diff --git a/src/arch/x86/mmap_boot.c b/src/arch/x86/mmap_boot.c
index 6c98954ff2..53973f9bb4 100644
--- a/src/arch/x86/mmap_boot.c
+++ b/src/arch/x86/mmap_boot.c
@@ -24,7 +24,7 @@
 #include <stdlib.h>
 
 /* The ROM is memory mapped just below 4GiB. Form a pointer for the base. */
-#define rom_base ((void *)(uintptr_t)(-(int32_t)CONFIG_ROM_SIZE))
+#define rom_base ((void *)(uintptr_t)(0x100000000ULL-CONFIG_ROM_SIZE))
 
 static const struct mem_region_device boot_dev =
 	MEM_REGION_DEV_INIT(rom_base, CONFIG_ROM_SIZE);