处理器不同编址方式、指令/数据处理方式区别

    每个外设都是通过读写其寄存器来控制的。外设寄存器也称为I/O端口，通常包括：控制寄存器、状态寄存器和数据寄存器三大类。根据访问外设寄存器的不同方式，可以把CPU分成两大类。一类CPU（如ARM，MIPS，M68K，Power PC等）把这些寄存器看作内存的一部分，寄存器参与内存统一编址，访问寄存器就通过访问一般的内存指令进行，所以，这种CPU没有专门用于设备I/O的指令。这就是所谓的“I/O内存”方式。另一类CPU（典型的如X86），将外设的寄存器看成一个独立的地址空间，所以访问内存的指令不能用来访问这些寄存器，而要为对外设寄存器的读／写设置专用指令，如IN和OUT指令。这就是所谓的“ I/O端口”方式。

    下面以X86的一种实现为例，该项目链接为http://zet.aluzina.org/index.php/Zet_processor。这个CPU的接口如下：

`include "defines.v"

module zet_core (
    input clk,
    input rst,

    // interrupts
    input  intr,
    output inta,
    input  nmi,
    output nmia,

    // interface
    output [19:0] cpu_adr_o,
    input  [15:0] iid_dat_i,
    input  [15:0] cpu_dat_i,
    output [15:0] cpu_dat_o,
    output        cpu_byte_o,
    input         cpu_block,
    output        cpu_mem_op,
    output        cpu_m_io,
    output        cpu_we_o,

    output [19:0] pc  // for debugging purposes
  );

    从上面总线接口的定义中我们可以看到，该CPU具有独立的IO总线，总线信号如 data_in，data_out，adr_out，we等都是共用的，也就是MEM总线与IO总线复用这些信号，通过cpu_m_io来区分当前的总线操作是IO还是MEM，实际上是分时复用的。当然也可以完全复用，就是各自有独立的总线信号，这个项目实现了兼容传统8086的架构，所以是复用的。

module zet_exec (
    input         clk,
    input         rst,

    input [`IR_SIZE-1:0] ir,
    input [15:0]  off,
    input [15:0]  imm,
    output [15:0] cs,
    output [15:0] ip,
    output        of,
    output        zf,
    output        cx_zero,
    input [15:0]  memout,

    output [15:0] wr_data,
    output [19:0] addr,
    output        we,
    output        m_io,
    output        byteop,
    input         block,
    output        div_exc,
    input         wrip0,

    output        ifl,
    output        tfl,
    output        wr_ss
  );

  // Net declarations
  wire [15:0] c;
  wire [15:0] omemalu;
  wire [ 3:0] addr_a;
  wire [ 3:0] addr_c;
  wire [ 3:0] addr_d;
  wire [ 8:0] flags;
  wire [15:0] a, b, s, alu_iflags, bus_b;
  wire [31:0] aluout;
  wire [3:0]  addr_b;
  wire [2:0]  t, func;
  wire [1:0]  addr_s;
  wire        wrfl, high, memalu, r_byte, c_byte;
  wire        wr, wr_reg;
  wire        wr_cnd;
  wire        jmp;
  wire        b_imm;
  wire  [8:0] iflags, oflags;
  wire  [4:0] logic_flags;
  wire        alu_word;
  wire        a_byte;
  wire        b_byte;
  wire        wr_high;
  wire        dive;

  // Module instances
  zet_alu alu( {c, a }, bus_b, aluout, t, func, alu_iflags, oflags,
               alu_word, s, off, clk, dive);
  zet_regfile regfile (
        a, b, c, cs, ip, {aluout[31:16], omemalu}, s, flags, wr_reg, wrfl,
                wr_high, clk, rst, addr_a, addr_b, addr_c, addr_d, addr_s, iflags,
                ~byteop, a_byte, b_byte, c_byte, cx_zero, wrip0);
  zet_jmp_cond jmp_cond (logic_flags, addr_b, addr_c[0], c, jmp);

  // Assignments
  assign addr_s = ir[1:0];
  assign addr_a = ir[5:2];
  assign addr_b = ir[9:6];
  assign addr_c = ir[13:10];
  assign addr_d = ir[17:14];
  assign wrfl   = ir[18];
  assign we     = ir[19];
  assign wr     = ir[20];
  assign wr_cnd = ir[21];
  assign high   = ir[22];
  assign t      = ir[25:23];
  assign func   = ir[28:26];
  assign byteop = ir[29];
  assign memalu = ir[30];
  assign m_io   = ir[32];
  assign b_imm  = ir[33];
  assign r_byte = ir[34];
  assign c_byte = ir[35];

  assign omemalu = memalu ? aluout[15:0] : memout;
  assign bus_b   = b_imm ? imm : b;

  assign addr = aluout[19:0];
  assign wr_data = c;
  assign wr_reg  = (wr | (jmp & wr_cnd)) && !block && !div_exc;
  assign wr_high = high && !block && !div_exc;
  assign of  = flags[8];
  assign ifl = flags[6];
  assign tfl = flags[5];
  assign zf  = flags[3];

  assign iflags = oflags;
  assign alu_iflags = { 4'b1111, flags[8:3], 1'b0, flags[2], 1'b0, flags[1],
                        1'b1, flags[0] };
  assign logic_flags = { flags[8], flags[4], flags[3], flags[1], flags[0] };

  assign alu_word = (t==3'b011) ? ~r_byte : ~byteop;
  assign a_byte = (t==3'b011 && func[1]) ? 1'b0 : r_byte;
  assign b_byte = r_byte;
  assign div_exc = dive && wr;

  assign wr_ss = (addr_d == 4'b1010) && wr;

endmodule

    这部分代码可以看出，addr的计算来源只有一个，也就是他并不区分指令地址和数据地址，因此是冯诺依曼结构的处理器。

    下面以OpenRISC无MMU和浮点运算单元的精简版本AltOR32处理器来说明统一编址的CPU总线接口形式。

    （1）冯诺依曼结构，读取数据的接口和读取指令的接口在一起的，代码如下：

module altor32_lite
(
    // General
    input               clk_i /*verilator public*/,
    input               rst_i /*verilator public*/,

    // Maskable interrupt    
    input               intr_i /*verilator public*/,

    // Unmaskable interrupt
    input               nmi_i /*verilator public*/,
    
    // Memory interface
    output reg [31:0]   mem_addr_o /*verilator public*/,
    input [31:0]        mem_dat_i /*verilator public*/,
    output reg [31:0]   mem_dat_o /*verilator public*/,
    output reg          mem_cyc_o /*verilator public*/,
    output reg          mem_stb_o /*verilator public*/,
    output reg          mem_we_o /*verilator public*/,
    output reg [3:0]    mem_sel_o /*verilator public*/,
    input               mem_ack_i/*verilator public*/ 
);
.
.
.
.
//-----------------------------------------------------------------
// Next State Logic
//-----------------------------------------------------------------
reg [3:0] next_state_r;
always @ *
begin
    next_state_r = state_q;

    case (state_q)    
    //-----------------------------------------
    // IDLE - 
    //-----------------------------------------
    STATE_IDLE :
    begin
        if (enable_i)
            next_state_r    = STATE_FETCH;
    end    
    //-----------------------------------------
    // FETCH - Fetch line from memory
    //-----------------------------------------
    STATE_FETCH :
    begin
        next_state_r    = STATE_FETCH_WAIT;
    end
    //-----------------------------------------
    // FETCH_WAIT - Wait for read responses
    //-----------------------------------------
    STATE_FETCH_WAIT:
    begin
        // Read from memory complete
        if (mem_ack_i)
            next_state_r = STATE_EXEC;
    end  
    //-----------------------------------------
    // EXEC
    //-----------------------------------------
    STATE_EXEC :
    begin
        if (load_inst_r || store_inst_r)
            next_state_r    = STATE_MEM;
        else
            next_state_r    = STATE_WRITE_BACK;
    end
    //-----------------------------------------
    // MEM
    //-----------------------------------------
    STATE_MEM :
    begin
        // Read from memory complete
        if (mem_ack_i)
            next_state_r = STATE_FETCH;
    end    
    //-----------------------------------------
    // WRITE_BACK
    //-----------------------------------------
    STATE_WRITE_BACK :
    begin
        if (enable_i)
            next_state_r    = STATE_FETCH;
    end    
    default:
        ;
   endcase
end
.
.
.
.
//-----------------------------------------------------------------
// Memory Access / Instruction Fetch
//-----------------------------------------------------------------
always @ (posedge rst_i or posedge clk_i )
begin
   if (rst_i == 1'b1)
   begin
        mem_addr_o      <= 32'h00000000;        
        mem_dat_o       <= 32'h00000000;
        mem_sel_o       <= 4'b0;
        mem_we_o        <= 1'b0;
        mem_stb_o       <= 1'b0;
        mem_cyc_o       <= 1'b0;

        opcode_q        <= 32'h00000000;
        mem_offset_q    <= 2'b0;
   end
   else
   begin
        case (state_q)

            //-----------------------------------------
            // FETCH - Issue instruction fetch
            //-----------------------------------------
            STATE_FETCH :
            begin
                // Start fetch from memory
                mem_addr_o  <= pc_q;
                mem_stb_o   <= 1'b1;
                mem_we_o    <= 1'b0;
                mem_cyc_o   <= 1'b1;
            end
            //-----------------------------------------
            // FETCH_WAIT - Wait for response
            //-----------------------------------------
            STATE_FETCH_WAIT :
            begin
                // Data ready from memory?
                if (mem_ack_i)
                begin
                    opcode_q    <= mem_dat_i;
                    mem_cyc_o   <= 1'b0;                            
                end
            end
            //-----------------------------------------
            // EXEC - Issue read / write
            //-----------------------------------------            
            STATE_EXEC :
            begin
            `ifdef CONF_CORE_TRACE
                $display("%08x: Execute 0x%08x", pc_q, opcode_q);
                $display(" rA[%d] = 0x%08x", ra_w, reg_ra_r);
                $display(" rB[%d] = 0x%08x", rb_w, reg_rb_r);
            `endif

                case (1'b1)
                 // l.lbs l.lhs l.lws l.lbz l.lhz l.lwz
                 load_inst_r:
                 begin
                     mem_addr_o      <= {mem_addr_r[31:2], 2'b0};
                     mem_offset_q    <= mem_addr_r[1:0];
                     mem_dat_o       <= 32'h00000000;
                     mem_sel_o       <= 4'b1111;
                     mem_we_o        <= 1'b0;
                     mem_stb_o       <= 1'b1;
                     mem_cyc_o       <= 1'b1;   

      `ifdef CONF_CORE_DEBUG
                     $display(" Load from 0x%08x to R%d", mem_addr_r, rd_w);
      `endif
                 end

                 inst_sb_w: // l.sb
                 begin
                     mem_addr_o      <= {mem_addr_r[31:2], 2'b0};
                     mem_offset_q    <= mem_addr_r[1:0];
                     case (mem_addr_r[1:0])
                         2'b00 :
                         begin
                             mem_dat_o       <= {reg_rb_r[7:0],24'h000000};
                             mem_sel_o       <= 4'b1000;
                             mem_we_o        <= 1'b1;
                             mem_stb_o       <= 1'b1;
                             mem_cyc_o       <= 1'b1;
                         end
                         2'b01 :
                         begin
                             mem_dat_o       <= {{8'h00,reg_rb_r[7:0]},16'h0000};
                             mem_sel_o       <= 4'b0100;
                             mem_we_o        <= 1'b1;
                             mem_stb_o       <= 1'b1;
                             mem_cyc_o       <= 1'b1;
                         end
                         2'b10 :
                         begin
                             mem_dat_o       <= {{16'h0000,reg_rb_r[7:0]},8'h00};
                             mem_sel_o       <= 4'b0010;
                             mem_we_o        <= 1'b1;
                             mem_stb_o       <= 1'b1;
                             mem_cyc_o       <= 1'b1;
                         end
                         2'b11 :
                         begin
                             mem_dat_o       <= {24'h000000,reg_rb_r[7:0]};
                             mem_sel_o       <= 4'b0001;
                             mem_we_o        <= 1'b1;
                             mem_stb_o       <= 1'b1;
                             mem_cyc_o       <= 1'b1;
                         end
                         default :
                            ;
                     endcase
                 end

                inst_sh_w: // l.sh
                begin
                     mem_addr_o      <= {mem_addr_r[31:2], 2'b0};
                     mem_offset_q    <= mem_addr_r[1:0];
                     case (mem_addr_r[1:0])
                         2'b00 :
                         begin
                             mem_dat_o       <= {reg_rb_r[15:0],16'h0000};
                             mem_sel_o       <= 4'b1100;
                             mem_we_o        <= 1'b1;
                             mem_stb_o       <= 1'b1;
                             mem_cyc_o       <= 1'b1;
                         end
                         2'b10 :
                         begin
                             mem_dat_o       <= {16'h0000,reg_rb_r[15:0]};
                             mem_sel_o       <= 4'b0011;
                             mem_we_o        <= 1'b1;
                             mem_stb_o       <= 1'b1;
                             mem_cyc_o       <= 1'b1;
                         end
                         default :
                            ;
                     endcase
                end

                inst_sw_w: // l.sw
                begin
                     mem_addr_o      <= {mem_addr_r[31:2], 2'b0};
                     mem_offset_q    <= mem_addr_r[1:0];
                     mem_dat_o       <= reg_rb_r;
                     mem_sel_o       <= 4'b1111;
                     mem_we_o        <= 1'b1;
                     mem_stb_o       <= 1'b1;
                     mem_cyc_o       <= 1'b1;

      `ifdef CONF_CORE_DEBUG
                     $display(" Store R%d to 0x%08x = 0x%08x", rb_w, {mem_addr_r[31:2],2'b00}, reg_rb_r);
      `endif
                end
                default:
                    ;
             endcase
            end
            //-----------------------------------------
            // MEM - Wait for response
            //-----------------------------------------
            STATE_MEM :
            begin
                // Data ready from memory?
                if (mem_ack_i)
                begin
                    mem_cyc_o   <= 1'b0;
                end
            end            
            default:
                ;
           endcase
   end
end

    该实现是一个典型的5级流水线处理器，下方的部分是访存部分的状态机逻辑实现，这部分的代码描述了 Memory Access / Instruction Fetch的操作，是通过唯一的总线端口进行的。

    （2）哈佛结构，读取数据的接口和读取指令的接口分开，代码如下：

//-----------------------------------------------------------------
// Module - AltOR32 CPU (Pipelined Wishbone Interfaces)
//-----------------------------------------------------------------
module altor32
(
    // General
    input               clk_i,
    input               rst_i,

    input               intr_i,
    input               nmi_i,
    output              fault_o,
    output              break_o,

    // Instruction memory
    output [31:0]       imem_addr_o,
    input [31:0]        imem_dat_i,
    output [2:0]        imem_cti_o,
    output              imem_cyc_o,
    output              imem_stb_o,
    input               imem_ack_i,  

    // Data memory
    output [31:0]       dmem_addr_o,
    output [31:0]       dmem_dat_o,
    input [31:0]        dmem_dat_i,
    output [3:0]        dmem_sel_o,
    output [2:0]        dmem_cti_o,
    output              dmem_cyc_o,
    output              dmem_we_o,
    output              dmem_stb_o,
    input               dmem_ack_i
);
.
.
.
//-----------------------------------------------------------------
// Module - Wishbone fetch unit
//-----------------------------------------------------------------
always @ (posedge rst_i or posedge clk_i )
begin
   if (rst_i == 1'b1)
   begin
        wbm_addr_o      <= 32'h00000000;
        wbm_cti_o       <= 3'b0;
        wbm_stb_o       <= 1'b0;
        wbm_cyc_o       <= 1'b0;

        fetch_word_q    <= {FETCH_WORDS_W{1'b0}};
        resp_word_q     <= {FETCH_WORDS_W{1'b0}};
   end
   else
   begin
        // Idle
        if (!wbm_cyc_o)
        begin
            if (fetch_i)
            begin
                if (burst_i)
                begin
                    wbm_addr_o      <= {address_i[31:FETCH_BYTES_W], {FETCH_BYTES_W{1'b0}}};
                    fetch_word_q    <= {FETCH_WORDS_W{1'b0}};
                    resp_word_q     <= {FETCH_WORDS_W{1'b0}};
                    
                    // Incrementing linear burst
                    wbm_cti_o       <= WB_CTI_BURST;
                end
                else
                begin                    
                    wbm_addr_o      <= address_i;
                    resp_word_q     <= address_i[FETCH_BYTES_W-1:2];

                    // Single fetch
                    wbm_cti_o       <= WB_CTI_FINAL;                    
                end

                // Start fetch from memory
                wbm_stb_o           <= 1'b1;
                wbm_cyc_o           <= 1'b1;                        
            end
        end
        // Access in-progress
        else
        begin
            // Command accepted
            if (~wbm_stall_i)
            begin
                // Fetch next word for line
                if (wbm_cti_o != WB_CTI_FINAL)
                begin
                    wbm_addr_o      <= {wbm_addr_o[31:FETCH_BYTES_W], next_word_w, 2'b0};
                    fetch_word_q    <= next_word_w;
                    
                    // Final word to read?
                    if (penultimate_word_w)
                        wbm_cti_o   <= WB_CTI_FINAL;
                end
                // Fetch complete
                else
                    wbm_stb_o       <= 1'b0;
            end

            // Response
            if (wbm_ack_i)
                resp_word_q         <= resp_word_q + 1'b1;

            // Last response?
            if (final_o)
                wbm_cyc_o           <= 1'b0;
        end
   end
end
.
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//-----------------------------------------------------------------
// Module - Data Cache Memory Interface
//-----------------------------------------------------------------
always @ *
begin
    next_state_r = state;

    case (state)
    //-----------------------------------------
    // IDLE
    //-----------------------------------------
    STATE_IDLE :
    begin
        // Perform cache evict (write)     
        if (evict_i)
            next_state_r    = STATE_WRITE_SETUP;
        // Perform cache fill (read)
        else if (fill_i)
            next_state_r    = STATE_FETCH;
        // Read/Write single
        else if (rd_single_i | (|wr_single_i))
            next_state_r    = STATE_MEM_SINGLE;
    end
    //-----------------------------------------
    // FETCH - Fetch line from memory
    //-----------------------------------------
    STATE_FETCH :
    begin
        // Line fetch complete?
        if (~mem_stall_i && request_idx == {CACHE_LINE_WORDS_IDX_MAX{1'b1}})
            next_state_r    = STATE_FETCH_WAIT;
    end
    //-----------------------------------------
    // FETCH_WAIT - Wait for read responses
    //-----------------------------------------
    STATE_FETCH_WAIT:
    begin
        // Read from memory complete
        if (mem_ack_i && response_idx == {CACHE_LINE_WORDS_IDX_MAX{1'b1}})
            next_state_r = STATE_IDLE;
    end  
    //-----------------------------------------
    // WRITE_SETUP - Wait for data from cache
    //-----------------------------------------
    STATE_WRITE_SETUP :
        next_state_r    = STATE_WRITE;
    //-----------------------------------------
    // WRITE - Write word to memory
    //-----------------------------------------
    STATE_WRITE :
    begin
        // Line write complete?
        if (~mem_stall_i && request_idx == {CACHE_LINE_WORDS_IDX_MAX{1'b1}})
            next_state_r = STATE_WRITE_WAIT;
        // Fetch next word for line
        else if (~mem_stall_i | ~mem_stb_o)
            next_state_r = STATE_WRITE_SETUP;
    end
    //-----------------------------------------
    // WRITE_WAIT - Wait for write to complete
    //-----------------------------------------
    STATE_WRITE_WAIT:
    begin
        // Write to memory complete
        if (mem_ack_i && response_idx == {CACHE_LINE_WORDS_IDX_MAX{1'b1}})
            next_state_r = STATE_IDLE;
    end            
    //-----------------------------------------
    // MEM_SINGLE - Single access to memory
    //-----------------------------------------
    STATE_MEM_SINGLE:
    begin
        // Data ready from memory?
        if (mem_ack_i)
            next_state_r  = STATE_IDLE;
    end  
    default:
        ;
   endcase
end

    上面摘录了不同阶段的代码，首先是CPU总线接口，有专门的指令总线和数据总线，下面是指令读取的状态机逻辑中的操作，最后部分是数据读写的逻辑操作。