Thingamajig/emulator.pas

program Emulator;

{$MODE OBJFPC}

uses Crt{$ifdef printer}, Printer{$endif};

const
    IO = $ffff;

var
    Hlt: boolean; //Halt flag
    Op: 0 .. $f; //Opcode
    X, Y: 0 .. 3; //Register arguments
    Addr, IP, RP: word; //Address argument and instruction and return pointers
    R: array [0 .. 3] of byte; //General-purpose registers
    Mem: array [0 .. $ffef] of byte; //Memory
    Prog{$ifdef printer}, Prn{$endif}: file of byte; //Program file and printer
    Ch: ansichar; //Character for input and output

begin

    //Initialise the halt flag and the pointers
    Hlt := false;
    IP := 0;
    RP := $fff0;

    //Read a program file and check for errors
    if ParamCount <> 1 then begin
        writeln ('Usage: emulator program');
        halt;
    end;
    {$i-}
    assign (Prog, ParamStr (1));
    reset (Prog);
    {$i+}
    if IOResult <> 0 then begin
        writeln ('Error: program file cannot be read from');
        halt;
    end;
    repeat
        read (Prog, Mem [IP]);
        IP := IP + 1;
    until (eof (Prog)) or (IP = $fff0);
    if IP = $fff0 then begin
        writeln ('Error: memory overflow');
        halt;
    end;

    //Reinitialise the instruction pointer
    IP := 0;

    //Begin the main loop
    while Hlt = false do begin

        //Fetch the instruction and increment the instruction pointer
        //Opcode
        Op := Mem [IP] and $f0 shr 4;
        //Register arguments
        X := Mem [IP] and $c shr 2;
        Y := Mem [IP] and 3;
        IP := IP + 1;
        if IP > $ffef then begin
            writeln ('Error: illegal instruction pointer value');
            halt;
        end;
        //Address argument
        if Op >= $a then begin
            //High byte
            Addr := Mem [IP];
            Addr := Addr shl 8;
            IP := IP + 1;
            if IP > $ffef then begin
                writeln ('Error: illegal instruction pointer value');
                halt;
            end;
            //Low byte
            Addr := Addr + Mem [IP];
            IP := IP + 1;
            if IP > $ffef then begin
                writeln ('Error: illegal instruction pointer value');
                halt;
            end;
        end;

        //Decode and execute the instruction
        //Halt
        if Op = 0 then Hlt := true
        //Ret
        else if Op = 1 then begin
            //High byte of the return address
            IP := Mem [RP];
            IP := IP shl 8;
            RP := RP + 1;
            if RP > $fff0 then begin
                writeln ('Error: illegal return pointer value');
                halt;
            end;
            //Low byte of the return address
            IP := IP + Mem [RP];
            RP := RP + 1;
            if RP > $fff0 then begin
                writeln ('Error: illegal return pointer value');
                halt;
            end;
        end
        //Shl
        else if Op = 2 then R [X] := R [X] shl 1
        //Shr
        else if Op = 3 then R [X] := R [X] shr 1
        //Rol
        else if Op = 4 then R [X] := RolByte (R [X])
        //Ror
        else if Op = 5 then R [X] := RorByte (R [X])
        //Nand
        else if Op = 6 then R [X] := not (R [X] and R [Y])
        //And
        else if Op = 7 then R [X] := R [X] and R [Y]
        //Or
        else if Op = 8 then R [X] := R [X] or R [Y]
        //Xor
        else if Op = 9 then R [X] := R [X] xor R [Y]
        //Load
        else if Op = $a then begin
            //Input
            if Addr = IO then begin
                Ch := ReadKey;
                write (Ch);
                R [X] := byte (Ch);
            end
            //Regular load
            else R [X] := Mem [Addr];
        end
        //Store
        else if Op = $b then begin
            //Output
            if Addr = IO then begin
                Ch := ansichar (R [X]);
                write (Ch);
            end
            //Printer
            {$ifdef printer}
            else if Addr = $fffe then begin
                assign (Prn, '/dev/usb/lp0');
                try
                    rewrite (Prn);
                    write (Prn, R [X]);
                    close (Prn);
                except
                end;
            end
            {$endif}
            //Regular store
            else Mem [Addr] := R [X];
        end
        //Breq
        else if Op = $c then begin
            if R [X] = R [Y] then IP := Addr;
        end
        //Brneq
        else if Op = $d then begin
            if R [X] <> R [Y] then IP := Addr;
        end
        //Cleq
        else if Op = $e then begin
            if R [X] = R [Y] then begin
                //Low byte of the return address
                RP := RP - 1;
                if RP > $fff0 then begin
                    writeln ('Error: illegal return pointer value');
                    halt;
                end;
                Mem [RP] := IP and $ff;
                //High byte of the return address
                RP := RP - 1;
                if RP > $fff0 then begin
                    writeln ('Error: illegal return pointer value');
                    halt;
                end;
                Mem [RP] := IP shr 8;
                //Call
                IP := Addr;
                if IP > $ffef then begin
                    writeln ('Error: illegal instruction pointer value');
                    halt;
                end;
            end;
        end
        //Clneq
        else if Op = $f then begin
            if R [X] <> R [Y] then begin
                //Low byte of the return address
                RP := RP - 1;
                if RP > $fff0 then begin
                    writeln ('Error: illegal return pointer value');
                    halt;
                end;
                Mem [RP] := IP and $ff;
                //High byte of the return address
                RP := RP - 1;
                if RP > $fff0 then begin
                    writeln ('Error: illegal return pointer value');
                    halt;
                end;
                Mem [RP] := IP shr 8;
                //Call
                IP := Addr;
                if IP > $ffef then begin
                    writeln ('Error: illegal instruction pointer value');
                    halt;
                end;
            end;
        end;

    end;

end.