diff --git a/hw/pci.c b/hw/pci.c
index 99c206f..316d131 100644
--- a/hw/pci.c
+++ b/hw/pci.c
@@ -145,11 +145,17 @@ int pci_device_load(PCIDevice *s, QEMUFile *f)
     return 0;
 }
 
+/* Maps a vector of physical addresses and returns a vector that is directly
+   manipulatible.  The target may not see the changes to memory until the
+   vector is unmapped. */
 IOVector *pci_device_dma_map(PCIDevice *s, PhysIOVector *phys)
 {
     return cpu_translate_physical_page_vector(phys);
 }
 
+/* Unmaps a vector of physical addresses that was previously mapped with
+   pci_device_dma_map.  The write flag indicates whether the data was written
+   to while mapped. */
 void pci_device_dma_unmap(PCIDevice *s, PhysIOVector *phys, IOVector *virt,
                          int write)
 {