diff --git a/exec.c b/exec.c
index aafd8147db..d9be5752c5 100644
--- a/exec.c
+++ b/exec.c
@@ -2291,8 +2291,9 @@ static void *subpage_init (target_phys_addr_t base, ram_addr_t *phys,
         }                                                               \
     } while (0)
 
-/* register physical memory. 'size' must be a multiple of the target
-   page size. If (phys_offset & ~TARGET_PAGE_MASK) != 0, then it is an
+/* register physical memory.
+   For RAM, 'size' must be a multiple of the target page size.
+   If (phys_offset & ~TARGET_PAGE_MASK) != 0, then it is an
    io memory page.  The address used when calling the IO function is
    the offset from the start of the region, plus region_offset.  Both
    start_addr and region_offset are rounded down to a page boundary