Add PHA instruction

src/cpu.verilog

@@ -574,6 +574,13 @@ always @(posedge clk)
                     P[P_V] <= 0;
                     state <= STATE_FETCH;
                 end
+              else if (decoded_instr == I_PHA)
+                begin
+                    mem_aux2_addr <= {8'h01, S};
+                    mem_sel <= DMUX_AUX2;
+                    S <= S - 8'h01;
+                    state <= STATE_EXECUTE_DATA_2;
+                end
           end
         else if (decoded_instr == I_BNE)
           begin
@@ -907,6 +914,11 @@ always_comb
                   mem_wr = 1'b1;
                   mem_data_wr = Y;
               end
+            else if (decoded_instr == I_PHA)
+              begin
+                  mem_wr = 1'b1;
+                  mem_data_wr = A;
+              end
         end
       else if (state == STATE_EXECUTE_ZP)
         begin
@@ -1058,10 +1070,15 @@ always @(posedge clk)
         f_prev_Y <= Y;
         f_prev_A <= A;
         f_prev_PC <= PC;
-        f_prev_S <= S;
         f_prev_P <= P;
     end
 
+always @(posedge clk)
+  if (f_past_valid && state == STATE_EXECUTE && $past(state) != STATE_EXECUTE)
+    begin
+        f_prev_S <= S;
+    end
+
 always @(*)
     f_prev_instruction_is_branch <=
                                    (f_prev_instruction == I_JMP ||
@@ -2083,6 +2100,27 @@ always @(posedge clk)
           assert(Y == f_prev_Y);
       end
 
+always @(posedge clk)
+  if (f_prev_valid && state == STATE_EXECUTE)
+    if (f_prev_instruction == I_PHA)
+      begin
+          cover($past(address_code) == ADDR_MODE_IMPLIED);
+
+          assert($past(PC) == $past(f_prev_PC) + 16'd1);
+          assert(f_prev_instr_cycles == 3);
+
+          assert($past(mem_addr_eff, 2) == 16'h0100 + f_prev_S);
+          assert($past(mem_data_wr, 2) == f_prev_A);
+          assert($past(mem_wr, 2) == 1'b1);
+
+          assert(S == f_prev_S - 8'h01);
+
+          assert(P == f_prev_P);
+          assert(A == f_prev_A);
+          assert(X == f_prev_X);
+          assert(Y == f_prev_Y);
+      end
+
 always @(posedge clk)
     if (f_past_valid)
 begin
@@ -2126,6 +2164,7 @@ begin
              decoded_instr == I_BPL ||
              decoded_instr == I_BVC ||
              decoded_instr == I_BVS ||
+             decoded_instr == I_PHA ||
              decoded_instr == I_JMP
 );
     assume(address_code != ADDR_MODE_INDIRECT);