/*-------------------
minisim.v included with the permission of Kluwer Academic Publishers

Copyright (r)

THE VERILOG(tm) HARDWARE DESCRIPTION LANGUAGE, Second Edition by
Donald E. Thomas and Philip Moorby, Copyright 1995 by Kluwer Academic
Publishers.  Used by permission.

Ordering information:
THE VERILOG(tm) HARDWARE DESCRIPTION LANGUAGE, Second Edition by
Donald E. Thomas and Philip R. Moorby, ISBN: 0-7923-9523-9, 304pp.,
 cloth, $98.00

Kluwer Academic Publishers           Phone 617-871-6600
101 Philip Drive                     fax 617-871-6528
Norwell, MA 02061                    email: kluwer@world.std.com

Disk included:
The Verilog(tm) Hardware Description Language, Second Edition includes
a disk that contains a DOS version of the Veriwell(tm) Verilog simulator
that can simulator up to 1000 lines of Verilog or 2000 total source lines
as well as other examples from the book. The simulator can also be used
to solve the exercices.
--------------------*/

//THE MINISIM EXAMPLE
module miniSim;

// element types being modeled
`define Nand 0
`define DEdgeFF 1
`define Wire 2

// literal values with strength:
//   format is 8 0-strength bits in decreasing strength order
//   followed by 8 1-strength bits in decreasing strength order
`define Strong0 16'b01000000_00000000
`define Strong1 16'b00000000_01000000
`define StrongX 16'b01111111_01111111
`define Pull0   16'b00100000_00000000
`define Pull1   16'b00000000_00100000
`define Highz0  16'b00000001_00000000
`define Highz1  16'b00000000_00000001

// three-valued logic set
`define Val0 3'd0
`define Val1 3'd1
`define ValX 3'd2

parameter
    DebugFlags =               'b11000, //set to 1 for message
//                               |||||
//  loading                   <--+||||
//  event value changes      <----+|||
//  wire calculation        <------+||
//  evaluation             <--------+|
//  scheduling           <-----------+

    IndexSize = 16,     //maximum size for index pointers
    MaxElements = 50,   //maximum number of elements
    TypeSize = 12;      //maximum number of types

reg [IndexSize-1:0]
    eventElement,             //output value change element
    evalElement,              //evaluation element on fanout
    fo0Index[1:MaxElements],  //first fanout index of eventElement
    fo1Index[1:MaxElements],  //second fanout index of eventElement
    currentList,              //current time scheduled event list
    nextList,                 //unit delay scheduled event list
    schedList[1:MaxElements]; //scheduled event list index
reg [TypeSize-1:0]
    eleType[1:MaxElements]; //element type
reg
    fo0TermNum[1:MaxElements],   //first fanout input terminal number
    fo1TermNum[1:MaxElements],   //second fanout input terminal number
    schedPresent[1:MaxElements]; //element is in scheduled event list flags
reg [15:0]
    eleStrength[1:MaxElements], //element strength indication
    outVal[1:MaxElements],      //element output value
    in0Val[1:MaxElements],      //element first input value
    in1Val[1:MaxElements],      //element second input value
    in0, in1, out, oldIn0;      //temporary value storage

integer pattern, simTime; //time keepers

initial
  begin
    // initialize variables
    pattern = 0;
    currentList = 0;
    nextList = 0;

    $display("Loading toggle circuit");
    loadElement(1, `DEdgeFF, 0, `Strong1,0,0, 4,0,0,0);
    loadElement(2, `DEdgeFF, 0, `Strong1,0,0, 3,0,0,0); 
    loadElement(3, `Nand, (`Strong0|`Strong1),
        `Strong0,`Strong1,`Strong1, 4,0,1,0); 
    loadElement(4, `DEdgeFF, (`Strong0|`Strong1),
        `Strong1,`Strong1,`Strong0, 3,0,1,0); 

    // apply stimulus and simulate
    $display("Applying 2 clocks to input element 1");
    applyClock(2, 1);
    $display("Changing element 2 to value 0 and applying 1 clock");
    setupStim(2, `Strong0);
    applyClock(1, 1);

    $display("\nLoading open-collector and pullup circuit");
    loadElement(1, `DEdgeFF, 0, `Strong1,0,0, 3,0,0,0);
    loadElement(2, `DEdgeFF, 0, `Strong0,0,0, 4,0,0,0);
    loadElement(3, `Nand, (`Strong0|`Highz1),
        `Strong0,`Strong1,`Strong1, 5,0,0,0);
    loadElement(4, `Nand, (`Strong0|`Highz1),
        `Highz1,`Strong0,`Strong1, 5,0,1,0);
    loadElement(5, `Wire, 0,
        `Strong0,`Strong0,`Highz1, 7,0,1,0);
    loadElement(6, `DEdgeFF, 0, `Pull1,0,0, 7,0,0,0);
    loadElement(7, `Wire, 0,
        `Strong0,`Pull1,`Strong0, 0,0,0,0);

    // apply stimulus and simulate
    $display("Changing element 1 to value 0");
    pattern = pattern + 1;
    setupStim(1, `Strong0);
    executeEvents;
    $display("Changing element 2 to value 1");
    pattern = pattern + 1;
    setupStim(2, `Strong1);
    executeEvents;
    $display("Changing element 2 to value X");
    pattern = pattern + 1;
    setupStim(2, `StrongX);
    executeEvents;
  end

// Initialize data structure for a given element.
task loadElement;
input [IndexSize-1:0] loadAtIndex; //element index being loaded
input [TypeSize-1:0] type;         //type of element
input [15:0] strengthCoercion;     //strength specification of element
input [15:0] oVal, i0Val, i1Val;   //output and input values
input [IndexSize-1:0] fo0, fo1;    //fanout element indexes
input fo0Term, fo1Term;            //fanout element input terminal indicators
  begin
    if (DebugFlags[4])
        $display(
            "Loading element %0d, type %0s, with initial value %s(%b_%b)",
            loadAtIndex, typeString(type),
            valString(oVal), oVal[15:8], oVal[7:0]);
    eleType[loadAtIndex] = type;
    eleStrength[loadAtIndex] = strengthCoercion;
    outVal[loadAtIndex] = oVal;
    in0Val[loadAtIndex] = i0Val;
    in1Val[loadAtIndex] = i1Val;
    fo0Index[loadAtIndex] = fo0;
    fo1Index[loadAtIndex] = fo1;
    fo0TermNum[loadAtIndex] = fo0Term;
    fo1TermNum[loadAtIndex] = fo1Term;
    schedPresent[loadAtIndex] = 0;
  end
endtask

// Given a type number, return a type string
function [32*8:1] typeString;
input [TypeSize-1:0] type;
    case (type)
      `Nand: typeString = "Nand";
      `DEdgeFF: typeString = "DEdgeFF";
      `Wire: typeString = "Wire";
      default: typeString = "*** Unknown element type";
    endcase
endfunction

// Setup a value change on an element.
task setupStim;
input [IndexSize-1:0] vcElement; //element index
input [15:0] newVal;           //new element value
  begin
    if (! schedPresent[vcElement])
      begin
        schedList[vcElement] = currentList;
        currentList = vcElement;
        schedPresent[vcElement] = 1;
      end
    outVal[vcElement] = newVal;
  end
endtask

// Setup and simulate a given number of clock pulses to a given element.
task applyClock;
input [7:0] nClocks;
input [IndexSize-1:0] vcElement;
    repeat(nClocks)
      begin
        pattern = pattern + 1;
        setupStim(vcElement, `Strong0);
        executeEvents;
        pattern = pattern + 1;
        setupStim(vcElement, `Strong1);
        executeEvents;
      end
endtask

// Execute all events in the current event list.
// Then move the events in the next event list to the current event
// list and loop back to execute these events. Continue this loop
// until no more events to execute.
// For each event executed, evaluate the two fanout elements if present.
task executeEvents;
reg [15:0] newVal;
  begin
    simTime = 0;
    while (currentList)
      begin
        eventElement = currentList;
        currentList = schedList[eventElement];
        schedPresent[eventElement] = 0;
        newVal = outVal[eventElement];
        if (DebugFlags[3])
            $display(
                "At %0d,%0d Element %0d, type %0s, changes to %s(%b_%b)",
                pattern, simTime,
                eventElement, typeString(eleType[eventElement]),
                valString(newVal), newVal[15:8], newVal[7:0]);
        if (fo0Index[eventElement]) evalFo(0);
        if (fo1Index[eventElement]) evalFo(1);
        if (! currentList) // if empty move to next time unit
          begin
            currentList = nextList;
            nextList = 0;
            simTime = simTime + 1;
          end
      end
  end
endtask

// Evaluate a fanout element by testing its type and calling the
// appropriate evaluation routine.
task evalFo;
input fanout; //first or second fanout indicator
  begin
    evalElement = fanout ? fo1Index[eventElement] : 
                           fo0Index[eventElement];
    if (DebugFlags[1])
        $display("Evaluating Element %0d type is %0s",
            evalElement, typeString(eleType[evalElement]));
    case (eleType[evalElement])
      `Nand: evalNand(fanout);
      `DEdgeFF: evalDEdgeFF(fanout);
      `Wire: evalWire(fanout);
    endcase
  end
endtask

// Store output value of event element into
// input value of evaluation element.
task storeInVal;
input fanout; //first or second fanout indicator
  begin
    // store new input value
    if (fanout ? fo1TermNum[eventElement] : fo0TermNum[eventElement])
        in1Val[evalElement] = outVal[eventElement];
    else
        in0Val[evalElement] = outVal[eventElement];
  end
endtask

// Convert a given full strength value to three-valued logic (0, 1 or X)
function [1:0] log3;
input [15:0] inVal;
    casez (inVal)
      16'b00000000_00000000: log3 = `ValX;
      16'b???????0_00000000: log3 = `Val0;
      16'b00000000_???????0: log3 = `Val1;
      default:               log3 = `ValX;
    endcase
endfunction

// Convert a given full strength value to four-valued logic (0, 1, X or Z),
// returning a 1 character string
function [8:1] valString;
input [15:0] inVal;
    case (log3(inVal))
      `Val0: valString = "0";
      `Val1: valString = "1";
      `ValX: valString = (inVal & 16'b11111110_11111110) ? "X" : "Z";
    endcase
endfunction

// Coerce a three-valued logic output value to a full output strength value
// for the scheduling of the evaluation element
function [15:0] strengthVal;
input [1:0] logVal;
    case (logVal)
      `Val0: strengthVal = eleStrength[evalElement] & 16'b11111111_00000000;
      `Val1: strengthVal = eleStrength[evalElement] & 16'b00000000_11111111;
      `ValX: strengthVal = fillBits(eleStrength[evalElement]);
    endcase
endfunction

// Given an incomplete strength value, fill the missing strength bits.
// The filling is only necessary when the value is unknown.
function [15:0] fillBits;
input [15:0] val;
  begin
    fillBits = val;
    if (log3(val) == `ValX)
      begin
        casez (val)
  16'b1???????_????????: fillBits = fillBits | 16'b11111111_00000001;
  16'b01??????_????????: fillBits = fillBits | 16'b01111111_00000001;
  16'b001?????_????????: fillBits = fillBits | 16'b00111111_00000001;
  16'b0001????_????????: fillBits = fillBits | 16'b00011111_00000001;
  16'b00001???_????????: fillBits = fillBits | 16'b00001111_00000001;
  16'b000001??_????????: fillBits = fillBits | 16'b00000111_00000001;
  16'b0000001?_????????: fillBits = fillBits | 16'b00000011_00000001;
        endcase
        casez (val)
  16'b????????_1???????: fillBits = fillBits | 16'b00000001_11111111;
  16'b????????_01??????: fillBits = fillBits | 16'b00000001_01111111;
  16'b????????_001?????: fillBits = fillBits | 16'b00000001_00111111;
  16'b????????_0001????: fillBits = fillBits | 16'b00000001_00011111;
  16'b????????_00001???: fillBits = fillBits | 16'b00000001_00001111;
  16'b????????_000001??: fillBits = fillBits | 16'b00000001_00000111;
  16'b????????_0000001?: fillBits = fillBits | 16'b00000001_00000011;
       endcase
     end
  end
endfunction

// Evaluate a 'Nand' gate primitive.
task evalNand;
input fanout; //first or second fanout indicator
  begin
    storeInVal(fanout);
    // calculate new output value
    in0 = log3(in0Val[evalElement]);
    in1 = log3(in1Val[evalElement]);
    out = ((in0 == `Val0) || (in1 == `Val0)) ?
        strengthVal(`Val1) :
        ((in0 == `ValX) || (in1 == `ValX)) ?
            strengthVal(`ValX):
            strengthVal(`Val0);
    // schedule if output value is different
    if (out != outVal[evalElement])
        schedule(out);
  end
endtask

// Evaluate a D positive edge-triggered flip flop
task evalDEdgeFF;
input fanout; //first or second fanout indicator
    // check value change is on clock input
    if (fanout ? (fo1TermNum[eventElement] == 0) :
                 (fo0TermNum[eventElement] == 0))
      begin
        // get old clock value
        oldIn0 = log3(in0Val[evalElement]);
        storeInVal(fanout);
        in0 = log3(in0Val[evalElement]);
        // test for positive edge on clock input
        if ((oldIn0 == `Val0) && (in0 == `Val1))
          begin
            out = strengthVal(log3(in1Val[evalElement]));
            if (out != outVal[evalElement])
              schedule(out);
          end
      end
    else
        storeInVal(fanout); // store data input value
endtask

// Evaluate a wire with full strength values
task evalWire;
input fanout;
reg [7:0] mask;
  begin
    storeInVal(fanout);

    in0 = in0Val[evalElement];
    in1 = in1Val[evalElement];
    mask = getMask(in0[15:8]) & getMask(in0[7:0]) &
           getMask(in1[15:8]) & getMask(in1[7:0]);
    out = fillBits((in0 | in1) & {mask, mask});

    if (out != outVal[evalElement])
        schedule(out);

    if (DebugFlags[2])
        $display("in0 = %b_%b\nin1 = %b_%b\nmask= %b %b\nout = %b_%b",
            in0[15:8],in0[7:0], in1[15:8],in1[7:0],
            mask,mask, out[15:8],out[7:0]);
  end
endtask

// Given either a 0-strength or 1-strength half of a strength value
// return a masking pattern for use in a wire evaluation.
function [7:0] getMask;
input [7:0] halfVal; //half a full strength value
    casez (halfVal)
      8'b???????1: getMask = 8'b11111111;
      8'b??????10: getMask = 8'b11111110;
      8'b?????100: getMask = 8'b11111100;
      8'b????1000: getMask = 8'b11111000;
      8'b???10000: getMask = 8'b11110000;
      8'b??100000: getMask = 8'b11100000;
      8'b?1000000: getMask = 8'b11000000;
      8'b10000000: getMask = 8'b10000000;
      8'b00000000: getMask = 8'b11111111;
    endcase
endfunction

// Schedule the evaluation element to change to a new value.
// If the element is already scheduled then just insert the new value.
task schedule;
input [15:0] newVal; // new value to change to
  begin
    if (DebugFlags[0])
        $display(
            "Element %0d, type %0s, scheduled to change to %s(%b_%b)",
            evalElement, typeString(eleType[evalElement]),
            valString(newVal), newVal[15:8], newVal[7:0]);
    if (! schedPresent[evalElement])
      begin
        schedList[evalElement] = nextList;
        schedPresent[evalElement] = 1;
        nextList = evalElement;
      end
    outVal[evalElement] = newVal;
  end
endtask
endmodule

/*******************  PLEASE NOTE:  ************************************

The values printed out in the book section 7.1.3 have the wrong
format. The 0 and 1 strength bits need to be swapped around. For
example, the first value printed in the book should read
1(00000000_01000000) and not 1(0100000_00000000). The value format you
get from running the above minisim description through Verilog should
be correct.

Here is a listing of the results:

Loading toggle circuit
Loading element 1, type DEdgeFF, with initial value 1(00000000_01000000)
Loading element 2, type DEdgeFF, with initial value 1(00000000_01000000)
Loading element 3, type Nand, with initial value 0(01000000_00000000)
Loading element 4, type DEdgeFF, with initial value 1(00000000_01000000)
Applying 2 clocks to input element 1
At 1,0 Element 1, type DEdgeFF, changes to 0(01000000_00000000)
At 2,0 Element 1, type DEdgeFF, changes to 1(00000000_01000000)
At 2,1 Element 4, type DEdgeFF, changes to 0(01000000_00000000)
At 2,2 Element 3, type Nand, changes to 1(00000000_01000000)
At 3,0 Element 1, type DEdgeFF, changes to 0(01000000_00000000)
At 4,0 Element 1, type DEdgeFF, changes to 1(00000000_01000000)
At 4,1 Element 4, type DEdgeFF, changes to 1(00000000_01000000)
At 4,2 Element 3, type Nand, changes to 0(01000000_00000000)
Changing element 2 to value 0 and applying 1 clock
At 5,0 Element 1, type DEdgeFF, changes to 0(01000000_00000000)
At 5,0 Element 2, type DEdgeFF, changes to 0(01000000_00000000)
At 5,1 Element 3, type Nand, changes to 1(00000000_01000000)
At 6,0 Element 1, type DEdgeFF, changes to 1(00000000_01000000)

Loading open-collector and pullup circuit
Loading element 1, type DEdgeFF, with initial value 1(00000000_01000000)
Loading element 2, type DEdgeFF, with initial value 0(01000000_00000000)
Loading element 3, type Nand, with initial value 0(01000000_00000000)
Loading element 4, type Nand, with initial value Z(00000000_00000001)
Loading element 5, type Wire, with initial value 0(01000000_00000000)
Loading element 6, type DEdgeFF, with initial value 1(00000000_00100000)
Loading element 7, type Wire, with initial value 0(01000000_00000000)
Changing element 1 to value 0
At 7,0 Element 1, type DEdgeFF, changes to 0(01000000_00000000)
At 7,1 Element 3, type Nand, changes to Z(00000000_00000001)
At 7,2 Element 5, type Wire, changes to Z(00000000_00000001)
At 7,3 Element 7, type Wire, changes to 1(00000000_00100000)
Changing element 2 to value 1
At 8,0 Element 2, type DEdgeFF, changes to 1(00000000_01000000)
At 8,1 Element 4, type Nand, changes to 0(01000000_00000000)
At 8,2 Element 5, type Wire, changes to 0(01000000_00000000)
At 8,3 Element 7, type Wire, changes to 0(01000000_00000000)
Changing element 2 to value X
At 9,0 Element 2, type DEdgeFF, changes to X(01111111_01111111)
At 9,1 Element 4, type Nand, changes to X(01111111_00000001)
At 9,2 Element 5, type Wire, changes to X(01111111_00000001)
At 9,3 Element 7, type Wire, changes to X(01111111_00111111)
*****************************************************************/