library IEEE;
use IEEE.std_logic_1164.all;
use IEEE.std_logic_unsigned.all;

entity vgacore is
    port
    (
        rgb: out std_logic_vector(5 downto 0);    -- red,green,blue colors
        addr: out std_logic_vector(14 downto 0);    -- address into video RAM
        data: in std_logic_vector(7 downto 0);    -- data from video RAM
        csb: out std_logic;        -- video RAM chip enable
        oeb: out std_logic;        -- video RAM output enable
        web: out std_logic        -- video RAM write enable
    );
end vgacore;

architecture vgacore_arch of vgacore is
signal vsyncb:  std_logic:= '0';
signal hsyncb:std_logic:= '0';
signal clock:  std_logic:= '0';
signal reset:std_logic:= '0';
signal hcnt: std_logic_vector(8 downto 0);    -- horizontal pixel counter
signal vcnt: std_logic_vector(9 downto 0);    -- vertical line counter
signal pixrg: std_logic_vector(7 downto 0);    -- byte-wide register for 4 pixels
signal blank: std_logic;                    -- video blanking signal
signal pblank: std_logic;                    -- pipelined video blanking signal
begin

A: process(clock,reset)
begin
    -- reset asynchronously clears pixel counter
    if reset='1' then
        hcnt <= "000000000";
    -- horiz. pixel counter increments on rising edge of dot clock
    elsif (clock'event and clock='1') then
        -- horiz. pixel counter rolls-over after 381 pixels
        if hcnt<380 then
            hcnt <= hcnt + 1;
        else
            hcnt <= "000000000";
        end if;
    end if;
end process;

B: process(hsyncb,reset)
begin
    -- reset asynchronously clears line counter
    if reset='1' then
        vcnt <= "0000000000";
    -- vert. line counter increments after every horiz. line
    elsif (hsyncb'event and hsyncb='1') then
        -- vert. line counter rolls-over after 528 lines
        if vcnt<527 then
            vcnt <= vcnt + 1;
        else
            vcnt <= "0000000000";
        end if;
    end if;
end process;

C: process(clock,reset)
begin
    -- reset asynchronously sets horizontal sync to inactive
    if reset='1' then
        hsyncb <= '1';
    -- horizontal sync is recomputed on the rising edge of every dot clock
    elsif (clock'event and clock='1') then
        -- horiz. sync is low in this interval to signal start of a new line
        if (hcnt>=291 and hcnt<337) then
            hsyncb <= '0';
        else
            hsyncb <= '1';
        end if;
    end if;
end process;

D: process(hsyncb,reset)
begin
    -- reset asynchronously sets vertical sync to inactive
    if reset='1' then
        vsyncb <= '1';
    -- vertical sync is recomputed at the end of every line of pixels
    elsif (hsyncb'event and hsyncb='1') then
        -- vert. sync is low in this interval to signal start of a new frame
        if (vcnt>=490 and vcnt<492) then
            vsyncb <= '0';
        else
            vsyncb <= '1';
        end if;
    end if;
end process;

-- blank video outside of visible region: (0,0) -> (255,479)
E: blank <= '1' when (hcnt>=256 or vcnt>=480) else '0';
-- store the blanking signal for use in the next pipeline stage
F: process(clock,reset)
begin
    if reset='1' then
        pblank <= '0';
    elsif (clock'event and clock='1') then
        pblank <= blank;
    end if;
end process;

-- video RAM control signals
G:
csb <= '0';        -- enable the RAM
web <= '1';        -- disable writing to the RAM
oeb <= blank;    -- enable the RAM outputs when video is not blanked

-- The video RAM address is built from the lower 9 bits of the vertical
-- line counter and bits 7-2 of the horizontal pixel counter.
-- Each byte of the RAM contains four 2-bit pixels.  As an example,
-- the byte at address ^h1234=^b0001,0010,0011,0100 contains the pixels
-- at (row,col) of (^h048,^hD0),(^h048,^hD1),(^h048,^hD2),(^h048,^hD3).
H: addr <= vcnt(8 downto 0) & hcnt(7 downto 2);

I: process(clock,reset)
begin
    -- clear the pixel register on reset
    if reset='1' then
        pixrg <= "00000000";
    -- pixel clock controls changes in pixel register
    elsif (clock'event and clock='1') then
        -- the pixel register is loaded with the contents of the video
        -- RAM location when the lower two bits of the horiz. counter
        -- are both zero.  The active pixel is in the lower two bits
        -- of the pixel register.  For the next 3 clocks, the pixel
        -- register is right-shifted by two bits to bring the other
        -- pixels in the register into the active position.
        if hcnt(1 downto 0)="00" then
            pixrg <= data;    -- load 4 pixels from RAM
        else
            pixrg <= "00" & pixrg(7 downto 2);    -- right-shift pixel register
        end if;
    end if;
end process;

-- the color mapper translates each 2-bit pixel into a 6-bit
-- color value.  When the video signal is blanked, the color
-- is forced to zero (black).
J: process(clock,reset)
begin
    -- blank the video on reset
    if reset='1' then
        rgb <= "000000";
    -- update the color outputs on every dot clock
    elsif (clock'event and clock='1') then
        -- map the pixel to a color if the video is not blanked
        if pblank='0' then
            case pixrg(1 downto 0) is
                when "00" =>   rgb <= "110000";    -- red
                when "01" =>   rgb <= "001100";    -- green
                when "10" =>   rgb <= "000011";    -- blue
                when others => rgb <= "111111";    -- white
            end case;
        -- otherwise, output black if the video is blanked
        else
            rgb <= "000000";    -- black
        end if;
    end if;
end process;

end vgacore_arch;