diff --git a/hw/fpga/hvl/cs_testbench.sv b/hw/fpga/hvl/cs_testbench.sv
index b6ba598..731443e 100644
--- a/hw/fpga/hvl/cs_testbench.sv
+++ b/hw/fpga/hvl/cs_testbench.sv
@@ -7,7 +7,9 @@ timeprecision 1ns;
 logic [15:0] addr;
 logic ram_cs;
 logic rom_cs;
-logic io_cs;
+logic hex_cs;
+
+int cs_count = ram_cs + rom_cs + hex_cs;
 
 addr_decode dut(.*);
 
@@ -16,13 +18,16 @@ initial begin : TEST_VECTORS
     for (int i = 0; i < 2**16; i++) begin
         addr <= i;
         #1
+        assert(cs_count < 2)
+        else
+            $error("Multiple chip selects present!");
         if (i < 16'h7ff0) begin
             assert(ram_cs == '1)
             else
                 $error("Bad CS! addr=%4x should have ram_cs!", addr);
         end
-        if (i >= 16'h7ff0 && i < 16'h8000) begin
-            assert(io_cs == '1)
+        if (i >= 16'h7ff0 && i < 16'h7ff2) begin
+            assert(hex_cs == '1)
             else
                 $error("Bad CS! addr=%4x should have io_cs!", addr);
         end