library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;
entity cpu is
port(
clk: in std_logic;
rst: in std_logic;
code_data: in std_logic_vector(15 downto 0);
code_addr: out std_logic_vector(15 downto 0);
mem_in: in std_logic_vector(15 downto 0);
mem_out: out std_logic_vector(15 downto 0);
mem_addr: out std_logic_vector(15 downto 0);
mem_write: out std_logic;
mem_read: out std_logic
);
end entity cpu;
architecture behavior of cpu is
component alu is
port(
a: in std_logic_vector(15 downto 0);
b: in std_logic_vector(15 downto 0);
sel: in std_logic_vector(3 downto 0);
flag: out std_logic;
q: out std_logic_vector(15 downto 0)
);
end component;
component reg is
port(
clk : in std_logic;
rst : in std_logic;
d : in std_logic_vector(15 downto 0);
q : out std_logic_vector(15 downto 0)
);
end component;
signal alu_a: std_logic_vector(15 downto 0);
signal alu_b: std_logic_vector(15 downto 0);
signal alu_q: std_logic_vector(15 downto 0);
signal alu_sel: std_logic_vector(3 downto 0);
signal alu_flag: std_logic;
signal load_reg_next, load_reg: std_logic_vector(15 downto 0);
signal load_addr_next, load_addr: std_logic_vector(15 downto 0);
type regbank is array(0 to 15) of std_logic_vector(15 downto 0);
signal reg_d: regbank;
signal reg_q: regbank;
type cpu_state_t is (RUN, LOAD, BRANCH);
signal cpu_state, cpu_state_next: cpu_state_t;
begin
cpu_alu: alu port map(a => alu_a, b => alu_b, sel => alu_sel, flag => alu_flag, q => alu_q);
load_reg_r: reg port map(clk => clk, rst => rst, d => load_reg_next, q => load_reg);
load_addr_r: reg port map(clk => clk, rst => rst, d => load_addr_next, q => load_addr);
allregs:
for i in 0 to 15 generate
regx: reg port map(clk => clk, rst => rst, d => reg_d(i), q => reg_q(i));
end generate allregs;
process(clk, rst)
begin
if rst = '1' then
cpu_state <= BRANCH; -- wait a cycle at first
elsif rising_edge(clk) then
cpu_state <= cpu_state_next;
end if;
end process;
code_addr <= reg_q(14);
process(code_data, reg_q, mem_in, alu_q, alu_flag, cpu_state, load_addr, load_reg) is
variable inst: std_logic_vector(15 downto 0);
variable regn_0: natural;
variable regn_1: natural;
variable regn_2: natural;
variable do_alu: std_logic;
begin
mem_write <= '0';
mem_read <= '0';
mem_addr <= x"0000";
mem_out <= x"0000";
alu_sel <= "0000";
alu_a <= x"0000";
alu_b <= x"0000";
do_alu := '0';
for i in 0 to 15 loop
reg_d(i) <= reg_q(i);
end loop;
cpu_state_next <= RUN;
load_reg_next <= load_reg;
load_addr_next <= load_addr;
case cpu_state is
when RUN =>
reg_d(14) <= std_logic_vector(unsigned(reg_q(14)) + 2);
inst := code_data;
when LOAD =>
inst := x"0000"; -- NOP
mem_addr <= load_addr; -- maintain this until we're done reading
if load_reg(3 downto 0) = x"e" then
cpu_state_next <= BRANCH;
else
reg_d(14) <= std_logic_vector(unsigned(reg_q(14)) + 2);
end if;
regn_0 := to_integer(unsigned(load_reg(3 downto 0)));
reg_d(regn_0) <= mem_in;
when BRANCH =>
inst := x"0000"; -- NOP
reg_d(14) <= std_logic_vector(unsigned(reg_q(14)) + 2);
end case;
regn_0 := to_integer(unsigned(inst(11 downto 8)));
regn_1 := to_integer(unsigned(inst(7 downto 4)));
regn_2 := to_integer(unsigned(inst(3 downto 0)));
case inst(15 downto 12) is
when "0000" => -- NOP
when "0001" => -- LOAD rn, [rm, imm] (imm is signed 4 bits)
mem_read <= '1';
cpu_state_next <= LOAD;
reg_d(14) <= reg_q(14); -- halt the prefetcher
load_addr_next <= std_logic_vector(signed(reg_q(regn_1)) + signed(inst(3 downto 0) & '0'));
mem_addr <= std_logic_vector(signed(reg_q(regn_1)) + signed(inst(3 downto 0) & '0'));
load_reg_next(3 downto 0) <= inst(11 downto 8);
when "0010" => -- STORE rn, [rm, imm]
mem_write <= '1';
mem_addr <= std_logic_vector(signed(reg_q(regn_1)) + signed(inst(3 downto 0) & '0'));
mem_out <= reg_q(regn_0);
--- ALU stuff
when "0011" => do_alu := '1'; -- ADD rd, rn, rm (rd := rn + rm)
when "0100" => do_alu := '1'; -- SUB rd, rn, rm (rd := rn - rm)
when "0101" => do_alu := '1'; -- OR rd, rn, rm (rd := rn or rm)
when "0110" => do_alu := '1'; -- AND rd, rn, rm (rd := rn and rm)
when "0111" => do_alu := '1'; -- NOT rd, rn (rd := not rn)
when "1000" => do_alu := '1'; -- XOR rd, rn, rm (rd := rn xor rm)
when "1001" => -- SETH rd, imm
reg_d(regn_0)(15 downto 8) <= inst(7 downto 0);
when "1010" => -- SHR rd, rn, imm (rd := rn >> imm)
alu_sel <= inst(15 downto 12);
alu_a <= reg_q(regn_1);
alu_b <= x"000" & inst(3 downto 0);
reg_d(regn_0) <= alu_q;
when "1011" => do_alu := '1'; -- MUL rd, rn, rm (rd := rn * rm)
when "1100" => -- CMP rn, rm (flag := 1 if equal)
alu_sel <= "1100";
alu_a <= reg_q(regn_0);
alu_b <= reg_q(regn_1);
reg_d(15)(0) <= alu_flag;
when "1101" => -- BEQ [rn, imm] (jump to [rn, imm] if flag is set, imm is signed 8 bits)
if reg_q(15)(0) = '1' then
reg_d(14) <= std_logic_vector(signed(reg_q(regn_0)) + signed(inst(7 downto 0) & '0'));
cpu_state_next <= BRANCH;
end if;
when "1110" => -- SET rd, imm (rd := imm, imm is 8 bit)
reg_d(regn_0) <= x"00" & inst(7 downto 0);
when "1111" => -- BNEQ [rn, imm]
if reg_q(15)(0) = '0' then
reg_d(14) <= std_logic_vector(signed(reg_q(regn_0)) + signed(inst(7 downto 0) & '0'));
cpu_state_next <= BRANCH;
end if;
when others => -- do nothing
end case;
if do_alu = '1' then
-- 1:1 mapping
alu_sel <= inst(15 downto 12);
alu_a <= reg_q(regn_1);
alu_b <= reg_q(regn_2);
reg_d(regn_0) <= alu_q;
reg_d(15)(0) <= alu_flag;
if inst(11 downto 8) = x"e" then
cpu_state_next <= BRANCH;
end if;
end if;
end process;
end behavior;