repo2/atari_chips/gtia/gtia_player.vhdl
| 1 | markw | ---------------------------------------------------------------------------
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-- (c) 2013 mark watson
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-- I am happy for anyone to use this for non-commercial use.
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-- If my vhdl files are used commercially or otherwise sold,
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-- please contact me for explicit permission at scrameta (gmail).
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-- This applies for source and binary form and derived works.
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---------------------------------------------------------------------------
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LIBRARY ieee;
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USE ieee.std_logic_1164.all;
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use ieee.numeric_std.all;
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ENTITY gtia_player IS
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PORT
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(
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CLK : IN STD_LOGIC;
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RESET_N : in std_logic;
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COLOUR_ENABLE : IN STD_LOGIC;
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LIVE_POSITION : in std_logic_vector(7 downto 0); -- counter ticks as display is drawn
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PLAYER_POSITION : in std_logic_vector(7 downto 0); -- requested position
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SIZE : in std_logic_vector(1 downto 0);
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bitmap : in std_logic_vector(7 downto 0);
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output : out std_logic
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);
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END gtia_player;
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ARCHITECTURE vhdl OF gtia_player IS
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-- pmgs
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signal shift_next : std_logic_vector(7 downto 0);
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signal shift_reg : std_logic_vector(7 downto 0);
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signal count_next : std_logic_vector(1 downto 0);
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signal count_reg : std_logic_vector(1 downto 0);
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begin
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-- register
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process(clk,reset_n)
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begin
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if (reset_n = '0') then
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shift_reg <= (others=>'0');
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count_reg <= (others=>'0');
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elsif (clk'event and clk='1') then
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shift_reg <= shift_next;
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count_reg <= count_next;
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end if;
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end process;
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process(shift_next,COLOUR_ENABLE, live_position, player_position, size, bitmap, shift_reg, count_reg)
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begin
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-- size is 2 bits (00 normal, 01 twice, 10 normal(bugged), 11 four times)
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-- grafp0 is 8 bits of data (2 bits for missiles)
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-- hposp0_reg is position - 0x30 is left side of normal playfield, 0xd0 is right size of normal playfield
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-- 'or' into shift register, so if not empty we get extra pixels
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-- counter before we shift
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shift_next <= shift_reg;
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count_next <= count_reg;
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output <= shift_next(7);
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if (COLOUR_ENABLE = '1') then
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case size is
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when "00" => -- normal size
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count_next <= "00";
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shift_next <= shift_reg(6 downto 0) &'0';
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when "10" => -- normal size (with bug)
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case count_reg is
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when "00"|"11" =>
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count_next <= "00";
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shift_next <= shift_reg(6 downto 0) &'0';
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when "01"|"10" =>
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count_next <= "10";
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when others=>
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--hang!
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end case;
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when "01" =>
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case count_reg is
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when "01"|"11" =>
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count_next <= "00";
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shift_next <= shift_reg(6 downto 0) &'0';
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when "00"|"10" =>
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count_next <= "01";
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when others=>
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--hang!
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end case;
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when "11" =>
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case count_reg is
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when "00" =>
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count_next <= "01";
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when "01" =>
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count_next <= "10";
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when "10" =>
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count_next <= "11";
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when "11" =>
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shift_next <= shift_reg(6 downto 0) &'0';
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count_next <= "00";
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when others=>
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--hang!
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end case;
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when others=>
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--hang!
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end case;
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if (live_position = player_position) then
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shift_next <= shift_reg(6 downto 0) &'0' or bitmap;
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count_next <= (others=>'0');
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end if;
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end if;
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end process;
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end vhdl;
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