eater_alu: Use 2complneg instead of subtract

1 files changed, 8 insertions, 1 deletions

diff --git a/eater_cpu/eater_alu.sv b/eater_cpu/eater_alu.sv
index b0fa9ae..01b62ae 100644
--- a/eater_cpu/eater_alu.sv
+++ b/eater_cpu/eater_alu.sv
@@ -6,12 +6,19 @@ module eater_alu (
     input clk_in,
     input en_output_in,
 
+    input subtract_n_add_in,
+
     input [7:0] A_in,
     input [7:0] B_in,
 
     output [7:0] bus_out
 );
 
-assign bus_out = en_output_in ? 0 : 8'bz;
+// wire [7:0] result = subtract_n_add_in ? (A_in - B_in) : (A_in + B_in);
+wire [7:0] xormask = {8{subtract_n_add_in}};
+wire [7:0] B_neg_if = B_in ^ xormask;
+wire [7:0] result2 = A_in + B_neg_if + subtract_n_add_in;
+
+assign bus_out = en_output_in ? result2 : 8'bz;
     
 endmodule