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`timescale 1ns / 1ps

////////////////////////////////////////////////////////////////////////////////

// Company:                                                                   //  

// Engineer:       Scott Moore                                                //

//                                                                            //

// Create Date:    11:45:32 09/04/2006                                        // 

// Design Name:                                                               // 

// Module Name:    cpu8080                                                    //

// Project Name:   cpu8080                                                    //

// Target Devices: xc3c200, xc3s1000                                          //

// Tool versions:                                                             //

// Description:                                                               //

//                                                                            //

//     Executes the 8080 instruction set. It is designed to be an internal    //

//     cell. Each of the I/Os are positive logic, and all signals are         //

//     constant with the exception of the data bus. The control signals are   //

//     fully decoded (unlike the orignal 8080), and features read and write   //

//     signals for both memory and I/O space. The I/O space is an 8 bit       //

//     address as in the original 8080. It does NOT echo the lower 8 bits to  //

//     the higher 8 bits, as was the practice in some systems.                //

//                                                                            //

//     Like the original 8080, the interrupt vectoring is fully external. The //

//     the external controller forces a full instruction onto the data bus.   //

//     The sequence begins with the assertion of interrupt request. The CPU   //

//     will then assert interrupt acknowledge, then it will run a special     //

//     read cycle with inta asserted for each cycle of a possibly             //

//     multibyte instruction. This matches the original 8080, which typically //

//     used single byte restart instructions to form a simple interrupt       //

//     controller, but was capable of full vectoring via insertion of a jump, //

//     call or similar instruction.                                           //

//     Note that the interrupt vector instruction should branch. This is      //

//     because the PC gets changed by the vector instruction, so if it does   //

//     not branch, it will have skipped a number of bytes after the interrupt //

//     equivalent to the vector instruction. The only instructions that       //

//     should really be used to vector are jmp, rst and call instructions.    //

//     Specifically, rst and call instruction compensate for the pc movement  //

//     by putting the pc unmodified on the stack.                             //

//                                                                            //

//     The memory, I/O and interrupt fetches all obey a simple clocking       //

//     sequence as follows. The CPU uses the positive clock edge to assert    //

//     and sample signals and data. The external logic theoretically uses the //

//     negative edge to check signal assertions and sample data, but it can   //

//     either use the negative edge, or actually be asynronous logic.         //

//     The only caution here is that a read device that samples read signals  //

//     from the CPU on a postive edge will present data too late, since read  //

//     cycles are only one cycle long. This can be fixed with a wait state.   //

//     Negative external clockers may also have an issue with the fact that   //

//     the net read time is 1/2 cycle, the negative mid cycle clock to the    //

//     next positive clock. Again, and perhaps unfortunately, the fix is to   //

//     add an external wait state.                                            //

//                                                                            //

//     A standard read sequence is as follows:                                //

//                                                                            //

//     1. At the positive clock edge, readmem, readio or readint is asserted. //

//     2. At the negative clock edge (or immediately), the external memory    //

//        places data onto the data bus.                                      //

//     3. At the next positive clock edge, the data is sampled, and the read  //

//        Signal is deasserted.                                               //

//                                                                            //

//     A standard write sequence is as follows:                               //

//                                                                            //

//     1. At the positive edge, data is asserted on the data bus.             //

//     2. At the next postive clock edge, writemem or writeio is asserted.    //

//     3. At the next positive clock edge, writemem or writeio is deasserted. //

//     4. At the next positive edge, the data is deasserted.                  //

//                                                                            //

// Dependencies:                                                              //

//                                                                            //

// Revision:                                                                  //

// Revision 0.01 - File Created                                               //

// Additional Comments:                                                       //

//                                                                            //

////////////////////////////////////////////////////////////////////////////////

//

// CPU states

//

`define cpus_idle     5'h00 // Idle

`define cpus_fetchi   5'h01 // Instruction fetch

`define cpus_fetchi2  5'h02 // Instruction fetch 2

`define cpus_fetchi3  5'h03 // Instruction fetch 3

`define cpus_halt     5'h04 // Halt (wait for interrupt)

`define cpus_alucb    5'h05 // alu cycleback

`define cpus_indcb    5'h06 // inr/dcr cycleback

`define cpus_movmtbc  5'h07 // Move memory to bc

`define cpus_movmtde  5'h08 // Move memory to de

`define cpus_movmthl  5'h09 // Move memory to hl

`define cpus_movmtsp  5'h0a // Move memory to sp

`define cpus_lhld     5'h0b // LHLD

`define cpus_jmp      5'h0c // JMP

`define cpus_write    5'h0d // write byte

`define cpus_write2   5'h0e // write byte #2

`define cpus_write3   5'h0f // write byte #3

`define cpus_write4   5'h10 // write byte #4

`define cpus_read     5'h11 // read byte

`define cpus_read2    5'h12 // read byte #2

`define cpus_pop      5'h13 // POP completion

`define cpus_in       5'h14 // IN

`define cpus_in2      5'h15 // IN #2

`define cpus_out      5'h16 // OUT

`define cpus_out2     5'h17 // OUT #2

`define cpus_out3     5'h18 // OUT #3

`define cpus_out4     5'h19 // OUT #4

`define cpus_movtr    5'h1a // move to register

`define cpus_movrtw   5'h1b // move read to write

`define cpus_movrtwa  5'h1c // move read to write address

`define cpus_movrtra  5'h1d // move read to read address

`define cpus_accimm   5'h1e // accumulator immediate operations

`define cpus_daa      5'h1f // DAA completion

//

// Register numbers

//

`define reg_b 3'b000 // B

`define reg_c 3'b001 // C

`define reg_d 3'b010 // D

`define reg_e 3'b011 // E

`define reg_h 3'b100 // H

`define reg_l 3'b101 // L

`define reg_m 3'b110 // M

`define reg_a 3'b111 // A

//

// ALU operations

//

`define aluop_add 3'b000 // add

`define aluop_adc 3'b001 // add with carry in

`define aluop_sub 3'b010 // subtract

`define aluop_sbb 3'b011 // subtract with borrow in

`define aluop_and 3'b100 // and

`define aluop_xor 3'b101 // xor

`define aluop_or  3'b110 // or

`define aluop_cmp 3'b111 // compare

//

// State macros

//

`define mac_writebyte  1  // write a byte

`define mac_readbtoreg 2  // read a byte, place in register

`define mac_readdtobc  4  // read double byte to BC

`define mac_readdtode  6  // read double byte to DE

`define mac_readdtohl  8 // read double byte to HL

`define mac_readdtosp  10 // read double byte to SP

`define mac_readbmtw   12 // read byte and move to write

`define mac_readbmtr   14 // read byte and move to register

`define mac_sta        16 // STA

`define mac_lda        20 // LDA

`define mac_shld       25 // SHLD

`define mac_lhld       30 // LHLD

`define mac_writedbyte 36 // write double byte

`define mac_pop        38 // POP

`define mac_xthl       40 // XTHL

`define mac_accimm     44 // accumulator immediate

`define mac_jmp        45 // JMP

`define mac_call       47 // CALL

`define mac_in         51 // IN

`define mac_out        52 // OUT

`define mac_rst        53 // RST

module cpu8080(addr,     // Address out

               data,     // Data bus

               readmem,  // Memory read   

               writemem, // Memory write

               readio,   // Read I/O space

               writeio,  // Write I/O space

               intr,     // Interrupt request 

               inta,     // Interrupt request 

               waitr,    // Wait request

               reset,    // Reset

               clock);   // System clock

   output [15:0] addr;

   inout  [7:0] data;

   output readmem;

   output writemem;

   output readio;

   output writeio;

   input  intr;

   output inta;

   input  waitr;

   input  reset;

   input  clock;

   // Output or input lines that need to be registered

   reg           readmem;

   reg           writemem;

   reg    [15:0] pc;

   reg    [15:0] addr;

   reg           readio;

   reg           writeio;

   reg           inta;

   reg    [15:0] sp;

   // Local registers

   reg    [4:0]  state;       // CPU state machine

   reg    [2:0]  regd;        // Destination register

   reg    [7:0]  datao;       // Data output register

   reg           dataeno;     // Enable output data

   reg    [15:0] waddrhold;   // address holding for write

   reg    [15:0] raddrhold;   // address holding for read

   reg    [7:0]  wdatahold;   // single byte write data holding

   reg    [7:0]  wdatahold2;  // single byte write data holding

   reg    [7:0]  rdatahold;   // single byte read data holding

   reg    [7:0]  rdatahold2;  // single byte read data holding

   reg    [1:0]  popdes;      // POP destination code

   reg    [5:0]  statesel;    // state map selector

   reg    [4:0]  nextstate;   // next state output

   reg           eienb;       // interrupt enable delay shift reg

   reg    [7:0]  opcode;      // opcode holding

   // Register file. Note that 3'b110 (6) is not used, and is the code for a

   // memory reference.

   reg    [7:0]  regfil[0:7];

   // The flags are represented individually

   reg           carry; // carry bit

   reg           auxcar; // auxiliary carry bit

   reg           sign; // sign bit

   reg           zero; // zero bit

   reg           parity; // parity bit

   reg           ei; // interrupt enable

   reg           intcyc; // in interrupt cycle

   // ALU communication

   wire   [7:0]  alures;  // result

   reg    [7:0]  aluopra; // left side operand

   reg    [7:0]  aluoprb; // right side operand

   reg           alucin;  // carry in

   wire          alucout; // carry out

   wire          alupar;  // parity out

   wire          aluaxc;  // auxiliary carry

   reg    [2:0]  alusel;  // alu operational select

   // Instantiate the ALU

   alu alu(alures, aluopra, aluoprb, alucin, alucout, aluzout, alusout, alupar,

           aluaxc, alusel);

   always @(posedge clock)

      if (reset) begin // syncronous reset actions

      state <= `cpus_fetchi; // Clear CPU state to initial fetch

      pc <= 0; // reset program counter to 1st location

      dataeno <= 0; // get off the data bus

      readmem <= 0; // all signals out false

      writemem <= 0;

      readio <= 0;

      writeio <= 0;

      inta <= 0;

      intcyc <= 0;

      ei <= 1;

      eienb <= 0;

   end else case (state)

      `cpus_fetchi: begin // start of instruction fetch

         // if interrupt request is on, enter interrupt cycle, else exit it now

         if (intr&&ei) begin

            intcyc <= 1; // enter interrupt cycle

            inta <= 1; // activate interrupt acknowledge

            ei <= 0; // disable interrupts

         end else begin

            intcyc <= 0; // leave interrupt cycle

            readmem <= 1; // activate instruction memory read

         end

         addr <= pc; // place current program count on output

         if (eienb) ei <=1; // process delayed interrupt enable

         eienb <=0; // reset interrupt enabler

         state <= `cpus_fetchi2; // next state

      end

      `cpus_fetchi2: begin // complete instruction memory read

         opcode <= data; // latch opcode

         readmem <= 0; // Deactivate instruction memory read

         inta <= 0; // and interrupt acknowledge

         state <= `cpus_fetchi3; // next state

      end

      `cpus_fetchi3: begin // complete instruction memory read

         // We split off the instructions into 4 groups. Most of the 8080

         // instructions are in the MOV and ACC operations class.

         case (opcode[7:6]) // Decode top level

            2'b00: begin // 00: Data transfers and others

               case (opcode[5:0]) // decode these instructions

                  6'b000000: begin // NOP

                     // yes, do nothing

                     state <= `cpus_fetchi; // Fetch next instruction

                     pc <= pc+1; // Next instruction byte

                  end

                  6'b110111: begin // STC

                     carry <= 1; // set carry flag

                     state <= `cpus_fetchi; // Fetch next instruction

                     pc <= pc+1; // Next instruction byte

                  end

                  6'b111111: begin // CMC

                     carry <= ~carry; // complement carry flag

                     state <= `cpus_fetchi; // Fetch next instruction

                     pc <= pc+1; // Next instruction byte

                  end

                  6'b101111: begin // CMA

                     regfil[`reg_a] <= ~regfil[`reg_a]; // complement accumulator

                     state <= `cpus_fetchi; // Fetch next instruction

                     pc <= pc+1; // Next instruction byte

                  end

                  6'b100111: begin // DAA

                     // decimal adjust accumulator, or remove by carry any 

                     // results in nybbles greater than 9

                     if (regfil[`reg_a][3:0] > 9 || auxcar) begin

                        { carry, regfil[`reg_a] } <= regfil[`reg_a]+6;

                        auxcar <= (regfil[`reg_a][3:0]+6 >> 4) & 1;

                     end

                     state <= `cpus_daa; // finish DAA

                     pc <= pc+1; // Next instruction byte

                  end

                  6'b000100, 6'b001100, 6'b010100, 6'b011100, 6'b100100,

                  6'b101100, 6'b110100, 6'b111100, 6'b000101, 6'b001101,

                  6'b010101, 6'b011101, 6'b100101, 6'b101101, 6'b110101,

                  6'b111101: begin // INR/DCR

                     regd <= opcode[5:3]; // get source/destination reg

                     aluopra <= regfil[opcode[5:3]]; // load as alu a

                     aluoprb <= 1; // load 1 as alu b

                     if (opcode[0]) alusel <= `aluop_sub; // set subtract

                     else alusel <= `aluop_add; // set add

                     state <= `cpus_indcb; // go inr/dcr cycleback

                     pc <= pc+1; // Next instruction byte

                  end

                  6'b000010, 6'b010010: begin // STAX

                     wdatahold <= regfil[`reg_a]; // place A as source

                     if (opcode[4]) // use DE pair

                        waddrhold <= regfil[`reg_d]<<8|regfil[`reg_d];

                     else // use BC pair

                        waddrhold <= regfil[`reg_b] << 8|regfil[`reg_c];

                     statesel <= `mac_writebyte; // write byte

                     state <= `cpus_write;

                     pc <= pc+1; // Next instruction byte

                  end

                  6'b001010, 6'b011010: begin // LDAX

                     regd <= `reg_a; // set A as destination

                     if (opcode[4]) // use DE pair

                        raddrhold <= regfil[`reg_d]<<8|regfil[`reg_d];

                     else // use BC pair

                        raddrhold <= regfil[`reg_b]<<8|regfil[`reg_c];

                     statesel <= `mac_readbtoreg; // read byte to register

                     state <= `cpus_read;

                     pc <= pc+1; // Next instruction byte

                  end

                  6'b000111: begin // RLC

                     // rotate left circular

                     { carry, regfil[`reg_a] } <=

                        (regfil[`reg_a] << 1)+regfil[`reg_a][7];

                     state <= `cpus_fetchi; // Fetch next instruction

                     pc <= pc+1; // Next instruction byte

                  end

                  6'b010111: begin // RAL

                     // rotate left through carry

                     { carry, regfil[`reg_a] } <= (regfil[`reg_a] << 1)+carry;

                     state <= `cpus_fetchi; // Fetch next instruction

                     pc <= pc+1; // Next instruction byte

                  end

                  6'b001111: begin // RRC

                     // rotate right circular

                     regfil[`reg_a] <=

                        (regfil[`reg_a] >> 1)+(regfil[`reg_a][0] << 7);

                     carry <= regfil[`reg_a][0];

                     state <= `cpus_fetchi; // Fetch next instruction

                     pc <= pc+1; // Next instruction byte

                  end

                  6'b011111: begin // RAR

                     // rotate right through carry

                     regfil[`reg_a] <= (regfil[`reg_a] >> 1)+(carry << 7);

                     carry <= regfil[`reg_a][0];

                     state <= `cpus_fetchi; // Fetch next instruction

                     pc <= pc+1; // Next instruction byte

                  end

                  6'b001001: begin // DAD B

                     // add BC to HL

                     { carry, regfil[`reg_h], regfil[`reg_l] } <=

                        (regfil[`reg_h] << 8)+regfil[`reg_l]+

                        (regfil[`reg_b] << 8)+regfil[`reg_c];

                     state <= `cpus_fetchi; // Fetch next instruction

                     pc <= pc+1; // Next instruction byte

                  end

                  6'b011001: begin // DAD D

                     // add DE to HL

                     { carry, regfil[`reg_h], regfil[`reg_l] } <=

                        (regfil[`reg_h] << 8)+regfil[`reg_l]+

                        (regfil[`reg_d] << 8)+regfil[`reg_e];

                     state <= `cpus_fetchi; // Fetch next instruction

                     pc <= pc+1; // Next instruction byte

                  end

                  6'b101001: begin // DAD H

                     // add HL to HL

                     { carry, regfil[`reg_h], regfil[`reg_l] } <=

                        (regfil[`reg_h] << 8)+regfil[`reg_l]+

                        (regfil[`reg_h] << 8)+regfil[`reg_l];

                     state <= `cpus_fetchi; // Fetch next instruction

                     pc <= pc+1; // Next instruction byte

                  end

                  6'b111001: begin // DAD SP

                     // add SP to HL

                     { carry, regfil[`reg_h], regfil[`reg_l] } <=

                        (regfil[`reg_h] << 8)+regfil[`reg_l]+sp;

                     state <= `cpus_fetchi; // Fetch next instruction

                     pc <= pc+1; // Next instruction byte

                  end

                  6'b000011: begin // INX B

                     // increment BC, no flags set

                     regfil[`reg_b] <=

                        (((regfil[`reg_b] << 8)+regfil[`reg_c])+1)>>8;

                     regfil[`reg_c] <=

                        ((regfil[`reg_b] << 8)+regfil[`reg_c])+1;

                     state <= `cpus_fetchi; // Fetch next instruction

                     pc <= pc+1; // Next instruction byte

                  end

                  6'b010011: begin // INX D

                     // increment DE, no flags set

                     regfil[`reg_d] <=

                        (((regfil[`reg_d] << 8)+regfil[`reg_e])+1)>>8;

                     regfil[`reg_e] <=

                        ((regfil[`reg_d] << 8)+regfil[`reg_e])+1;

                     state <= `cpus_fetchi; // Fetch next instruction

                     pc <= pc+1; // Next instruction byte

                  end

                  6'b100011: begin // INX H

                     // increment HL, no flags set

                     regfil[`reg_h] <=

                        (((regfil[`reg_h] << 8)+regfil[`reg_l])+1)>>8;

                     regfil[`reg_l] <=

                        ((regfil[`reg_h] << 8)+regfil[`reg_l])+1;

                     state <= `cpus_fetchi; // Fetch next instruction

                     pc <= pc+1; // Next instruction byte

                  end

                  6'b110011: begin // INX SP

                     // increment SP, no flags set

                     sp <= sp+1;

                     state <= `cpus_fetchi; // Fetch next instruction

                     pc <= pc+1; // Next instruction byte

                  end

                  6'b001011: begin // DCX B

                     // decrement BC, no flags set

                     regfil[`reg_b] <=

                        (((regfil[`reg_b] << 8)+regfil[`reg_c])-1)>>8;

                     regfil[`reg_c] <=

                        ((regfil[`reg_b] << 8)+regfil[`reg_c])-1;

                     state <= `cpus_fetchi; // Fetch next instruction

                     pc <= pc+1; // Next instruction byte

                  end

                  6'b011011: begin // DCX D

                     // decrement DE, no flags set

                     regfil[`reg_d] <=

                        (((regfil[`reg_d] << 8)+regfil[`reg_e])-1)>>8;

                     regfil[`reg_e] <=

                        ((regfil[`reg_d] << 8)+regfil[`reg_e])-11;

                     state <= `cpus_fetchi; // Fetch next instruction

                     pc <= pc+1; // Next instruction byte

                  end

                  6'b101011: begin // DCX H

                     // decrement HL, no flags set

                     regfil[`reg_h] <=

                        (((regfil[`reg_h] << 8)+regfil[`reg_l])-1)>>8;

                     regfil[`reg_l] <=

                        ((regfil[`reg_h] << 8)+regfil[`reg_l])-1;

                     state <= `cpus_fetchi; // Fetch next instruction

                     pc <= pc+1; // Next instruction byte

                  end

                  6'b111011: begin // DCX SP

                     // decrement SP, no flags set

                     sp <= sp-1;

                     state <= `cpus_fetchi; // Fetch next instruction

                     pc <= pc+1; // Next instruction byte

                  end

                  6'b000001: begin // LXI B

                     raddrhold <= pc+1; // pick up after instruction

                     statesel <= `mac_readdtobc; // read double to BC

                     state <= `cpus_read;

                     pc <= pc+3; // skip

                  end

                  6'b010001: begin // LXI D

                     raddrhold <= pc+1; // pick up after instruction

                     statesel <= `mac_readdtode; // read double to DE

                     state <= `cpus_read;

                     pc <= pc+3; // skip

                  end

                  6'b100001: begin // LXI H

                     raddrhold <= pc+1; // pick up after instruction

                     statesel <= `mac_readdtohl; // read double to HL

                     state <= `cpus_read;

                     pc <= pc+3; // skip

                  end

                  6'b110001: begin // LXI SP

                     raddrhold <= pc+1; // pick up after instruction

                     pc <= pc+3; // skip

                     statesel <= `mac_readdtosp; // read double to SP

                     state <= `cpus_read;

                     pc <= pc+3; // skip

                  end

                  6'b000110, 6'b001110, 6'b010110, 6'b011110, 6'b100110,

                  6'b101110, 6'b110110, 6'b111110: begin // MVI

                     // move immediate to register

                     regd <= opcode[5:3]; // set destination register

                     raddrhold <= pc+1; // set pickup address

                     if (opcode[5:3] == `reg_m) begin // it's mvi m,imm

                        regd <= opcode[5:3]; // set destination register

                        // set destination address

                        waddrhold <= { regfil[`reg_h], regfil[`reg_l] };

                        statesel <= `mac_readbmtw; // read byte and move to write

                     end else

                        statesel <= `mac_readbmtr; // read byte and move to register

                     state <= `cpus_read;

                     pc <= pc+2; // advance over byte

                  end

                  6'b110010: begin // STA

                     wdatahold <= regfil[`reg_a]; // set write data

                     raddrhold <= pc+1; // set read address

                     statesel <= `mac_sta; // perform sta

                     state <= `cpus_read;

                     pc <= pc+3; // next

                  end

                  6'b111010: begin // LDA

                     raddrhold <= pc+1; // set read address

                     regd <= `reg_a; // set destination

                     statesel <= `mac_lda; // perform lda

                     state <= `cpus_read;

                     pc <= pc+3; // next

                  end