utils/sim: Document memory-mapped I/O interface.

utils/sim/mos-sim.c

@@ -25,9 +25,16 @@ static const char usage[] =
     "The simulated 6502 will execute a reset sequence through the vector at\n"
     "$FFFC like a real 6502.\n"
     "\n"
-    "Writing to $FFF7 aborts.\n"
-    "Writing to $FFF8 quits normally.\n"
-    "Writing to $FFF9 writes to stdout.\n"
+    "Memory-mapped I/O:\n"
+    "\n"
+    " Addr | Len | Description\n"
+    "$FFF0 |  4  | Read: CPU clock cycles from program start.\n"
+    "      |     | Write: Reset counter.\n"
+    "$FFF5 |  1  | Read: Character from standard input.\n"
+    "$FFF6 |  1  | Read: 1 if last $FFF5 read was EOF.\n"
+    "$FFF7 |  1  | Write: Aborts.\n"
+    "$FFF8 |  1  | Write: Exits, returning the written exit code.\n"
+    "$FFF9 |  1  | Write: Character to standard output.\n"
     "\n"
     "OPTIONS:\n"
     "\t--cycles: Print cycle count to stderr.\n"