Subversion Repositories light8080

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-- Light8080 simulation test bench.

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-- This test bench was built from a generic template. The details on what tests

-- are performed by this test bench can be found in the assembly source for the 

-- 8080 program, in file asm\exer.asm.

-------------------------------------------------------------------------------- 

-- 

-- This test bench provides a simulated CPU system to test programs. This test 

-- bench does not do any assertions or checks, all assertions are left to the 

-- software.

--

-- The simulated environment has 64KB of RAM covering the whole address map.

-- The simulated RAM is initialized with the contents of constant 'obj_code'.

-- This constant's contents are generated from some .HEX object code file using 

-- a helper script. See the perl script 'util\hexconv.pl' and BAT files in the 

-- asm directory.

--

-- This simulated system provides some means to trigger hardware irq from 

-- software, including the specification of the instructions fed to the CPU as 

-- interrupt vectors during inta cycles. This is only meant to test interrupt 

-- response of the CPU.

--

-- We will simulate 8 possible irq sources. The software can trigger any one of 

-- them by writing at ports 0x010 to 0x011. Port 0x010 holds the irq source to 

-- be triggered (0 to 7) and port 0x011 holds the number of clock cycles that 

-- will elapse from the end of the instruction that writes to the register to 

-- the assertion of intr. Port 0x012 holds the number of cycles intr will remain 

-- high. Intr will be asserted for 1 cycle at least, so writing a 0 here is the 

-- same as writing 1.

--

-- When the interrupt is acknowledged and inta is asserted, the test bench reads

-- the value at register 0x010 as the irq source, and feeds an instruction to 

-- the CPU starting from the RAM address 0040h+source*4.

-- That is, address range 0040h-005fh is reserved for the simulated 'interrupt

-- vectors', a total of 4 bytes for each of the 8 sources. This allows the 

-- software to easily test different interrupt vectors without any hand 

-- assembly. All of this is strictly simulation-only stuff.

--

-- Upon completion, the software must write a value to register 0x020. Writing 

-- a 0x055 means 'success', writing a 0x0aa means 'failure'. The write operation

-- will stop the simulation. Success and failure conditions are defined by the 

-- software.

--

-- If a time period defined as constant MAX_SIM_LENGTH passes before anything

-- is written to io address 0x020, the test bench assumes the software ran away

-- and quits with an error message.

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library ieee;

use ieee.std_logic_1164.ALL;

use ieee.std_logic_unsigned.all;

use ieee.numeric_std.ALL;

use work.obj_code_pkg.all;

use work.l80pkg.all;

entity light8080_tb is

end entity light8080_tb;

architecture behavior of light8080_tb is

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-- Simulation parameters

-- T: simulated clock period

constant T : time := 100 ns;

-- MAX_SIM_LENGTH: maximum simulation time

-- Enough for the tb0, you may need to modify this for other tests.

constant MAX_SIM_LENGTH : time := T * 7000;

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signal data_i :           std_logic_vector(7 downto 0) := (others=>'0');

signal vma_o  :           std_logic;

signal rd_o :             std_logic;

signal wr_o :             std_logic;

signal io_o :             std_logic;

signal data_o :           std_logic_vector(7 downto 0);

signal data_mem :         std_logic_vector(7 downto 0);

signal addr_o :           std_logic_vector(15 downto 0);

signal fetch_o :          std_logic;

signal inta_o :           std_logic;

signal inte_o :           std_logic;

signal intr_i :           std_logic := '0';

signal halt_o :           std_logic;

signal reset :            std_logic := '0';

signal clk :              std_logic := '1';

signal done :             std_logic := '0';

signal irq_vector_byte:   std_logic_vector(7 downto 0);

signal irq_source :       integer range 0 to 7;

signal cycles_to_intr :   integer range -10 to 255;

signal intr_width :       integer range 0 to 255;

signal int_vector_index : integer range 0 to 3;

signal addr_vector_table: integer range 0 to 65535;

signal con_line_buf :     string(1 to 80);

signal con_line_ix :      integer;

-- Using shared variables for big memory arrays speeds up simulation a lot;

-- see Modelsim 6.3 User Manual, section on 'Modelling Memory'.

-- WARNING: I have only tested this construct with Modelsim SE 6.3.

shared variable ram : ram_t(0 to 65536-1) := objcode_to_bram(obj_code, 65536);

begin

  -- Instantiate the Unit Under Test (UUT)

  cpu: entity work.light8080

  port map (

    clk => clk,

    reset => reset,

    vma => vma_o,

    rd => rd_o,

    wr => wr_o,

    io => io_o,

    fetch => fetch_o,

    addr_out => addr_o,

    data_in => data_i,

    data_out => data_o,

    intr => intr_i,

    inte => inte_o,

    inta => inta_o,

    halt => halt_o

  );

-- clock: run clock until test is done

clock:

process(done, clk)

begin

  if done = '0' then

    clk <= not clk after T/2;

  end if;

end process clock;

-- Drive reset and done 

main_test:

process

begin

  -- Load object code on memory -- note this comsumes no simulated time

  --load_object_code(obj_code, ram);

  -- Assert reset for at least one full clk period

  reset <= '1';

  wait until clk = '1';

  wait for T/2;

  reset <= '0';

  -- Remember to 'cut away' the preceding 3 clk semiperiods from 

  -- the wait statement...

  wait for (MAX_SIM_LENGTH - T*1.5);

  -- Maximum sim time elapsed, assume the program ran away and

  -- stop the clk process asserting 'done' (which will stop the simulation)

  done <= '1';

  assert (done = '1')

  report "Test timed out."

  severity failure;

  wait;

end process main_test;

-- Synchronous RAM covering the whole address map

synchronous_ram:

process(clk)

begin

  if (clk'event and clk='1') then

    data_mem <= ram(conv_integer(addr_o));

    if wr_o = '1' then

      ram(conv_integer(addr_o)) := data_o;

    end if;

  end if;

end process synchronous_ram;

irq_trigger_register:

process(clk)

begin

  if (clk'event and clk='1') then

    if reset='1' then

      cycles_to_intr <= -10; -- meaning no interrupt pending

    else

      if io_o='1' and wr_o='1' and addr_o(7 downto 0)=X"11" then

        cycles_to_intr <= conv_integer(data_o) + 1;

      else

        if cycles_to_intr >= 0 then

          cycles_to_intr <= cycles_to_intr - 1;

        end if;

      end if;

    end if;

  end if;

end process irq_trigger_register;

irq_pulse_width_register:

process(clk)

variable intr_pulse_countdown : integer;

begin

  if (clk'event and clk='1') then

    if reset='1' then

      intr_width <= 1;

      intr_pulse_countdown := 0;

      intr_i <= '0';

    else

      if io_o='1' and wr_o='1' and addr_o(7 downto 0)=X"12" then

        intr_width <= conv_integer(data_o) + 1;

      end if;

      if cycles_to_intr = 0 then

        intr_i <= '1';

        intr_pulse_countdown := intr_width;

      elsif intr_pulse_countdown <= 1 then

        intr_i <= '0';

      else

        intr_pulse_countdown := intr_pulse_countdown - 1;

      end if;

    end if;

  end if;

end process irq_pulse_width_register;

irq_source_register:

process(clk)

begin

  if (clk'event and clk='1') then

    if reset='1' then

      irq_source <= 0;

    else

      if io_o='1' and wr_o='1' and addr_o(7 downto 0)=X"10" then

        irq_source <= conv_integer(data_o(2 downto 0));

      end if;

    end if;

  end if;

end process irq_source_register;

-- 'interrupt vector' logic.

irq_vector_table:

process(clk)

begin

  if (clk'event and clk='1') then

    if vma_o = '1' and rd_o='1' then

      if inta_o = '1' then

        int_vector_index <= int_vector_index + 1;

      else

        int_vector_index <= 0;

      end if;

    end if;

    -- this is the address of the byte we'll feed to the CPU

    addr_vector_table <= 64+irq_source*4+int_vector_index;

  end if;

end process irq_vector_table;

irq_vector_byte <= ram(addr_vector_table);

data_i <= data_mem when inta_o='0' else irq_vector_byte;

test_outcome_register:

process(clk)

variable outcome : std_logic_vector(7 downto 0);

begin

  if (clk'event and clk='1') then

    if io_o='1' and wr_o='1' and addr_o(7 downto 0)=X"20" then

    assert (data_o /= X"55") report "Software reports SUCCESS" severity failure;

    assert (data_o /= X"aa") report "Software reports FAILURE" severity failure;

    assert ((data_o = X"aa") or (data_o = X"55"))

    report "Software reports unexpected outcome value."

    severity failure;

    end if;

  end if;

end process test_outcome_register;

dummy_uart_output_reg:

process(clk)

variable outcome : std_logic_vector(7 downto 0);

begin

  if (clk'event and clk='1') then

    if io_o='1' and wr_o='1' and addr_o(7 downto 0)=X"21" then

      -- append char to output string

      if con_line_ix < con_line_buf'high then

        con_line_buf(con_line_ix) <= character'val(conv_integer(data_o));

        con_line_ix <= con_line_ix + 1;

      end if;

    end if;

  end if;

end process dummy_uart_output_reg;

end;