Implement subtraction in terms of addition

1 files changed, 6 insertions, 8 deletions

diff --git a/alu.v b/alu.v
index 5fbb8ba..63812a7 100644
--- a/alu.v
+++ b/alu.v
@@ -26,16 +26,17 @@ module alu (
 //  1111: result = x >> y (barrel shift)
 
 	wire [16:0] sum;
-	wire [16:0] diff;
 	wire [16:0] prod;
 	wire [16:0] quot;
 	wire [15:0] anded;
 	wire [15:0] ored;
 	wire [15:0] xored;
 	wire [15:0] shifted;
+	wire issum;
 
-	assign sum = x + y;
-	assign diff = x - y;
+	assign issum = ~|(op[3:1]);
+
+	assign sum = x + (op[0] ? ~y + 1 : y);
 	assign prod = x * y;
 	assign quot = x / y;
 	assign anded = x & y;
@@ -51,8 +52,7 @@ module alu (
 	);
 
 	assign result =
-		op == 4'b0000 ? sum[15:0] :
-		op == 4'b0001 ? diff[15:0] :
+		issum ? sum[15:0] :
 		op == 4'b0010 ? prod[15:0] :
 		op == 4'b0011 ? quot[15:0] :
 		op == 4'b0100 ? anded :
@@ -62,8 +62,6 @@ module alu (
 		shifted;
 
 	assign zero = ~(|result);
-	assign carry =
-		op == 4'b0000 ? sum[16] :
-		op == 4'b0001 ? diff[16] : 1'b0;
+	assign carry = issum ? sum[16] : 1'b0;
 
 endmodule