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-- The Potato Processor - A simple processor for FPGAs

-- (c) Kristian Klomsten Skordal 2014 - 2015 <kristian.skordal@wafflemail.net>

-- Report bugs and issues on <http://opencores.org/project,potato,bugtracker>

library ieee;

use ieee.std_logic_1164.all;

use work.pp_constants.all;

use work.pp_csr.all;

use work.pp_types.all;

use work.pp_utilities.all;

--! @brief Unit decoding instructions and setting control signals apropriately.

entity pp_control_unit is

        port(

                -- Inputs, indices correspond to instruction word indices:

                opcode  : in std_logic_vector( 4 downto 0);

                funct3  : in std_logic_vector( 2 downto 0);

                funct7  : in std_logic_vector( 6 downto 0);

                funct12 : in std_logic_vectoR(11 downto 0);

                -- Control signals:

                rd_write            : out std_logic;

                branch              : out branch_type;

                -- Exception signals:

                decode_exception       : out std_logic;

                decode_exception_cause : out csr_exception_cause;

                -- Control register signals:

                csr_write : out csr_write_mode;

                csr_imm   : out std_logic; --! Indicating an immediate variant of the csrr* instructions.

                -- Sources of operands to the ALU:

                alu_x_src, alu_y_src : out alu_operand_source;

                -- ALU operation:

                alu_op : out alu_operation;

                -- Memory transaction parameters:

                mem_op   : out memory_operation_type;

                mem_size : out memory_operation_size

        );

end entity pp_control_unit;

architecture behaviour of pp_control_unit is

        signal exception       : std_logic;

        signal exception_cause : csr_exception_cause;

        signal alu_op_temp     : alu_operation;

begin

        csr_imm <= funct3(2);

        alu_op <= alu_op_temp;

        decode_exception <= exception or to_std_logic(alu_op_temp = ALU_INVALID);

        decode_exception_cause <= exception_cause when alu_op_temp /= ALU_INVALID

                else CSR_CAUSE_INVALID_INSTR;

        alu_control: entity work.pp_alu_control_unit

                port map(

                        opcode => opcode,

                        funct3 => funct3,

                        funct7 => funct7,

                        alu_x_src => alu_x_src,

                        alu_y_src => alu_y_src,

                        alu_op => alu_op_temp

                );

        decode_ctrl: process(opcode, funct3, funct12)

        begin

                case opcode is

                        when b"01101" => -- Load upper immediate

                                rd_write <= '1';

                                exception <= '0';

                                exception_cause <= CSR_CAUSE_NONE;

                                branch <= BRANCH_NONE;

                        when b"00101" => -- Add upper immediate to PC

                                rd_write <= '1';

                                exception <= '0';

                                exception_cause <= CSR_CAUSE_NONE;

                                branch <= BRANCH_NONE;

                        when b"11011" => -- Jump and link

                                rd_write <= '1';

                                exception <= '0';

                                exception_cause <= CSR_CAUSE_NONE;

                                branch <= BRANCH_JUMP;

                        when b"11001" => -- Jump and link register

                                rd_write <= '1';

                                exception <= '0';

                                exception_cause <= CSR_CAUSE_NONE;

                                branch <= BRANCH_JUMP_INDIRECT;

                        when b"11000" => -- Branch operations

                                rd_write <= '0';

                                exception <= '0';

                                exception_cause <= CSR_CAUSE_NONE;

                                branch <= BRANCH_CONDITIONAL;

                        when b"00000" => -- Load instructions

                                rd_write <= '1';

                                exception <= '0';

                                exception_cause <= CSR_CAUSE_NONE;

                                branch <= BRANCH_NONE;

                        when b"01000" => -- Store instructions

                                rd_write <= '0';

                                exception <= '0';

                                exception_cause <= CSR_CAUSE_NONE;

                                branch <= BRANCH_NONE;

                        when b"00100" => -- Register-immediate operations

                                rd_write <= '1';

                                exception <= '0';

                                exception_cause <= CSR_CAUSE_NONE;

                                branch <= BRANCH_NONE;

                        when b"01100" => -- Register-register operations

                                rd_write <= '1';

                                exception <= '0';

                                exception_cause <= CSR_CAUSE_NONE;

                                branch <= BRANCH_NONE;

                        when b"00011" => -- Fence instructions, ignored

                                rd_write <= '0';

                                exception <= '0';

                                exception_cause <= CSR_CAUSE_NONE;

                                branch <= BRANCH_NONE;

                        when b"11100" => -- System instructions

                                if funct3 = b"000" then

                                        rd_write <= '0';

                                        if funct12 = x"000" then

                                                exception <= '1';

                                                exception_cause <= CSR_CAUSE_ECALL;

                                                branch <= BRANCH_NONE;

                                        elsif funct12 = x"001" then

                                                exception <= '1';

                                                exception_cause <= CSR_CAUSE_BREAKPOINT;

                                                branch <= BRANCH_NONE;

                                        elsif funct12 = CSR_EPC_ERET then

                                                exception <= '0';

                                                exception_cause <= CSR_CAUSE_NONE;

                                                branch <= BRANCH_SRET;

                                        else

                                                exception <= '1';

                                                exception_cause <= CSR_CAUSE_INVALID_INSTR;

                                                branch <= BRANCH_NONE;

                                        end if;

                                else

                                        rd_write <= '1';

                                        exception <= '0';

                                        exception_cause <= CSR_CAUSE_NONE;

                                        branch <= BRANCH_NONE;

                                end if;

                        when others =>

                                rd_write <= '0';

                                exception <= '1';

                                exception_cause <= CSR_CAUSE_INVALID_INSTR;

                                branch <= BRANCH_NONE;

                end case;

        end process decode_ctrl;

        decode_csr: process(opcode, funct3)

        begin

                if opcode = b"11100" then

                        case funct3 is

                                when b"001" | b"101" => -- csrrw/i

                                        csr_write <= CSR_WRITE_REPLACE;

                                when b"010" | b"110" => -- csrrs/i

                                        csr_write <= CSR_WRITE_SET;

                                when b"011" | b"111" => -- csrrc/i

                                        csr_write <= CSR_WRITE_CLEAR;

                                when others =>

                                        csr_write <= CSR_WRITE_NONE;

                        end case;

                else

                        csr_write <= CSR_WRITE_NONE;

                end if;

        end process decode_csr;

        decode_mem: process(opcode, funct3)

        begin

                case opcode is

                        when b"00000" => -- Load instructions

                                case funct3 is

                                        when b"000" => -- lw

                                                mem_size <= MEMOP_SIZE_BYTE;

                                                mem_op <= MEMOP_TYPE_LOAD;

                                        when b"001" => -- lh

                                                mem_size <= MEMOP_SIZE_HALFWORD;

                                                mem_op <= MEMOP_TYPE_LOAD;

                                        when b"010" | b"110" => -- lw

                                                mem_size <= MEMOP_SIZE_WORD;

                                                mem_op <= MEMOP_TYPE_LOAD;

                                        when b"100" => -- lbu

                                                mem_size <= MEMOP_SIZE_BYTE;

                                                mem_op <= MEMOP_TYPE_LOAD_UNSIGNED;

                                        when b"101" => -- lhu

                                                mem_size <= MEMOP_SIZE_HALFWORD;

                                                mem_op <= MEMOP_TYPE_LOAD_UNSIGNED;

                                        when others => -- FIXME: Treat others as lw.

                                                mem_size <= MEMOP_SIZE_WORD;

                                                mem_op <= MEMOP_TYPE_INVALID;

                                end case;

                        when b"01000" => -- Store instructions

                                case funct3 is

                                        when b"000" =>

                                                mem_op <= MEMOP_TYPE_STORE;

                                                mem_size <= MEMOP_SIZE_BYTE;

                                        when b"001" =>

                                                mem_op <= MEMOP_TYPE_STORE;

                                                mem_size <= MEMOP_SIZE_HALFWORD;

                                        when b"010" =>

                                                mem_op <= MEMOP_TYPE_STORE;

                                                mem_size <= MEMOP_SIZE_WORD;

                                        when others =>

                                                mem_op <= MEMOP_TYPE_INVALID;

                                                mem_size <= MEMOP_SIZE_WORD;

                                end case;

                        when others =>

                                mem_op <= MEMOP_TYPE_NONE;

                                mem_size <= MEMOP_SIZE_WORD;

                end case;

        end process decode_mem;

end architecture behaviour;