fix operand order of subtraction. add rollover counter UART test.

1 files changed, 9 insertions, 4 deletions

diff --git a/PIC/ALU.vhd b/PIC/ALU.vhd
index ac770a6..b7c0344 100644
--- a/PIC/ALU.vhd
+++ b/PIC/ALU.vhd
@@ -78,7 +78,11 @@ architecture rtl of alu is
     end if;
     res <= t;
     if do_skip then
-      do_skip_out <= '1' when unsigned(t) = 0 else '0';
+      if unsigned(t) = 0 then
+        do_skip_out <= '1';
+      else
+        do_skip_out <= '0';
+      end if;
     else
       stat <= set_flag(FLAG_Z, sel(unsigned(t) = 0, '1', '0'), stat_in);
     end if;
@@ -117,10 +121,11 @@ begin
         result <= tmp_extended(7 downto 0);
 
       when SUBLW | SUBWF =>
-        tmp_extended := std_logic_vector(unsigned('0' & a) - unsigned('0' & b));
+        -- PIC16F84A: SUBWF = f - W, SUBLW = k - W  →  dest = b - a
+        tmp_extended := std_logic_vector(unsigned('0' & b) - unsigned('0' & a));
 
-        -- DC
-        tmp(4 downto 0) := std_logic_vector(('0' & unsigned(a(3 downto 0))) - ('0' & unsigned(b(3 downto 0))));
+        -- DC: borrow out of lower nibble
+        tmp(4 downto 0) := std_logic_vector(('0' & unsigned(b(3 downto 0))) - ('0' & unsigned(a(3 downto 0))));
 
         tmp_status := set_flag(FLAG_Z, sel(unsigned(tmp_extended(7 downto 0)) = 0, '1', '0'), status_in);
         tmp_status := set_flag(FLAG_C, not tmp_extended(8), tmp_status);