livingcomputermuseum.ContrAlto/Contralto/CPU/Shifter.cs

/*  
    This file is part of ContrAlto.

    ContrAlto is free software: you can redistribute it and/or modify
    it under the terms of the GNU Affero General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.

    ContrAlto 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 Affero General Public License for more details.

    You should have received a copy of the GNU Affero General Public License
    along with ContrAlto.  If not, see <http://www.gnu.org/licenses/>.
*/

using System;

namespace Contralto.CPU
{
    public enum ShifterOp
    {        
        None,
        ShiftLeft,
        ShiftRight,
        RotateLeft,             
    }

    public enum ShifterModifier
    {
        None,
        Magic,
        DNS,
    }

    // NOTE: FOR NOVA (NOVEL) SHIFTS (from aug '76 manual):
    // The emulator has two additional bits of state, the SKIP and CARRY flip flops.CARRY is identical
    // to the Nova carry bit, and is set or cleared as appropriate when the DNS+- (do Nova shifts)
    // function is executed.  DNS also addresses R from (R[3 - 4] XOR 3), and sets the SKIP flip flop if 
    // appropriate.The PC is incremented by 1 at the beginning of the next emulated instruction if
    // SKIP is set, using ALUF DB.  IR<- clears SKIP.
    public static class Shifter
    {
        static Shifter()
        {
            Reset();
        }

        public static void Reset()
        {
            _op = ShifterOp.None;            
            _output = 0;
            _modifier = ShifterModifier.None;
            _dnsCarry = 0;
        }

        /// <summary>
        /// Returns the result of the last Shifter operation (via DoOperation).
        /// </summary>
        public static ushort Output
        {
            get { return _output; }
            set { _output = value; }
        }

        public static ShifterOp Op
        {
            get { return _op; }
        }

        /// <summary>
        /// Returns the last DNS-style Carry bit from the last operation (via DoOperation),
        /// or sets the carry-in for the next DNS-style shift.
        /// </summary>
        public static int DNSCarry
        {
            get { return _dnsCarry; }
            set
            {
                // Sanity check
                if (value != 0 && value != 1)
                {
                    throw new InvalidOperationException("Invalid DNSCarry value.");
                }
                _dnsCarry = value;
            }
        }

        public static void SetOperation(ShifterOp op)
        {
            _op = op;            
        }

        public static void SetModifier(ShifterModifier mod)
        {
            _modifier = mod;
        }        

        /// <summary>
        /// Does the last specified operation to the specified inputs; the result
        /// can be read from Output.
        /// </summary>
        /// <param name="input">Normal input to be shifted</param>
        /// <param name="t">CPU t register, used for MAGIC shifts only</param>        
        public static ushort DoOperation(ushort input, ushort t)
        {           
            switch(_op)
            {               
                case ShifterOp.None:
                    _output = input;
                    break;

                case ShifterOp.ShiftLeft:
                    _output = (ushort)(input << 1);                   

                    switch (_modifier)
                    {
                        case ShifterModifier.Magic:
                            // "MAGIC places the high order bit of T into the low order bit of the
                            // shifter output on left shifts..."
                            _output |= (ushort)((t & 0x8000) >> 15);
                            break;

                        case ShifterModifier.DNS:
                            //
                            // "Rotate the 17 input bits left by one bit.  This has the effect of rotating
                            // bit 0 left into the carry position and the carry bit into bit 15."
                            //

                            // Put input carry into bit 15.
                            _output |= (ushort)(_dnsCarry);

                            // update carry
                            _dnsCarry = ((input & 0x8000) >> 15);
                            break;
                    }
                    
                    break;

                case ShifterOp.ShiftRight:
                    _output = (ushort)(input >> 1);                    

                    switch (_modifier)
                    {
                        case ShifterModifier.Magic:                    
                            _output |= (ushort)((t & 0x1) << 15);
                            break;

                        case ShifterModifier.DNS:
                            //
                            // "Rotate the 17 bits right by one bit.  Bit 15 is rotated into the carry position
                            // and the carry bit into bit 0."
                            //

                            // Put input carry into bit 0.
                            _output |= (ushort)(_dnsCarry << 15);

                            // update carry
                            _dnsCarry = input & 0x1;
                            break;
                    }
                    break;

                case ShifterOp.RotateLeft:                                        
                    //
                    // "Swap the 8-bit halves of the 16-bit result.  The carry is not affected."
                    // NOTE:  The hardware reference (Section 2) seems to indicate that L LCY 8 is modified by MAGIC and/or DNS,
                    // but this does not appear to actually be the case.  Nothing in the documentation elsewhere, the microcode,
                    // or the schematics indicates that L LCY 8 ever does anything other than a simple swap.
                    //
                    _output = (ushort)(((input & 0xff00) >> 8) | ((input & 0x00ff) << 8));                                        
                    break;                   

                default:
                    throw new InvalidOperationException(String.Format("Unhandled shift operation {0}", _op));
            }            

            return _output;
        }

        private static ShifterOp _op;
        private static ushort _output;        
        private static ShifterModifier _modifier;
        private static int _dnsCarry;
    }
}