diff --git a/hw/pci.c b/hw/pci.c
index 1debdab..19b8cc6 100644
--- a/hw/pci.c
+++ b/hw/pci.c
@@ -583,18 +583,14 @@ uint32_t pci_default_read_config(PCIDevice *d,
 
 void pci_default_write_config(PCIDevice *d, uint32_t addr, uint32_t val, int l)
 {
-    uint8_t orig[PCI_CONFIG_SPACE_SIZE];
     int i;
 
     /* not efficient, but simple */
-    memcpy(orig, d->config, PCI_CONFIG_SPACE_SIZE);
     for(i = 0; i < l && addr < PCI_CONFIG_SPACE_SIZE; val >>= 8, ++i, ++addr) {
         uint8_t wmask = d->wmask[addr];
         d->config[addr] = (d->config[addr] & ~wmask) | (val & wmask);
     }
-    if (memcmp(orig + PCI_BASE_ADDRESS_0, d->config + PCI_BASE_ADDRESS_0, 24)
-        || ((orig[PCI_COMMAND] ^ d->config[PCI_COMMAND])
-            & (PCI_COMMAND_MEMORY | PCI_COMMAND_IO)))
+    if (ranges_overlap(addr, l, PCI_COMMAND, PCI_ROM_ADDRESS + 3))
         pci_update_mappings(d);
 }
 
diff --git a/hw/pci.h b/hw/pci.h
index 93f93fb..b9374d1 100644
--- a/hw/pci.h
+++ b/hw/pci.h
@@ -366,4 +366,27 @@ PCIBus *pci_apb_init(target_phys_addr_t special_base,
 PCIBus *sh_pci_register_bus(pci_set_irq_fn set_irq, pci_map_irq_fn map_irq,
                             void *pic, int devfn_min, int nirq);
 
+/* These are not pci specific. Should move into a separate header. */
+
+/* Check whether a given range covers a given byte. */
+static inline int range_covers_byte(uint64_t first, uint64_t last,
+				    uint64_t byte)
+{
+	return first <= byte && last >= byte;
+}
+
+/* Check whether 2 given ranges overlap. Undefined if last < first. */
+static inline int ranges_overlap(uint64_t first1, uint64_t last1,
+				 uint64_t first2, uint64_t last2)
+{
+	return first1 >= last2 && first2 >= last1;
+}
+
+/* Get last byte of a range from offset + length.
+ * Undefined for ranges that wrap around 0. */
+static inline uint64_t range_get_last(uint64_t offset, uint64_t len)
+{
+	return offset + len - 1;
+}
+
 #endif