VHDL microprocessor/microcontroller

This answer is about the 3rd part of your question.

You may find it useful to take a look at the MCPU project. It's an 8-bit CPU in 77 lines of VHDL code. Because the author has squeezed the entire design into 32 macrocells, the code is a little tricky in some places, but the design document helps.

I've also created a refactored version aiming at code readability, which is included below. Note that I'm not the original author of the project -- all kudos go to Tim Böscke.

library ieee;
use ieee.std_logic_1164.all;
use ieee.std_logic_unsigned.all;

entity mcpu is
    port (
        data_bus: inout std_logic_vector(7 downto 0);
        address: out std_logic_vector(5 downto 0);
        n_oe: out std_logic;
        -- Asynchronous memory interface
        n_we: out std_logic;    
        n_reset: in std_logic;
        clock: in std_logic
    );
end;

architecture refactored of mcpu is
  signal accumulator: std_logic_vector(8 downto 0);
  alias carry is accumulator(8);  
  alias result is accumulator(7 downto 0);
  alias opcode is data_bus(7 downto 6);

  signal address_register: std_logic_vector(5 downto 0);
  signal pc: std_logic_vector(5 downto 0);
  signal states: std_logic_vector(2 downto 0);

  type cpu_state_type is (FETCH, WRITE, ALU_ADD, ALU_NOR, BRANCH_NOT_TAKEN);
  signal cpu_state: cpu_state_type;

  type state_encoding_type is array (cpu_state_type) of std_logic_vector(2 downto 0);
  constant STATE_ENCODING: state_encoding_type := (
      FETCH => "000", 
      WRITE => "001", 
      ALU_ADD => "010", 
      ALU_NOR => "011", 
      BRANCH_NOT_TAKEN => "101"
  );

begin
    process (clock, n_reset)
    begin
        if not n_reset then
            -- start execution at memory location 0
            address_register <= (others => '0');  
            states <= "000";
            cpu_state <= FETCH;
            accumulator <= (others => '0');
            pc <= (others => '0');
        elsif rising_edge(clock) then

            -- PC / Adress path
            if cpu_state = FETCH then
              pc <= address_register + 1;
              address_register <= data_bus(5 downto 0);
            else
              address_register <= pc;
            end if;

            -- ALU / Data Path
            case cpu_state is
                when ALU_ADD => 
                    accumulator <= ('0' & result) + ('0' & data_bus);
                when ALU_NOR => 
                    result <= result nor data_bus;
                when BRANCH_NOT_TAKEN => 
                    carry <= '0';
                when others => null;
            end case;

            -- State machine
            if cpu_state /= FETCH then 
                cpu_state <= FETCH;
            elsif opcode ?= "11" and carry then 
                cpu_state <= BRANCH_NOT_TAKEN;
            else
                states <= "0" & not opcode;       -- execute instruction
                case opcode is
                    when "00" => cpu_state <= ALU_NOR;  -- 011
                    when "01" => cpu_state <= ALU_ADD;  -- 010                    
                    when "10" => cpu_state <= WRITE;    -- 001
                    when "11" => cpu_state <= FETCH;    -- 000
                    when others => null;                     
                end case;
            end if;
        end if;
    end process;

    -- output
    address <= address_register;    
    data_bus <= result when (cpu_state = WRITE) else (others => 'Z');

    -- output enable is active low, asserted only when
    -- rst=1, clk=0, and state!=001(wr_acc) and state!=101(read_pc)
    n_oe <= '1' when (clock='1' or cpu_state = WRITE or n_reset = '0' or cpu_state = BRANCH_NOT_TAKEN) else '0';

    -- write enable is active low, asserted only when
    -- rst=1, clk=0, and state=001(wr_acc)
    n_we <= '1' when (clock = '1' or cpu_state /= WRITE or n_reset = '0') else '0';

end;