diff --git a/firmware/alu/main.vhd b/firmware/alu/main.vhd
index 1352b07..318000a 100644
--- a/firmware/alu/main.vhd
+++ b/firmware/alu/main.vhd
@@ -5,7 +5,8 @@ use ieee.numeric_std.ALL;
 
 entity main is
     generic (
-        WIDTH: integer := 8
+        WIDTH: integer := 8;
+        FIRST: boolean := true
     );
     port (
         func: in std_logic_vector(3 downto 0);
@@ -77,13 +78,21 @@ begin
             when "1001" => -- INC
                 x <= accu;
                 y <= (others => '0');
-                cin <= '1';
+                if FIRST then
+                    cin <= '1';
+                else
+                    cin <= carry_in;
+                end if;
                 result <= sum;
                 carry_out <= cout;
             when "1010" => -- DEC
                 x <= accu;
                 y <= (others => '1');
-                cin <= '0';
+                if FIRST then
+                    cin <= '0';
+                else
+                    cin <= not carry_in;
+                end if;
                 result <= sum;
                 carry_out <= not cout;
             when "1011" => -- ZERO
diff --git a/firmware/alu/tests/toy_16.vhd b/firmware/alu/tests/toy_16.vhd
index ee0fc1d..84f0ccf 100644
--- a/firmware/alu/tests/toy_16.vhd
+++ b/firmware/alu/tests/toy_16.vhd
@@ -11,6 +11,9 @@ architecture test of toy_16 is
     constant period: time := 1us;
 
     component main
+        generic (
+            FIRST: boolean
+        );
         port (
             func: in std_logic_vector(3 downto 0);
             accu: in std_logic_vector(7 downto 0);
@@ -43,7 +46,9 @@ architecture test of toy_16 is
 
 begin
 
-    alu1: main port map (
+    alu1: main generic map (
+        FIRST => true
+    ) port map (
         func => func,
         accu => accu(7 downto 0),
         ram => ram(7 downto 0),
@@ -52,7 +57,9 @@ begin
         carry_out => carry_propagation
     );
 
-    alu2: main port map (
+    alu2: main generic map (
+        FIRST => false
+    ) port map (
         func => func,
         accu => accu(15 downto 8),
         ram => ram(15 downto 8),