diff --git a/usr.sbin/acpi/acpidump/acpi_user.c b/usr.sbin/acpi/acpidump/acpi_user.c
index a8e56a6a7757..91017f14d3db 100644
--- a/usr.sbin/acpi/acpidump/acpi_user.c
+++ b/usr.sbin/acpi/acpidump/acpi_user.c
@@ -96,19 +96,24 @@ static struct ACPIrsdp *
 acpi_get_rsdp(u_long addr)
 {
 	struct ACPIrsdp rsdp;
-	size_t		len;
 
+	/* Read in the table signature and check it. */
 	pread(acpi_mem_fd, &rsdp, 8, addr);
 	if (memcmp(rsdp.signature, "RSD PTR ", 8))
 		return (NULL);
+
 	/* Read the entire table. */
-	len = sizeof(rsdp);
-	pread(acpi_mem_fd, &rsdp, len, addr);
-	if (acpi_checksum(&rsdp, len))
+	pread(acpi_mem_fd, &rsdp, sizeof(rsdp), addr);
+
+	/* Run the checksum only over the version 1 header. */
+	if (acpi_checksum(&rsdp, 20))
 		return (NULL);
-	if (rsdp.revision > 0)
-		len = rsdp.length;
-	return (acpi_map_physical(addr, len));
+	if (rsdp.revision == 0)
+		return (NULL);
+
+	/* XXX Should handle ACPI 2.0 RSDP extended checksum here. */
+
+	return (acpi_map_physical(addr, rsdp.length));
 }
 
 /*