using System;

public class Program {

    public static void Main() {

        var rf = new RegisterFile();

        rf["ah"] = 0x12;

        rf["al"] = 0x34;

        Console.WriteLine($"ax: {rf["ax"]:x4}");

        rf["ax"] = 0x5678;

        Console.WriteLine($"ah, al: {rf["ah"]:x2},{rf["al"]:x2}");

    }

}

​

class RegisterFile {

    ushort ax, bx, cx, dx;

    public ushort this[string name] {

        get {

            switch(name) {

                case "ax": return ax;

                case "bx": return bx;

                case "cx": return cx;

                case "dx": return dx;

                case "ah": return (byte) (ax >> 8);

                case "al": return (byte) (ax & 0xff);

                // ... and so on ...

                default: throw new ArgumentException($"Unknown register {name}.");

            }

        }

        set {

            switch(name) {

                case "ax": ax = value; break;

                case "bx": bx = value; break;

                case "cx": cx = value; break;

                case "dx": dx = value; break;

                case "ah": ax = (ushort) ((ax & 0xff) | (value << 8)); break;   // should check if value < 256, omitted here

                case "al": ax = (ushort) ((ax & 0xff00) | value); break;    // ditto

                // ... and so on ...

                default: throw new ArgumentException($"Unknown register {name}.");

            }

        }

    }

}