Implement halting

1 files changed, 6 insertions, 1 deletions

diff --git a/nandgame/instruction_decode.sv b/nandgame/instruction_decode.sv
index f81f251..b78bee8 100644
--- a/nandgame/instruction_decode.sv
+++ b/nandgame/instruction_decode.sv
@@ -34,7 +34,9 @@ module instruction_decode  #(
   // whether result should be stored to D
   output dst_D_out,
   // whether result should be stored in memory at address in A register
-  output dst_pA_out
+  output dst_pA_out,
+  // Invalid instruction
+  output invalid_ins
 );
 
 wire is_immediate_int;
@@ -42,6 +44,9 @@ wire is_immediate_int;
 // bit 15 unset = immediate
 assign is_immediate_int = !instruction_in[15];
 
+// bit 14 must be 1
+assign invalid_ins = instruction_in[15] == 1 && instruction_in[14] == 0;
+
 assign dst_A_out = is_immediate_int || instruction_in[5];
 assign dst_D_out = !is_immediate_int && instruction_in[4];
 assign dst_pA_out = !is_immediate_int && instruction_in[3];