diff --git a/abstract-machine/am/src/platform/nemu/ioe/gpu.c b/abstract-machine/am/src/platform/nemu/ioe/gpu.c
index 6d1df6b..80fcf12 100644
--- a/abstract-machine/am/src/platform/nemu/ioe/gpu.c
+++ b/abstract-machine/am/src/platform/nemu/ioe/gpu.c
@@ -12,7 +12,7 @@ void __am_gpu_init() {
   // int w = inw(VGACTL_ADDR + 2);  // TODO: get the correct width
   // int h = inw(VGACTL_ADDR);  // TODO: get the correct height
   uint32_t *fb = (uint32_t *)(uintptr_t)FB_ADDR;
-  for (i = 0; i < w * h; i ++) fb[i] = 0x00FFFFFF;
+  for (i = 0; i < w * h; i ++) fb[i] = i;
   outl(SYNC_ADDR, 1);
 }
 
diff --git a/nemu/src/device/vga.c b/nemu/src/device/vga.c
index 0e4118c..7b9c26d 100644
--- a/nemu/src/device/vga.c
+++ b/nemu/src/device/vga.c
@@ -76,6 +76,7 @@ void vga_update_screen() {
   // TODO: call `update_screen()` when the sync register is non-zero,
   // then zero out the sync register
   if (*sync_reg != 0) {
+    printf("Update Screen\n");
     update_screen();
     *sync_reg = 0;
   }