diff --git a/src/dtb.cpp b/src/dtb.cpp
index 4d1ef758e..1ef881d5d 100644
--- a/src/dtb.cpp
+++ b/src/dtb.cpp
@@ -139,6 +139,20 @@ void dtb_init(const machine_config &c, unsigned char *dtb_start, uint64_t dtb_le
             fdt.end_node();
         }
 
+        { // reserved memory
+            fdt.begin_node("reserved-memory");
+            fdt.prop_u32("#address-cells", 2);
+            fdt.prop_u32("#size-cells", 2);
+            fdt.prop_empty("ranges");
+            { // reserve 256KB for firmware M-mode code (such as OpenSBI)
+                fdt.begin_node_num("fw_resv", PMA_RAM_START);
+                fdt.prop_u64_list<2>("reg", {PMA_RAM_START, 0x40000});
+                fdt.prop_empty("no-map");
+                fdt.end_node();
+            }
+            fdt.end_node();
+        }
+
         // drives
         int drive_index = 0;
         for (const auto &f : c.flash_drive) {