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-- Company: CPE 233
-- Engineer: Zach Bunce, Kaioli Bessert, and Garrett Maxon
--
-- Create Date: 02/11/2017 03:59:55 PM
-- Design Name: Stack Pointer
-- Module Name: SP - Counter
-- Project Name: Experiment 8 - Get 'er Done
-- Description: 8-bit Stack Pointer which either loads in a value, increments or
-- decrements the previous pointer value, or resets the pointer to 0 based off of the
-- CPU control lines.
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LIBRARY IEEE;
USE IEEE.STD_LOGIC_1164.ALL;
USE IEEE.STD_LOGIC_ARITH.ALL;
USE IEEE.STD_LOGIC_UNSIGNED.ALL;


ENTITY SP is
    PORT (
            RST   : in  STD_LOGIC;
            LD    : in  STD_LOGIC;
            INCR  : in  STD_LOGIC;
            DECR  : in  STD_LOGIC;
            D_IN  : in  STD_LOGIC_VECTOR (7 downto 0);
            CLK   : in  STD_LOGIC;
            D_OUT : out STD_LOGIC_VECTOR (7 downto 0)
         );
END SP;

ARCHITECTURE Counter of SP is
    -- Sets up a intermediary wire to allow for incrementation.
    SIGNAL D_TEMP : STD_LOGIC_VECTOR (7 downto 0) := (others => '0');

BEGIN
    -- Links the temporary data wire to the output line.
    D_OUT <= D_TEMP;

    -- Determines the pointer value based off of the control lines.
    countProc: PROCESS(D_IN, LD, INCR, DECR, RST, CLK)
    BEGIN

        if(rising_edge(CLK)) then
            -- Resets the pointer to 0x00 when the reset line is high.
            if (RST = '1') then
                D_TEMP <= x"00";
            -- Sets the output to the input when the load line is high.
            elsif (LD = '1') then
                D_TEMP <= D_IN;
            -- Increments the output by one when the increment line is high.
            elsif (INCR = '1') then
                D_TEMP <= (D_TEMP + 1);
            -- Decrements the output by one when the decrement line is high.
            elsif (DECR = '1') then
                D_TEMP <= (D_TEMP - 1);
            -- Otherwise, holds the output.
            else
                D_TEMP <= D_TEMP;
            end if;
        end if;
    END PROCESS countProc;
END Counter;