diff --git a/nemu/include/common.h b/nemu/include/common.h
index ee3e429..fbffaa5 100644
--- a/nemu/include/common.h
+++ b/nemu/include/common.h
@@ -40,6 +40,7 @@ typedef MUXDEF(CONFIG_ISA64, int64_t, int32_t)  sword_t;
 static const word_t WORD_T_MAX = MUXDEF(CONFIG_ISA64, UINT64_MAX, UINT32_MAX);
 static const sword_t SWORD_T_MAX = MUXDEF(CONFIG_ISA64, INT64_MAX, INT32_MAX);
 static const sword_t SWORD_T_MIN = MUXDEF(CONFIG_ISA64, INT64_MIN, INT32_MIN);
+#define WORD_BYTES MUXDEF(CONFIG_ISA64, 8, 4)
 #define FMT_WORD MUXDEF(CONFIG_ISA64, "0x%016" PRIx64, "0x%08" PRIx32)
 
 typedef word_t vaddr_t;
diff --git a/nemu/src/monitor/sdb/sdb.c b/nemu/src/monitor/sdb/sdb.c
index 369784b..d1e00ec 100644
--- a/nemu/src/monitor/sdb/sdb.c
+++ b/nemu/src/monitor/sdb/sdb.c
@@ -189,10 +189,10 @@ static int cmd_x(char *args) {
   word_t addr = parse_expr(arg, &res);
   if (!res)
     goto wrong_usage;
-  printf("%d, %d\n", addr, MUXDEF(CONFIG_ISA64, 4, 8));
+  printf("%d, %d\n", addr, WORD_BYTES);
   for (paddr_t paddr = addr; paddr < addr + n;
-       paddr += MUXDEF(CONFIG_ISA64, 4, 8)) {
-    word_t value = paddr_read(addr, MUXDEF(CONFIG_ISA64, 4, 8));
+       paddr += WORD_BYTES) {
+    word_t value = paddr_read(addr, WORD_BYTES);
     printf(FMT_WORD "\n", value);
   }
   return 0;