS 1 
 #define MG_ENABLE_DRIVER_STM32H 1
-#define MG_ENABLE_STM32H5 1
 #define MG_ENABLE_LINES 1
diff --git a/examples/stm32/nucleo-h743zi-make-baremetal-builtin/mongoose_custom.h b/examples/stm32/nucleo-h743zi-make-baremetal-builtin/mongoose_custom.h
index 1316e8b1..c965bdca 100644
--- a/examples/stm32/nucleo-h743zi-make-baremetal-builtin/mongoose_custom.h
+++ b/examples/stm32/nucleo-h743zi-make-baremetal-builtin/mongoose_custom.h
@@ -2,11 +2,11 @@
 
 #define MG_ARCH MG_ARCH_NEWLIB
 #define MG_OTA MG_OTA_FLASH
+#define MG_SYS MG_SYS_STM32H7
 
 #define MG_ENABLE_TCPIP 1
 #define MG_ENABLE_CUSTOM_MILLIS 1
 #define MG_ENABLE_CUSTOM_RANDOM 1
 #define MG_ENABLE_PACKED_FS 1 
 #define MG_ENABLE_DRIVER_STM32H 1
-#define MG_ENABLE_STM32H7 1
 #define MG_ENABLE_LINES 1
diff --git a/mongoose.c b/mongoose.c
index 7b42365a..b1e451c6 100644
--- a/mongoose.c
+++ b/mongoose.c
@@ -4979,8 +4979,6 @@ size_t mg_ota_size(int fw) {
   (void) fw;
   return 0;
 }
-void mg_sys_reset(void) {
-}
 #endif
 
 #ifdef MG_ENABLE_LINES
@@ -6844,6 +6842,42 @@ bool mg_path_is_sane(const char *path) {
   return true;
 }
 
+#ifdef MG_ENABLE_LINES
+#line 1 "src/sys_dummy.c"
+#endif
+
+
+#if MG_SYS == MG_SYS_NONE
+void *mg_flash_start(void) {
+  return NULL;
+}
+size_t mg_flash_size(void) {
+  return 0;
+}
+size_t mg_flash_sector_size(void) {
+  return 0;
+}
+size_t mg_flash_write_align(void) {
+  return 0;
+}
+int mg_flash_bank(void) {
+  return 0;
+}
+bool mg_flash_erase(void *location) {
+  (void) location;
+  return false;
+}
+bool mg_flash_swap_bank(void) {
+  return true;
+}
+bool mg_flash_write(void *addr, const void *buf, size_t len) {
+  (void) addr, (void) buf, (void) len;
+  return false;
+}
+void mg_sys_reset(void) {
+}
+#endif
+
 #ifdef MG_ENABLE_LINES
 #line 1 "src/sys_stm32h5.c"
 #endif
@@ -6851,7 +6885,7 @@ bool mg_path_is_sane(const char *path) {
 
 
 
-#if MG_ENABLE_STM32H5
+#if MG_SYS == MG_SYS_STM32H5
 
 #define FLASH_BASE 0x40022000          // Base address of the flash controller
 #define FLASH_KEYR (FLASH_BASE + 0x4)  // See RM0481 7.11
@@ -6998,7 +7032,7 @@ void mg_sys_reset(void) {
 
 
 
-#if MG_ENABLE_STM32H7
+#if MG_SYS == MG_SYS_STM32H7
 
 #define FLASH_BASE1 0x52002000  // Base address for bank1
 #define FLASH_BASE2 0x52002100  // Base address for bank2
diff --git a/mongoose.h b/mongoose.h
index 354d8d02..31aed55c 100644
--- a/mongoose.h
+++ b/mongoose.h
@@ -815,14 +815,6 @@ struct timeval {
 #define MG_EPOLL_MOD(c, wr)
 #endif
 
-#ifndef MG_ENABLE_STM32H5
-#define MG_ENABLE_STM32H5 0
-#endif
-
-#ifndef MG_ENABLE_STM32H7
-#define MG_ENABLE_STM32H7 0
-#endif
-
 
 
 
@@ -1702,13 +1694,20 @@ size_t mg_ota_size(int firmware);         // Firmware size
 
 bool mg_ota_commit(void);    // Commit current firmware
 bool mg_ota_rollback(void);  // Rollback to the previous firmware
-
-void mg_sys_reset(void);  // Reboot device immediately
 // Copyright (c) 2023 Cesanta Software Limited
 // All rights reserved
 
 
 
+#define MG_SYS_NONE 0      // Dummy system
+#define MG_SYS_STM32H5 1   // STM32 H5
+#define MG_SYS_STM32H7 2   // STM32 H7
+#define MG_SYS_CUSTOM 100  // Custom implementation
+
+#ifndef MG_SYS
+#define MG_SYS MG_SYS_NONE
+#endif
+
 // Flash information
 void *mg_flash_start(void);         // Return flash start address
 size_t mg_flash_size(void);         // Return flash size
@@ -1726,6 +1725,8 @@ bool mg_flash_swap_bank(void);
 bool mg_flash_load(void *sector, uint32_t key, void *buf, size_t len);
 bool mg_flash_save(void *sector, uint32_t key, const void *buf, size_t len);
 
+void mg_sys_reset(void);  // Reboot device immediately
+
 
 
 
diff --git a/src/config.h b/src/config.h
index 7314105f..f69da657 100644
--- a/src/config.h
+++ b/src/config.h
@@ -149,11 +149,3 @@
 #define MG_EPOLL_ADD(c)
 #define MG_EPOLL_MOD(c, wr)
 #endif
-
-#ifndef MG_ENABLE_STM32H5
-#define MG_ENABLE_STM32H5 0
-#endif
-
-#ifndef MG_ENABLE_STM32H7
-#define MG_ENABLE_STM32H7 0
-#endif
diff --git a/src/ota.h b/src/ota.h
index 3a979e12..a39c79bf 100644
--- a/src/ota.h
+++ b/src/ota.h
@@ -33,5 +33,3 @@ size_t mg_ota_size(int firmware);         // Firmware size
 
 bool mg_ota_commit(void);    // Commit current firmware
 bool mg_ota_rollback(void);  // Rollback to the previous firmware
-
-void mg_sys_reset(void);  // Reboot device immediately
diff --git a/src/ota_dummy.c b/src/ota_dummy.c
index 25b81831..6aa106a3 100644
--- a/src/ota_dummy.c
+++ b/src/ota_dummy.c
@@ -35,6 +35,4 @@ size_t mg_ota_size(int fw) {
   (void) fw;
   return 0;
 }
-void mg_sys_reset(void) {
-}
 #endif
diff --git a/src/sys.h b/src/sys.h
index 94897220..457fccff 100644
--- a/src/sys.h
+++ b/src/sys.h
@@ -3,6 +3,15 @@
 
 #pragma once
 
+#define MG_SYS_NONE 0      // Dummy system
+#define MG_SYS_STM32H5 1   // STM32 H5
+#define MG_SYS_STM32H7 2   // STM32 H7
+#define MG_SYS_CUSTOM 100  // Custom implementation
+
+#ifndef MG_SYS
+#define MG_SYS MG_SYS_NONE
+#endif
+
 // Flash information
 void *mg_flash_start(void);         // Return flash start address
 size_t mg_flash_size(void);         // Return flash size
@@ -19,3 +28,5 @@ bool mg_flash_swap_bank(void);
 // If `sector` is NULL, then the last sector of flash is used
 bool mg_flash_load(void *sector, uint32_t key, void *buf, size_t len);
 bool mg_flash_save(void *sector, uint32_t key, const void *buf, size_t len);
+
+void mg_sys_reset(void);  // Reboot device immediately
diff --git a/src/sys_dummy.c b/src/sys_dummy.c
new file mode 100644
index 00000000..80834e6e
--- /dev/null
+++ b/src/sys_dummy.c
@@ -0,0 +1,32 @@
+#include "sys.h"
+
+#if MG_SYS == MG_SYS_NONE
+void *mg_flash_start(void) {
+  return NULL;
+}
+size_t mg_flash_size(void) {
+  return 0;
+}
+size_t mg_flash_sector_size(void) {
+  return 0;
+}
+size_t mg_flash_write_align(void) {
+  return 0;
+}
+int mg_flash_bank(void) {
+  return 0;
+}
+bool mg_flash_erase(void *location) {
+  (void) location;
+  return false;
+}
+bool mg_flash_swap_bank(void) {
+  return true;
+}
+bool mg_flash_write(void *addr, const void *buf, size_t len) {
+  (void) addr, (void) buf, (void) len;
+  return false;
+}
+void mg_sys_reset(void) {
+}
+#endif
diff --git a/src/sys_stm32h5.c b/src/sys_stm32h5.c
index 63185338..f9f9ee7b 100644
--- a/src/sys_stm32h5.c
+++ b/src/sys_stm32h5.c
@@ -2,7 +2,7 @@
 #include "log.h"
 #include "ota.h"
 
-#if MG_ENABLE_STM32H5
+#if MG_SYS == MG_SYS_STM32H5
 
 #define FLASH_BASE 0x40022000          // Base address of the flash controller
 #define FLASH_KEYR (FLASH_BASE + 0x4)  // See RM0481 7.11
diff --git a/src/sys_stm32h7.c b/src/sys_stm32h7.c
index 1bdc585a..3566dcb4 100644
--- a/src/sys_stm32h7.c
+++ b/src/sys_stm32h7.c
@@ -2,7 +2,7 @@
 #include "log.h"
 #include "ota.h"
 
-#if MG_ENABLE_STM32H7
+#if MG_SYS == MG_SYS_STM32H7
 
 #define FLASH_BASE1 0x52002000  // Base address for bank1
 #define FLASH_BASE2 0x52002100  // Base address for bank2