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//---------------------------------------------------------------------------

// RISC 16F84 "small" core

//

// This file is part of the "risc_16F84" project.

// http://www.opencores.org/cores/risc_16F84

// 

//

// Description: See description below (which suffices for IP core

//                                     specification document.)

//

// Copyright (C) 1999 Sumio Morioka (original VHDL design version)

// Copyright (C) 2001 John Clayton and OPENCORES.ORG (this Verilog version)

//

// NOTE: This source code is free for educational/hobby use only.  It cannot

// be used for commercial purposes without the consent of Microchip

// Technology incorporated.

//

// This source file may be used and distributed without restriction provided

// that this copyright statement is not removed from the file and that any

// derivative work contains the original copyright notice and the associated

// disclaimer.

//

// This source file is free software; you can redistribute it and/or modify

// it under the terms of the GNU Lesser General Public License as published

// by the Free Software Foundation;  either version 2.1 of the License, or

// (at your option) any later version.

//

// This source is distributed in the hope that it will be useful, but WITHOUT

// ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or 

// FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public

// License for more details.

//

// You should have received a copy of the GNU Lesser General Public License

// along with this source.

// If not, download it from http://www.opencores.org/lgpl.shtml

//

//---------------------------------------------------------------------------

//

// Author: John Clayton

// Date  : January 29, 2002

//

// (NOTE: Date formatted as day/month/year.)

// Update: 29/01/02 copied this file from memory_sizer.v (pared down).

//                  Translated the module and signal declarations.

//                  Transformed the instruction wires to lowercase.

//                  Transformed the addressing wires to lowercase.

// Update: 31/01/02 Translated the instruction decoder.

// Update:  5/02/02 Determined that stack is simply a circular buffer of

//                  8 locations, 13 bits per location.  Started translating

//                  "main_efsm" process.  Added all code from piccore.vhd

//                  into this file for eventual translation.  Concluded that

//                  "stack_full_node" is not needed.

// Update:  6/02/02 Translated the "ram_i_node" if/else precedural assignment.

// Update:  7/02/02 Changed all := to <=, changed all '0' to 0 and '1' to 1.

//                  Replaced all " downto " with ":".

//                  Finished translating QRESET state.

// Update: 20/02/02 Replaced all instances of Qreset with QRESET_PP.  Also

//                  replaced other state designations with their new names.

//                  Finished translating Q1, Q2 states.

// Update: 22/02/02 Translated section 2-4-1-1 (aluout register)

// Update: 27/02/02 Replaced all "or" with "||" in if statements

//                  Replaced all "and" with "&&" in if statements.

//                  Replaced all "not" with "~" in if statements.

//                  Finished translating Q3,Q4 states.

//                  Translated output signal assignments at end of code.

//                  Translated interrupt trigger processes.

// Update: 28/02/02 Finished translation of WDT and TMR0 prescaler.

//                  Trimmed line length to 80 characters throughout.

//                  Prepared to attempt initial syntax checking.

//                  Cleaned up some naming conventions, and verified that

//                  all I/O pins have _i or _o appended in the body of the

//                  code.

// Update: 03/04/02 Changed "progdata_i" to "prog_dat_i" Also changed

//                  "progadr_o" to "prog_adr_o"

// Update: 04/04/02 Created new file "risc16f84_lite.v"  This file is reduced

//                  and simplified from the original "risc16f84.v" file.

//                  Specifically, I am removing EEPROM support, and 

//                  consolidating porta and portb I/O pins so that they

//                  are bidirectional.

// Update: 04/04/02 Created a new file "risc16f84_small.v"  This file is

//                  further reduced and simplified from "risc16f84_lite.v"

//                  Specifically, I am removing the prescaler, TMR0 and WDT.

//                  Also, I am removing support for portb interrupts, leaving

//                  only rb0/int as an interrupt source.  This pin will be

//                  the only way to wake up from the SLEEP instruction...

//                  Obviously, the CLEARWDT instruction will no longer do

//                  anything.

// Update: 05/04/02 Removed the "powerdown_o", "startclk_o" and "clk_o" pins

//                  from the small design.  Also removed "rbpu_o", so if you

//                  want pullups, you have to add them explicitly in the

//                  constraints file, and option_reg[7] doesn't control them.

// Update: 26/04/02 Fixed bug in aluout_reg logic, so that bitwise operators

//                  are used for AND/OR.  (They had been wrongly coded using

//                  logical operators!)

// Update: 01/05/02 Fixed another bug -- the rrf and rlf instructions were

//                  coded incorrectly.

// Update: 03/05/02 Fixed another bug -- the carry bit was incorrect (the

//                  problem was discovered while performing SUBWF X,W where

//                  W contained 0 and X contained 1. (1-0).  The logic for

//                  the carry bit appears to have been incorrect even in

//                  the original VHDL code by Sumio Morioka.

// Update: 10/30/02 Fixed syntax error pointed out by Cheol-Kyoo Lee, who got

//                  the source code from opencores.com.  Removed semicolon

//                  from "endcase" statements.

//

// Description

//---------------------------------------------------------------------------

// This logic module implements a small RISC microcontroller, with functions

// and instruction set very similar to those of the Microchip 16F84 chip.

// This work is a translation (from VHDL to Verilog) of the "CQPIC" design

// published in 1999 by Sumio Morioka of Japan, and published in the December

// 1999 issue of "Transistor Gijutsu Magazine."  The translation was performed

// by John Clayton, without the use of any translation tools.

//

// Original version used as basis for translation:  CQPIC version 1.00b

//                                                  (December 10, 2000)

//---------------------------------------------------------------------------

`define STATEBIT_SIZE 3      // Size of state machine register (bits)

module risc16f84_small (

  prog_dat_i,           // [13:0] ROM read data

  prog_adr_o,           // [12:0] ROM address

  ram_dat_i,            // [7:0] RAM read data

  ram_dat_o,            // [7:0] RAM write data

  ram_adr_o,            // [8:0] RAM address; ram_adr[8:7] indicates RAM-BANK

  readram_o,            // RAM read strobe (H active)

  writeram_o,           // RAM write strobe (H active)

  porta_io,             // [4:0] PORT-A

  portb_io,             // [7:0] PORT-B

  int0_i,               // PORT-B(0) INT

  pon_rst_n_i,          // Power-on reset (L active)

  clk_i                 // Clock input

);

// You can change the following parameters as you would like

parameter STACK_SIZE_PP      = 8;   // Size of PC stack

parameter LOG2_STACK_SIZE_PP = 3;   // Log_2(stack_size)

parameter WDT_SIZE_PP        = 255; // Size of watch dog timer (WDT)

parameter WDT_BITS_PP        = 8;   // Bits needed for watch dog timer (WDT)

// State definitions for state machine, provided as parameters to allow

// for redefinition of state values by the instantiator if desired.

parameter QRESET_PP = 3'b100; // reset state

parameter Q1_PP     = 3'b000; // state Q1

parameter Q2_PP     = 3'b001; // state Q2

parameter Q3_PP     = 3'b011; // state Q3

parameter Q4_PP     = 3'b010; // state Q4

// I/O declarations

       // program ROM data bus/address bus

input  [13:0] prog_dat_i;   // ROM read data

output [12:0] prog_adr_o;   // ROM address

       // data RAM data bus/address bus/control signals

input  [7:0] ram_dat_i;     // RAM read data

output [7:0] ram_dat_o;     // RAM write data

output [8:0] ram_adr_o;     // RAM address; ram_adr[8:7] indicates RAM-BANK

output readram_o;           // RAM read  strobe (H active)

output writeram_o;          // RAM write strobe (H active)

       // I/O ports

inout  [4:0] porta_io;      // PORT-A

inout  [7:0] portb_io;      // PORT-B

       // PORT-B interrupt input

input  int0_i;              // PORT-B(0) INT

       // CPU reset

input  pon_rst_n_i;         // Power-on reset (LOW active)

       // CPU clock

input  clk_i;               // Clock input

// Internal signal declarations

     // User registers

reg  [7:0] w_reg;            // W

reg  [12:0] pc_reg;          // PCH/PCL

reg  [7:0] status_reg;       // STATUS

reg  [7:0] fsr_reg;          // FSR

reg  [4:0] porta_i_sync_reg; // PORTA IN (synchronizer)

reg  [4:0] porta_o_reg;      // PORTA OUT

reg  [7:0] portb_i_sync_reg; // PORTB IN (synchronizer)

reg  [7:0] portb_o_reg;      // PORTB OUT

reg  [4:0] pclath_reg;       // PCLATH

reg  [7:0] intcon_reg;       // INTCON

reg  [7:0] option_reg;       // OPTION

reg  [4:0] trisa_reg;        // TRISA

reg  [7:0] trisb_reg;        // TRISB

     // Internal registers for controlling instruction execution

reg  [13:0] inst_reg;        // Hold fetched op-code/operand

reg  [7:0] aluinp1_reg;      // data source (1 of 2)

reg  [7:0] aluinp2_reg;      // data source (2 of 2)

reg        c_in;             // Used with ALU data sources.

reg  [7:0] aluout_reg;       // result of calculation

reg  exec_op_reg;            // if L (i.e. GOTO instruction etc), stall exec.

reg  intstart_reg;           // if H (i.e. interrupt), stall instr. exec.

reg  sleepflag_reg;          // if H, sleeping.

     // Stack

                             // stack (array of data-registers)

reg  [12:0] stack_reg [STACK_SIZE_PP-1:0];

                             // stack pointer (binary encoded)

reg  [LOG2_STACK_SIZE_PP-1:0] stack_pnt_reg;

     // Interrupt registers/nodes

reg  intrise_reg;            // detect positive edge of PORT-B inputs

reg  intdown_reg;            // detect negative edge of PORT-B inputs

wire rb0_int;                // Interrupt trigger

wire inte;                    // RB0   interrupt trigger

reg  intclr_reg;              // CPU; clear intrise_reg and intdown_reg

     // State register

reg  [`STATEBIT_SIZE-1:0] state_reg;

     // Result of decoding instruction -- only 1 is active at a time

wire inst_addlw;

wire inst_addwf;

wire inst_andlw;

wire inst_andwf;

wire inst_bcf;

wire inst_bsf;

wire inst_btfsc;

wire inst_btfss;

wire inst_call;

wire inst_clrf;

wire inst_clrw;

wire inst_comf;

wire inst_decf;

wire inst_decfsz;

wire inst_goto;

wire inst_incf;

wire inst_incfsz;

wire inst_iorlw;

wire inst_iorwf;

wire inst_movlw;

wire inst_movf;

wire inst_movwf;

wire inst_retfie;

wire inst_retlw;

wire inst_ret;

wire inst_rlf;

wire inst_rrf;

wire inst_sleep;

wire inst_sublw;

wire inst_subwf;

wire inst_swapf;

wire inst_xorlw;

wire inst_xorwf;

     // Result of calculating RAM access address

wire [8:0] ram_adr_node;      // RAM access address

     // These wires indicate accesses to special registers... 

     // Only 1 is active at a time.

wire addr_pcl;

wire addr_stat;

wire addr_fsr;

wire addr_porta;

wire addr_portb;

wire addr_pclath;

wire addr_intcon;

wire addr_option;

wire addr_trisa;

wire addr_trisb;

wire addr_sram;

     // Other output registers (for removing hazards)

reg  writeram_reg;      // data-sram write strobe

reg  [8:0] ram_adr_reg; // data-sram address

     // Synchronizers

reg  inte_sync_reg;

reg  reset_condition;

     // Signals used in "main_efsm" procedure

     // (Intermediate nodes used for resource sharing.)

reg  [7:0] ram_i_node;   // result of reading RAM/Special registers

reg  [12:0] inc_pc_node; // value of PC + 1

wire [7:0] mask_node;    // bit mask for logical operations

reg  [8:0] add_node;     // result of 8bit addition

reg  [4:0] addlow_node;  // result of low-4bit addition

wire temp;               // Placeholder wire

wire dtemp;              // Placeholder wire

reg  aluout_zero_node;   // H if ALUOUT = 0

reg  writew_node;        // H if destination is W register

reg  writeram_node;      // H if destination is RAM/Special registers

reg  int_node;           // H if interrupt request comes

//--------------------------------------------------------------------------

// Instantiations

//--------------------------------------------------------------------------

//--------------------------------------------------------------------------

// Functions & Tasks

//--------------------------------------------------------------------------

function [7:0] bitwise_tristate;

  input [7:0] port;

  input [7:0] tris;

  integer k;

  begin

    for (k=0; k<8; k=k+1) bitwise_tristate[k] <= (tris[k])?1'bZ:port[k];

  end

endfunction

//--------------------------------------------------------------------------

// Module code

//--------------------------------------------------------------------------

// CPU synchronizers

always @(posedge clk_i)

begin

  inte_sync_reg   <= inte;

  reset_condition <= ~pon_rst_n_i;

end

// Decode OPcode    (see pp.54 of PIC16F84 data sheet)

// only 1 signal of the following signals will be '1'

assign inst_call     = (inst_reg[13:11] ==  3'b100           );

assign inst_goto     = (inst_reg[13:11] ==  3'b101           );

assign inst_bcf      = (inst_reg[13:10] ==  4'b0100          );

assign inst_bsf      = (inst_reg[13:10] ==  4'b0101          );

assign inst_btfsc    = (inst_reg[13:10] ==  4'b0110          );

assign inst_btfss    = (inst_reg[13:10] ==  4'b0111          );

assign inst_movlw    = (inst_reg[13:10] ==  4'b1100          );

assign inst_retlw    = (inst_reg[13:10] ==  4'b1101          );

assign inst_sublw    = (inst_reg[13:9]  ==  5'b11110         );

assign inst_addlw    = (inst_reg[13:9]  ==  5'b11111         );

assign inst_iorlw    = (inst_reg[13:8]  ==  6'b111000        );

assign inst_andlw    = (inst_reg[13:8]  ==  6'b111001        );

assign inst_xorlw    = (inst_reg[13:8]  ==  6'b111010        );

assign inst_subwf    = (inst_reg[13:8]  ==  6'b000010        );

assign inst_decf     = (inst_reg[13:8]  ==  6'b000011        );

assign inst_iorwf    = (inst_reg[13:8]  ==  6'b000100        );

assign inst_andwf    = (inst_reg[13:8]  ==  6'b000101        );

assign inst_xorwf    = (inst_reg[13:8]  ==  6'b000110        );

assign inst_addwf    = (inst_reg[13:8]  ==  6'b000111        );

assign inst_movf     = (inst_reg[13:8]  ==  6'b001000        );

assign inst_comf     = (inst_reg[13:8]  ==  6'b001001        );

assign inst_incf     = (inst_reg[13:8]  ==  6'b001010        );

assign inst_decfsz   = (inst_reg[13:8]  ==  6'b001011        );

assign inst_rrf      = (inst_reg[13:8]  ==  6'b001100        );

assign inst_rlf      = (inst_reg[13:8]  ==  6'b001101        );

assign inst_swapf    = (inst_reg[13:8]  ==  6'b001110        );

assign inst_incfsz   = (inst_reg[13:8]  ==  6'b001111        );

assign inst_movwf    = (inst_reg[13:7]  ==  7'b0000001       );

assign inst_clrw     = (inst_reg[13:7]  ==  7'b0000010       );

assign inst_clrf     = (inst_reg[13:7]  ==  7'b0000011       );

assign inst_ret      = (inst_reg[13:0]  == 14'b00000000001000);

assign inst_retfie   = (inst_reg[13:0]  == 14'b00000000001001);

assign inst_sleep    = (inst_reg[13:0]  == 14'b00000001100011);

// Calculate RAM access address (see pp.19 of PIC16F84 data sheet)

    // if "d"=0, indirect addressing is used, so RAM address is BANK+FSR

    // otherwise, RAM address is BANK+"d"

    // (see pp.19 of PIC16F84 data sheet)

assign ram_adr_node = (inst_reg[6:0]==0)?{status_reg[7],fsr_reg[7:0]}:

                               {status_reg[6:5],inst_reg[6:0]};

    // check if this is an access to external RAM or not

assign addr_sram   = (ram_adr_node[6:0] > 7'b0001011); //0CH-7FH,8CH-FFH

    // check if this is an access to special register or not

    // only 1 signal of the following signals will be '1'

assign addr_pcl     = (ram_adr_node[6:0] ==  7'b0000010); // 02H, 82H

assign addr_stat    = (ram_adr_node[6:0] ==  7'b0000011); // 03H, 83H

assign addr_fsr     = (ram_adr_node[6:0] ==  7'b0000100); // 04H, 84H

assign addr_porta   = (ram_adr_node[7:0] == 8'b00000101); // 05H

assign addr_portb   = (ram_adr_node[7:0] == 8'b00000110); // 06H

assign addr_pclath  = (ram_adr_node[6:0] ==  7'b0001010); // 0AH, 8AH

assign addr_intcon  = (ram_adr_node[6:0] ==  7'b0001011); // 0BH, 8BH

assign addr_option  = (ram_adr_node[7:0] == 8'b10000001); // 81H

assign addr_trisa   = (ram_adr_node[7:0] == 8'b10000101); // 85H

assign addr_trisb   = (ram_adr_node[7:0] == 8'b10000110); // 86H

// construct bit-mask for logical operations and bit tests

assign mask_node = 1 << inst_reg[9:7];

// MAIN EFSM: description of register value changes in each clock cycle

always @(posedge clk_i)

begin

  // 1. Intermediate nodes for resource sharing

  // This is a long if/else chain.  Consider pulling in the decoded signals

  // addr_tmr0 etc., and using a case statement instead?

  // 1-1. Reading RAM/data sources  (see pp.13 of PIC16F84 data sheet)

  if (addr_sram)         ram_i_node <= ram_dat_i;   // data from ext. SRAM

  else if (addr_pcl)     ram_i_node <= pc_reg[7:0]; // data from pcl

  else if (addr_stat)    ram_i_node <= status_reg;  // data from status

  else if (addr_fsr)     ram_i_node <= fsr_reg;     // data from fsr

  else if (addr_porta)

  begin

    // Logic implements a 2:1 mux for each bit [4:0] of ram_i_node

    ram_i_node[4:0] <= (

                           (~trisa_reg[4:0] & porta_o_reg[4:0])

                        || ( trisa_reg[4:0] & porta_i_sync_reg[4:0])

                        );

    ram_i_node[7:5] <= 3'b0;

  end

  else if (addr_portb)

  begin

    // Logic implements a 2:1 mux for each bit [7:0] of ram_i_node

    ram_i_node[7:0] <= (

                           (~trisb_reg[7:0] & portb_o_reg[7:0])

                        || ( trisb_reg[7:0] & portb_i_sync_reg[7:0])

                        );

  end

  else if (addr_pclath)  ram_i_node <= {3'b0,pclath_reg}; // pclath (5bit)

  else if (addr_intcon)  ram_i_node <= intcon_reg;        // data from intcon

  else if (addr_option)  ram_i_node <= option_reg;        // data from option

  else if (addr_trisa)   ram_i_node <= {3'b0,trisa_reg};  // trisa (5bit)

  else if (addr_trisb)   ram_i_node <= trisb_reg;         // data from trisb

  else ram_i_node <= 0;

  // 1-2. PC + 1

  inc_pc_node  <= pc_reg + 1;

  // 1-3. Adder (ALU)

  // full 8bit-addition, with carry in/out.

  {add_node,temp}     <=    {1'b0,aluinp1_reg,1'b1}

                          + {1'b0,aluinp2_reg,c_in};

  // lower 4bit-addition

  {addlow_node,dtemp} <=    {1'b0,aluinp1_reg[3:0],1'b1}

                          + {1'b0,aluinp2_reg[3:0],c_in};

  // 1-4. Test if aluout = 0

  aluout_zero_node <= (aluout_reg == 0)?1:0;

  // 1-5. Determine destination

  if (intstart_reg)

  begin

    writew_node     <= 0;

    writeram_node   <= 0;

  end

  else if (inst_movwf || inst_bcf || inst_bsf || inst_clrf)

  begin

    writew_node     <= 0;

    writeram_node   <= 1;

  end

  else if (   inst_movlw || inst_addlw || inst_sublw || inst_andlw

           || inst_iorlw || inst_xorlw || inst_retlw || inst_clrw)

  begin

    writew_node     <= 1;

    writeram_node   <= 0;

  end

  else if (   inst_movf   || inst_swapf || inst_addwf || inst_subwf

           || inst_andwf  || inst_iorwf || inst_xorwf || inst_decf

           || inst_incf   || inst_rlf   || inst_rrf   || inst_decfsz

           || inst_incfsz || inst_comf)

  begin

    writew_node     <= ~inst_reg[7];  // ("d" field of fetched instruction)

    writeram_node   <=  inst_reg[7];  // ("d" field of fetched instruction)

  end

  else

  begin

    writew_node     <= 0;

    writeram_node   <= 0;

  end

  // 1-6. Interrupt request   (see pp.17 of PIC16F84 data sheet)

  int_node <= intcon_reg[7]        // GIE

              && (intcon_reg[4] && intcon_reg[1]); // INTE,INTF

  // 2. EFSM body

  case (state_reg)

    // 2-1. Reset state (see pp.14 and pp.42 of PIC16F84 data sheet)

    QRESET_PP :

    begin

      pc_reg          <= 0;     // 0

      status_reg[7:5] <= 3'b0;

      pclath_reg      <= 0;     // 0

      intcon_reg[7:1] <= 7'b0;

      option_reg      <= -1;    // Set to all ones, like vhdl (others => 1)

      trisa_reg       <= -1;    // Set to all ones, like vhdl (others => 1)

      trisb_reg       <= -1;    // Set to all ones, like vhdl (others => 1)

      exec_op_reg     <= 0;

      intclr_reg      <= 1;     // clear int

      intstart_reg    <= 0;

      writeram_reg    <= 0;

      sleepflag_reg   <= 0;

      status_reg[4]   <= 1;     // /T0 = 1

      status_reg[3]   <= 1;     // /PD = 1

      stack_pnt_reg   <= 0;     // Reset stack pointer

      // go to Q1 state if reset signal is de-asserted

      if (~reset_condition) state_reg <= Q1_PP;

    end  // End of QRESET_PP state

    // 2-2. Q1 cycle

    Q1_PP :

    begin

      // 2-2-1. Clear external interrupt registers if GIE=0

      if (intcon_reg[7]) intclr_reg <= 0;

      else intclr_reg <= 1;     // clear interrupt

      // 2-2-2. Read I/O port

      porta_i_sync_reg <= porta_io;

      portb_i_sync_reg <= portb_io;

      // 2-2-3. Read RAM read address, if not stalled

      if (exec_op_reg && ~intstart_reg) ram_adr_reg <= ram_adr_node;

      // 2-2-5. Goto next cycle

      if (reset_condition) state_reg <= QRESET_PP;

      else

        // if in the sleep mode, wait until wake-up trigger comes

        if (sleepflag_reg && ~intstart_reg)

        begin

          if (inte_sync_reg)

          begin

          // if rb0/inte interrupt comes, then resume execution

            sleepflag_reg <= 0;

            state_reg     <= Q2_PP;

          end

        end

        // if not in sleep mode, or if stalled, continue execution

        else state_reg   <= Q2_PP;

// Alternate rendition of 2-2-5...

//      // 2-2-5. Goto next cycle

//      if (reset_condition) state_reg <= QRESET_PP; // Reset

//      else if (~sleepflag_reg) state_reg <= Q2_PP; // Not sleeping, move on

//      else if (intstart_reg)   state_reg <= Q2_PP; // Stalled, move on

//      else if (~inte_sync_reg) state_reg <= Q1_PP; // No wakeup, stay sleeping

//      else // Not stalled and wake-up trigger comes...

//      begin

//        // if rb0/int interrupt comes, then resume execution

//        sleepflag_reg <= 0;    // Exit sleep mode condition

//        state_reg     <= Q2_PP;

//      end

    end   // End of Q1 state

    // 2-3. Q2 cycle

    Q2_PP :

    begin

      // 2-3-1. Read data-RAM and substitute source values to alu-input regs

      if (exec_op_reg && ~intstart_reg)  // if NOT STALLED

      begin

        // 2-3-1-1. Set aluinp1 register (source #1)

        if (   inst_movf   || inst_swapf || inst_addwf || inst_subwf

            || inst_andwf  || inst_iorwf || inst_xorwf || inst_decf

            || inst_incf   || inst_rlf   || inst_rrf   || inst_bcf

            || inst_bsf    || inst_btfsc || inst_btfss || inst_decfsz

            || inst_incfsz || inst_comf)

            aluinp1_reg <= ram_i_node;       // RAM/Special registers

        else

        if (   inst_movlw || inst_addlw || inst_sublw || inst_andlw

            || inst_iorlw || inst_xorlw || inst_retlw)

            aluinp1_reg <= inst_reg[7:0];    // Immediate value ("k")

        else

        if (   inst_clrf  || inst_clrw) aluinp1_reg <= 0; // 0

        else aluinp1_reg <= w_reg;                        // W register

        // 2-3-1-2. Set aluinp2 register (source #2)

        if      (inst_decf || inst_decfsz) aluinp2_reg <= -1; // for decr.

        else if (inst_incf || inst_incfsz) aluinp2_reg <=  1; // for incr.

                // -1 * W register (for subtract)

        else if (inst_sublw || inst_subwf) aluinp2_reg <= ~w_reg + 1;

                // operation of BCF: AND with inverted mask ("1..101..1")

                // mask for BCF: value of only one position is 0

        else if (inst_bcf) aluinp2_reg <= ~mask_node;

                // operation of BSF: OR with mask_node ("0..010..0")

                // operation of FSC and FSS: AND with mask_node, compare to 0

        else if (inst_btfsc || inst_btfss || inst_bsf)

                                      aluinp2_reg <= mask_node;

        else aluinp2_reg <= w_reg; // W register

        // 2-3-1-3. Set stack pointer register (pop stack)

        if (inst_ret || inst_retlw || inst_retfie)

             stack_pnt_reg   <= stack_pnt_reg - 1; // cycles 3,2,1,0,7,6...

        // 2-3-1-4. Set ram_adr register (set RAM write address)

        ram_adr_reg  <= ram_adr_node;     // RAM write address

      end

      // 2-3-4. Goto next cycle

      if (reset_condition) state_reg <= QRESET_PP;

      else state_reg <= Q3_PP;

    end   // End of Q2 state

    // 2-4. Q3 cycle

    Q3_PP :

    begin

      // 2-4-1. Calculation and store result into alu-output register

      if (exec_op_reg && ~intstart_reg) // if NOT STALLED

      begin

        // 2-4-1-1. Set aluout register

                // Rotate left

        if      (inst_rlf)

                aluout_reg <= {aluinp1_reg[6:0],status_reg[0]};

                // Rotate right

        else if (inst_rrf)

                aluout_reg  <= {status_reg[0],aluinp1_reg[7:1]};

                // Swap nibbles

        else if (inst_swapf)

                aluout_reg <= {aluinp1_reg[3:0],aluinp1_reg[7:4]};

                // Logical inversion

        else if (inst_comf)

                aluout_reg  <= ~aluinp1_reg;

                // Logical AND, bit clear/bit test

        else if (   inst_andlw || inst_andwf || inst_bcf || inst_btfsc

                 || inst_btfss)

                aluout_reg  <= (aluinp1_reg & aluinp2_reg);

                // Logical OR, bit set

        else if (inst_bsf || inst_iorlw || inst_iorwf)

                aluout_reg  <= (aluinp1_reg | aluinp2_reg);

                // Logical XOR

        else if (inst_xorlw || inst_xorwf)