diff --git a/console.c b/console.c
index 766dc30..46c867b 100644
--- a/console.c
+++ b/console.c
@@ -143,6 +143,9 @@ cgaputc(int c)
     if(pos > 0) --pos;
   } else
     crt[pos++] = (c&0xff) | 0x0700;  // black on white
+
+  if(pos > 25*80)
+    panic("pos overflow");
   
   if((pos/80) >= 24){  // Scroll up.
     memmove(crt, crt+80, sizeof(crt[0])*23*80);