RD4_Int<='0';

WR0<=NOT (PHI2 AND NOT RW);
WR1<=NOT (PHI2 AND NOT RW);
RD<=NOT (PHI2 AND RW);
CLK<=NOT CCTL AND PHI2;

ROM<=NOT (NOT S5 AND ENABLED);
CS0<='1';

ROM_Mapper:                process (SDX_REG,OSS_REG,EXTERN)
    begin
        if EXTERN='1' then
            AB(5)<='1';
            AB(4 downto 2)<=CAR_TYPE;
            AB(1)<=OSS_REG(1);
            AB(0)<=OSS_REG(0);
        else
            AB(5)<='0';
            AB(4)<='0';
            AB(3)<=not SDX_REG(4);
            AB(2)<=not SDX_REG(2);
            AB(1)<=not SDX_REG(1);
            AB(0)<=not SDX_REG(0);
        end if;
    end process;
    
CAR_TYPE_Register:    process (clk, A_Bus,DataIN)
    begin
        if A_Bus(7 downto 5)="110" and RW='0' and falling_edge(clk) then
            CAR_TYPE<=DataIN(2 downto 0);
        end if;
    end process;
    
SDX_Register:            process (clk, A_Bus)
    begin
        if A_Bus(7 downto 5)="111" and RW='0' and falling_edge(clk) then
                SDX_REG<=A_Bus(4 downto 0);
            end if;
    end process;

SDX_Mux:                    process (SDX_REG,S4,S5,RD4_Int,RD5_Int)
    begin
        if    SDX_REG(3)='1' and SDX_REG(2)='1' then                    -- "11" - SDX Off, Ext Off
            S4_Int<='1';
            S5_Int<='1';
            RD4<='1';
            RD5<='1';
            EXTERN<='0';
            ENABLED<='0';
        elsif SDX_REG(3)='1' and SDX_REG(2)='0' then                -- "10" - SDX Off, Ext On
            S4_Int<=S4;
            S5_Int<=S5;
            RD4<=RD4_Int;
            RD5<=RD5_Int;
            EXTERN<='1';
            ENABLED<='1';
        else                                                                    -- "01" or "00" - SDX On, Ext Off
            S4_Int<='1';
            S5_Int<='1';
            RD4<='0';
            RD5<='1';
            EXTERN<='0';
            ENABLED<='1';
        end if;
    end process;
    
OSS_Register:            process (clk, A_Bus)
    begin
        if A_Bus(7 downto 5)="000" and RW='0' and falling_edge(clk) then
                OSS_REG(1)<=A_Bus(3);
                OSS_REG(0)<=A_Bus(0);
            end if;
    end process;

OSS_Enable:                process (OSS_REG)
    begin
        if OSS_REG="10" then
            RD5_Int<='0';
        else
            RD5_Int<='1';
        end if;

    end process;