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library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;
use work.alu_types.all;
use work.decoder.all;
entity state_machine is
port(
clk : in std_logic;
opcode : in std_logic_vector(13 downto 0);
reset : in std_logic;
op : out alu_op;
we_mem : out std_logic := '0';
re_mem : out std_logic := '0';
we_w : out std_logic := '0';
instr_ret : out std_logic := '0';
bit_select : out std_logic_vector(2 downto 0) := (others => '0');
data : out std_logic_vector(7 downto 0) := (others => '0');
pc : out std_logic_vector(12 downto 0);
addr : out std_logic_vector(6 downto 0) := (others => '0');
use_literal : out std_logic;
we_status : out std_logic := '0';
stack_push : out std_logic := '0';
stack_pop : out std_logic := '0';
stack_din : out std_logic_vector(12 downto 0) := (others => '0');
stack_dout : in std_logic_vector(12 downto 0);
alu_skip : in std_logic;
alu_result : in std_logic_vector(7 downto 0)
);
end entity state_machine;
architecture rtl of state_machine is
type state_t is (IFetch, MRead, Execute, MWrite);
signal pc_internal : std_logic_vector(12 downto 0) := (others => '0');
signal state : state_t := IFetch;
signal pclath : std_logic_vector(4 downto 0) := (others => '0');
begin
pc <= pc_internal;
fsm : process(clk)
variable instr : instruction_t;
-- do we do a phantom nop after this instruction?
variable do_phantom_nop : boolean := false;
variable is_phantom : boolean := false;
-- true when phantom came from DECFSZ/INCFSZ skip (must still increment PC)
-- false when phantom came from GOTO/CALL/RETURN (PC already at target)
variable is_skip_phantom : boolean := false;
begin
if rising_edge(clk) then
re_mem <= '0';
we_mem <= '0';
we_w <= '0';
we_status <= '0';
instr_ret <= '0';
stack_push <= '0';
stack_pop <= '0';
if reset = '1' then
pc_internal <= (others => '0');
pclath <= (others => '0');
state <= IFetch;
do_phantom_nop := false;
else
case state is
when IFetch =>
-- if we had a branch before, this cycle is a NOP.
if do_phantom_nop then
instr := instruction_decode((others => '0'));
do_phantom_nop := false;
is_phantom := true;
-- is_skip_phantom carries over from whoever set do_phantom_nop
else
instr := instruction_decode(opcode);
is_phantom := false;
is_skip_phantom := false;
end if;
-- set use_literal based on decoded instruction, holds through MWrite
if instr.class = LITERAL_OP then
use_literal <= '1';
else
use_literal <= '0';
end if;
-- for literal operations
data <= instr.k(7 downto 0);
-- for memory-read/write operations
addr <= instr.f;
-- for bit-select operations
bit_select <= instr.b;
state <= Mread;
when MRead =>
-- read memory addressed by the opcode
-- MOVWF, CLRF, CLRW, NOP are write-only.
if instr.class = BIT_OP or
(instr.class = BYTE_OP and
instr.op /= MOVWF and instr.op /= CLRF and
instr.op /= CLRW and instr.op /= NOP) then
re_mem <= '1';
end if;
state <= Execute;
when Execute =>
op <= instr.op; -- move the decoded instruction to the ALU
if not is_phantom then
-- increment PC or change address, different for different instructions.
if instr.op = GOTO then
pc_internal <= "00" & instr.k;
do_phantom_nop := true;
is_skip_phantom := false;
elsif instr.op = CALL then
-- push PC+1 to the stack
stack_din <= std_logic_vector(unsigned(pc_internal) + 1);
stack_push <= '1';
-- go to the function we called
pc_internal <= "00" & instr.k;
do_phantom_nop := true;
is_skip_phantom := false;
elsif instr.op = RETUR or instr.op = RETLW then
-- pop the PC from stack.
pc_internal <= stack_dout;
stack_pop <= '1'; -- pop deferred to next cycle.
do_phantom_nop := true;
is_skip_phantom := false;
else
pc_internal <= std_logic_vector(unsigned(pc_internal) + 1);
end if;
elsif is_skip_phantom then
-- skip phantom: PC was at the skipped instruction, step past it
pc_internal <= std_logic_vector(unsigned(pc_internal) + 1);
end if;
-- branch phantom: PC already at target, no change needed
state <= MWrite;
when MWrite =>
-- write to correct memory based on class (skip for phantom NOP)
if not is_phantom then
case instr.class is
when BYTE_OP =>
if instr.op /= NOP then
if instr.d = '1' then
we_mem <= '1';
-- Write to PCLATH (addr 0x0A): latch upper PC bits
if instr.f = "0001010" then
pclath <= alu_result(4 downto 0);
end if;
-- Write to PCL (addr 0x02): redirect PC, like a GOTO
if instr.f = "0000010" then
pc_internal <= pclath & alu_result;
do_phantom_nop := true;
is_skip_phantom := false;
end if;
else
we_w <= '1';
end if;
end if;
when BIT_OP =>
-- NOT for the BTFSS or BTFSC instructions.
if instr.op = BCF or instr.op = BSF then
we_mem <= '1';
end if;
when LITERAL_OP =>
if instr.op /= NOP and instr.op /= RETUR then
we_w <= '1';
end if;
when others => null;
end case;
end if;
-- DECFSZ/INCFSZ/BTFSS/BTFSC: skip next instruction if ALU signals skip
if not is_phantom and
(instr.op = DECFSZ or instr.op = INCFSZ or instr.op = BTFSS or instr.op = BTFSC) and alu_skip = '1' then
do_phantom_nop := true;
is_skip_phantom := true;
end if;
if not is_phantom then
we_status <= '1';
-- branch instructions complete over 2 cycles; pulse instr_ret only
-- on the phantom NOP cycle (below), not here
if instr.op /= GOTO and instr.op /= CALL and
instr.op /= RETUR and instr.op /= RETLW then
instr_ret <= '1';
end if;
elsif not is_skip_phantom then
-- branch phantom NOP: this is the true end of the 2-cycle instruction
instr_ret <= '1';
end if;
state <= IFetch;
end case;
end if; -- reset
end if;
end process fsm;
end architecture rtl;
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