diff --git a/.gitignore b/.gitignore
index 99314f1..d4d99f8 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,3 +1,4 @@
 **.o
 **.img
 **.elf
+**.cpio
diff --git a/include/mbox.h b/include/mbox.h
index 563c2d9..605a549 100644
--- a/include/mbox.h
+++ b/include/mbox.h
@@ -40,9 +40,12 @@ extern volatile unsigned int mbox[36];
 #define MBOX_CH_PROP    8
 
 /* tags */
-#define MBOX_TAG_GETSERIAL      0x10004
 #define MBOX_TAG_LAST           0
 #define MBOX_TAG_BOARD_REVISION 0x00010002
+#define MBOX_TAG_GETSERIAL      0x00010004
+#define MBOX_TAG_ARM_MEMORY     0x00010005
 
 int mbox_call(unsigned char ch);
 unsigned int get_board_revision(void);
+unsigned int get_memory_base_addr(void);
+unsigned int get_memory_size(void);
diff --git a/kernel/Start.S b/kernel/Start.S
index df2cb8f..4d2b7f0 100644
--- a/kernel/Start.S
+++ b/kernel/Start.S
@@ -17,13 +17,9 @@ _start:
 	mov     sp, x1
 
 	// clear bss
-	ldr     x1, =__bss_start
-	ldr     w2, =__bss_size
-3:  
-	cbz     w2, 4f
-	str     xzr, [x1], #8
-	sub     w2, w2, #1
-	cbnz    w2, 3b
+	adr x0, __bss_start
+	adr x1, __bss_end
+	bl memzero
 
 4:
 	// jump to C code, should not return
diff --git a/lib/mbox.c b/lib/mbox.c
index ac3f4fd..a0240de 100644
--- a/lib/mbox.c
+++ b/lib/mbox.c
@@ -42,11 +42,9 @@ volatile unsigned int  __attribute__((aligned(16))) mbox[36];
 
 #define MAILBOX_BASE    MMIO_BASE + 0xb880
 
-#define REQUEST_CODE        0x00000000
 #define REQUEST_SUCCEED     0x80000000
 #define REQUEST_FAILED      0x80000001
-#define TAG_REQUEST_CODE    0x00000000
-#define END_TAG             0x00000000
+#define TAG_MBOX_REQUEST    0x00000000
 
 /**
  * Make a mailbox call. Returns 0 on failure, non-zero on success
@@ -73,17 +71,55 @@ int mbox_call(unsigned char ch)
 unsigned int get_board_revision(void)
 {
   mbox[0] = 36 * 4;
-  mbox[1] = REQUEST_CODE;
+  mbox[1] = MBOX_REQUEST;
 
   // tags
   mbox[2] = MBOX_TAG_BOARD_REVISION;
   mbox[3] = 4;
-  mbox[4] = TAG_REQUEST_CODE;
+  mbox[4] = TAG_MBOX_REQUEST;
   mbox[5] = 0; // value buffer
 
-  mbox[6] = END_TAG;
+  mbox[6] = MBOX_TAG_LAST;
 
   mbox_call(MBOX_CH_PROP);
 
   return mbox[5];
 }
+
+unsigned int get_memory_base_addr(void)
+{
+  mbox[0] = 36 * 4;
+  mbox[1] = MBOX_REQUEST;
+
+  // tags
+  mbox[2] = MBOX_TAG_ARM_MEMORY;
+  mbox[3] = 8;
+  mbox[4] = 8;
+  mbox[5] = 0;
+  mbox[6] = 0;
+
+  mbox[7] = MBOX_TAG_LAST;
+
+  mbox_call(MBOX_CH_PROP);
+
+  return mbox[5];
+}
+
+unsigned int get_memory_size(void)
+{
+  mbox[0] = 36 * 4;
+  mbox[1] = MBOX_REQUEST;
+
+  // tags
+  mbox[2] = MBOX_TAG_ARM_MEMORY;
+  mbox[3] = 8;
+  mbox[4] = 8;
+  mbox[5] = 0;
+  mbox[6] = 0;
+
+  mbox[7] = MBOX_TAG_LAST;
+
+  mbox_call(MBOX_CH_PROP);
+
+  return mbox[6];
+}
diff --git a/lib/shell.c b/lib/shell.c
index ab7a4d9..bd951ce 100644
--- a/lib/shell.c
+++ b/lib/shell.c
@@ -6,6 +6,11 @@
 
 #define INPUT_BUFLEN 1000
 
+#define PM_PASSWORD 0x5a000000
+
+#define PM_RSTC 0x3F10001c
+#define PM_WDOG 0x3F100024
+
 void help (void)
 {
   uart_puts(
@@ -23,16 +28,24 @@ void hello (void)
 
 void hwinfo (void)
 {
-  unsigned int val = get_board_revision();
-  uart_puts("hwinfo: ");
-  uart_hex(val);
+  uart_puts(
+    "hwinfo: " ENDL
+    "board revision: "
+  );
+  uart_hex(get_board_revision());
+  uart_puts(
+    ENDL
+    "memory base addr: "
+  );
+  uart_hex(get_memory_base_addr());
+  uart_puts(
+    ENDL
+    "memory size: "
+  );
+  uart_hex(get_memory_size());
   uart_puts(ENDL);
 }
 
-#define PM_PASSWORD 0x5a000000
-#define PM_RSTC 0x3F10001c
-#define PM_WDOG 0x3F100024
-
 void set(long addr, unsigned int value) {
     volatile unsigned int* point = (unsigned int*)addr;
     *point = value;