1 files changed, 39 insertions, 0 deletions
diff --git a/verilog/cin_choice.v b/verilog/cin_choice.v
new file mode 100755
index 0000000..e6ab92b
--- /dev/null
+++ b/verilog/cin_choice.v
@@ -0,0 +1,39 @@
+`timescale 1ns / 1ps
+//////////////////////////////////////////////////////////////////////////////////
+// Company: 
+// Engineer: 
+// 
+// Create Date:    12:44:24 10/04/2012 
+// Design Name: 
+// Module Name:    cin_choice 
+// Project Name: 
+// Target Devices: 
+// Tool versions: 
+// Description: 
+//
+// Dependencies: 
+//
+// Revision: 
+// Revision 0.01 - File Created
+// Additional Comments: 
+//
+//////////////////////////////////////////////////////////////////////////////////
+module cin_choice(
+    output cin,
+    input [2:0] op
+    );
+
+// we only want non-zero cin if we're subtracting.
+// Subtraction is used in both sub and slt, so we have two cases
+// 011 and 111. output is 1 for these and 0 for others 
+
+// This is equivalent to a'bc + abc (where a, b, c are bits in op)
+
+wire notw, subw, sltw;
+not n1(notw, op[2]);
+
+and sub_choose(subw, notw, op[1], op[0]);
+and slt_choose(sltw, op[2], op[1], op[0]);
+or choose(cin, sltw, subw);
+
+endmodule