SP.vhd

library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.numeric_std.all; 

entity SP is
    Port ( 
           en, mem_read ,Clk, INTR : in STD_LOGIC;
           address_select : in STD_LOGIC;
           FunctionCode : in std_logic_vector(2 downto 0) ; 
           data_out : out STD_LOGIC_VECTOR (9 downto 0)        
       );
end SP;

architecture Behavioral of SP is
    signal stack_pointer : unsigned(9 downto 0) := "1111111111"; -- 1023 in decimal
begin
    process (Clk , mem_read, address_select , FunctionCode , INTR)
	    variable temp_stack_pointer : unsigned(9 downto 0);
    begin
        if falling_edge(clk) then 
	if en = '1' then
            if address_select = '1' then
                -- Interrupt case push PC + Flags 
                -- RTI pop PC + Flags 
                if (mem_read = '1' and FunctionCode ="111") then -- RTI
                    temp_stack_pointer := stack_pointer +2 ;   
                    stack_pointer <= temp_stack_pointer;
                    data_out <= std_logic_vector(temp_stack_pointer);
                elsif (mem_read = '1')  then -- pop 
                    temp_stack_pointer := stack_pointer +1 ;   
                    stack_pointer <= temp_stack_pointer;
                    data_out <= std_logic_vector(temp_stack_pointer);
                elsif (mem_read = '0' and INTR ='1' ) then  -- push INTERRUPT
                    temp_stack_pointer := stack_pointer;
                    data_out <= std_logic_vector(temp_stack_pointer);
                    temp_stack_pointer := stack_pointer -2 ;
                    stack_pointer <= temp_stack_pointer;
                elsif mem_read = '0' then -- push 
                    temp_stack_pointer := stack_pointer;
                    data_out <= std_logic_vector(temp_stack_pointer);
                    temp_stack_pointer := stack_pointer -1 ;
                    stack_pointer <= temp_stack_pointer;
		end if;
	    else data_out <= std_logic_vector(stack_pointer);
	    end if;
           end if;
        end if; 
    end process; 
 end Behavioral;