From 6d14b9a5bf448f9baae654b302245fc9a86414d4 Mon Sep 17 00:00:00 2001
From: CrazyEttin <>
Date: Fri, 26 Aug 2022 14:36:42 +0300
Subject: [PATCH] Remove limits on the pointer values, specify access to unused
 addresses and modify instruction fetching and calling to take into account
 unused addresses and memory mapped devices in the emulator, and write a
 separate section for the emulator in the readme

---
 emulator.pas | 476 ++++++++++++++++++++++++---------------------------
 readme.md    |  52 +++---
 2 files changed, 247 insertions(+), 281 deletions(-)

diff --git a/emulator.pas b/emulator.pas
index 6aeed38..fd418c7 100644
--- a/emulator.pas
+++ b/emulator.pas
@@ -16,6 +16,7 @@ type
 
 const
     IO = $ffff;
+    LastRAM = $ffef;
 
 var
     Hlt, ASCII, Verbose: boolean; //Halt, ASCII, and verbose flags
@@ -23,7 +24,7 @@ var
     X, Y: 0 .. 3; //Register arguments
     Addr, IP, RP: word; //Immediate or address argument and instruction and return pointers
     R: array [0 .. 3] of byte; //General-purpose registers
-    Mem: array [0 .. $ffef] of byte; //Memory
+    Mem: array [0 .. LastRAM] of byte; //Random access memory
     Prog{$ifdef printer}, Prn{$endif}{$ifdef tape}, TapeIn, TapeOut{$endif}: file of byte; //Program file, line printer, and tape reader and punch tapes
     {$ifdef tape}
     Reader, Punch: Tape; //States of the tape reader and punch
@@ -31,6 +32,7 @@ var
     {$endif}
     Ch, Scan: ansichar; //Character for input and output and scancode for non-ASCII keys
     IC, LFX: integer; //Instruction counter for CPU speed
+    Fetched: byte; //Fetched byte
 
 //Terminal output
 procedure Output;
@@ -60,12 +62,204 @@ begin
     IC := 0;
 end;
 
+//Load a byte from memory
+function LoadByte (W: word): byte;
+var
+    B: byte;
+begin
+    //Terminal input
+    if W = IO then begin
+        wait;
+        //Read a keypress
+        repeat
+        Ch := ReadKey;
+            //Check for non-ASCII keys
+            if Ch = ansichar (0) then begin
+                //Non-ASCII
+                if keypressed then begin
+                    Scan := ReadKey;
+                    //The delete key inserts the delete character
+                    if Scan = ansichar ($53) then begin
+                        Ch := ansichar ($7f);
+                        ASCII := true;
+                    end
+                    //Unused function keys insert a null
+                    else ASCII := true;
+                end
+                //Null
+                else ASCII := true;
+            end
+            //Other ASCII
+            else ASCII := true;
+        until ASCII = true;
+        //Bodge for the home and end keys
+        if Ch = ansichar ($37) then begin
+            if keypressed then begin
+                Scan := ReadKey;
+                Scan := ReadKey;
+                Scan := ReadKey;
+                Ch := ansichar (0);
+            end;
+        end
+        else if Ch = ansichar ($38) then begin
+            if keypressed then begin
+                Scan := ReadKey;
+                Scan := ReadKey;
+                Scan := ReadKey;
+                Ch := ansichar (0);
+            end;
+        end;
+        //Process the keypress
+        Output; //Local echo
+        B := byte (Ch);
+    end
+    //Tape reader
+    {$ifdef tape}
+    else if W = $fffd then begin
+        wait;
+        assign (State, ExpandFileName ('~/.tapes.thingamajig'));
+        //Check the reader state
+        if FileExists (ExpandFileName ('~/.tapes.thingamajig')) then begin
+            try
+                reset (State);
+                read (State, Reader);
+                read (State, Punch);
+                close (State);
+            except
+            end;
+        end;
+        //Read
+        assign (TapeIn, Reader.Path);
+        try
+            reset (TapeIn);
+            seek (TapeIn, Reader.Pos);
+            read (TapeIn, B);
+            close (TapeIn);
+            Reader.Pos := Reader.Pos + 1;
+        except
+            B := $ff;
+        end;
+        //Save the reader state
+        if FileExists (ExpandFileName ('~/.tapes.thingamajig')) then begin
+            try
+                rewrite (State);
+                write (State, Reader);
+                write (State, Punch);
+                close (State);
+            except
+            end;
+        end;
+    end
+    {$endif}
+    //Unused addresses
+    else if W > LastRAM then B := 0
+    //Regular load
+    else B := Mem [W];
+    //Result
+    LoadByte := B;
+end;
+
+procedure StoreByte (W: word; B: byte);
+begin
+    //Terminal output
+    if W = IO then begin
+        wait;
+        Ch := ansichar (B);
+        Output;
+    end
+    //Printer
+    {$ifdef printer}
+    else if W = $fffe then begin
+        wait;
+        assign (Prn, '/dev/usb/lp0');
+        try
+            rewrite (Prn);
+            write (Prn, B);
+            close (Prn);
+        except
+        end;
+    end
+    {$endif}
+    //Tape punch
+    {$ifdef tape}
+    else if W = $fffd then begin
+        wait;
+        assign (State, ExpandFileName ('~/.tapes.thingamajig'));
+        //Check the punch state
+        if FileExists (ExpandFileName ('~/.tapes.thingamajig')) then begin
+            try
+                reset (State);
+                read (State, Reader);
+                read (State, Punch);
+                close (State);
+            except
+            end;
+        end;
+        //Punch
+        if Punch.Path <> '' then begin
+            assign (TapeOut, Punch.Path);
+            if FileExists (Punch.Path) = false then begin
+                try
+                    rewrite (TapeOut);
+                    write (TapeOut, B);
+                    close (TapeOut);
+                    Punch.Reset := false;
+                except
+                end;
+            end
+            else if Punch.Reset then begin
+                try
+                    rewrite (TapeOut);
+                    write (TapeOut, B);
+                    close (TapeOut);
+                    Punch.Reset := false;
+                except
+                end;
+            end
+            else begin
+                try
+                    reset (TapeOut);
+                    seek (TapeOut, FileSize (TapeOut));
+                    write (TapeOut, B);
+                    close (TapeOut);
+                except
+                end;
+            end;
+        end;
+        //Save the punch state
+        if FileExists (ExpandFileName ('~/.tapes.thingamajig')) then begin
+            try
+                rewrite (State);
+                write (State, Reader);
+                write (State, Punch);
+                close (State);
+            except
+            end;
+        end;
+    end
+    {$endif}
+    //Regular store
+    else if W <= LastRAM then Mem [W] := B;
+end;
+
+procedure Call;
+begin
+    //Low byte of the return address
+    RP := RP - 1;
+    StoreByte (RP, IP and $ff);
+    //High byte of the return address
+    RP := RP - 1;
+    StoreByte (RP, IP shr 8);
+    //Call
+    IP := Addr;
+end;
+
 begin
 
     //Initialise the halt flag, the pointers, and the instruction counter
     Hlt := false;
     IP := 0;
-    RP := $fff0;
+    RP := LastRAM + 1;
     IC := 0;
 
     //Initialise the tape reader and punch
@@ -103,6 +297,10 @@ begin
     if Verbose = true then assign (Prog, ParamStr (2))
     else assign (Prog, ParamStr (1));
     reset (Prog);
+    if FileSize (Prog) > LastRAM + 1 then begin
+        writeln ('Error: program size cannot exceed ', LastRam + 1, ' bytes');
+        halt;
+    end;
     {$i+}
     if IOResult <> 0 then begin
         writeln ('Error: program file cannot be read from');
@@ -111,11 +309,7 @@ begin
     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;
+    until (eof (Prog));
 
     //Reinitialise the instruction pointer
     IP := 0;
@@ -127,45 +321,34 @@ begin
         if Verbose = true then writeln (StdErr, 'IR: ', IntToHex (Op, 1), IntToHex (Regs, 1), IntToHex (Addr, 4), '; IP: ', IntToHex (IP, 4), ', RP: ', IntToHex (RP, 4), '; R0: ', IntToHex (R[0], 2), ', R1: ', IntToHex (R[1], 2), ', R2: ', IntToHex (R[2], 2), ', R3: ', IntToHex (R[3], 2), ansichar ($d));
 
         //Fetch the instruction and increment the instruction pointer
-        //Opcode
-        Op := Mem [IP] and $f0 shr 4;
-        //Register arguments
-        Regs := Mem [IP] and $f;
-        X := Mem [IP] and $c shr 2;
-        Y := Mem [IP] and 3;
+        //Fetch the opcode and register arguments
+        Fetched := LoadByte (IP);
+        //Decode the opcode
+        Op := Fetched and $f0 shr 4;
+        //Decode the register arguments
+        Regs := Fetched and $f;
+        X := Fetched and $c shr 2;
+        Y := Fetched and 3;
         IP := IP + 1;
-        if IP > $ffef then begin
-            writeln ('Error: illegal instruction pointer value');
-            halt;
-        end;
         //Immediate or address argument
         if Op >= $a then begin
             //Immediate or high byte of address
-            Addr := Mem [IP];
+            Fetched := LoadByte (IP);
+            Addr := Fetched;
             Addr := Addr shl 8;
             IP := IP + 1;
-            if IP > $ffef then begin
-                writeln ('Error: illegal instruction pointer value');
-                halt;
-            end;
             //Low byte of address
             if Op = $a then begin
                 if Y = 0 then begin
-                    Addr := Addr + Mem [IP];
+                    Fetched := LoadByte (IP);
+                    Addr := Addr + Fetched;
                     IP := IP + 1;
-                    if IP > $ffef then begin
-                        writeln ('Error: illegal instruction pointer value');
-                        halt;
-                    end;
                 end;
             end
             else begin
-                Addr := Addr + Mem [IP];
+                Fetched := LoadByte (IP);
+                Addr := Addr + Fetched;
                 IP := IP + 1;
-                if IP > $ffef then begin
-                    writeln ('Error: illegal instruction pointer value');
-                    halt;
-                end;
             end;
         end
         else Addr := 0;
@@ -176,20 +359,12 @@ begin
         //Ret
         else if Op = 1 then begin
             //High byte of the return address
-            IP := Mem [RP];
+            IP := LoadByte (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];
+            IP := IP + LoadByte (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
@@ -212,177 +387,10 @@ begin
             //Immediate
             if Y <> 0 then R [X] := Addr shr 8
             //Address
-            else begin
-                //Terminal input
-                if Addr = IO then begin
-                    wait;
-                    //Read a keypress
-                    repeat
-                    Ch := ReadKey;
-                        //Check for non-ASCII keys
-                        if Ch = ansichar (0) then begin
-                            //Non-ASCII
-                            if keypressed then begin
-                                Scan := ReadKey;
-                                //The delete key inserts the delete character
-                                if Scan = ansichar ($53) then begin
-                                    Ch := ansichar ($7f);
-                                    ASCII := true;
-                                end
-                                //Unused function keys insert a null
-                                else ASCII := true;
-                            end
-                            //Null
-                            else ASCII := true;
-                        end
-                        //Other ASCII
-                        else ASCII := true;
-                    until ASCII = true;
-                    //Bodge for the home and end keys
-                    if Ch = ansichar ($37) then begin
-                        if keypressed then begin
-                            Scan := ReadKey;
-                            Scan := ReadKey;
-                            Scan := ReadKey;
-                            Ch := ansichar (0);
-                        end;
-                    end
-                    else if Ch = ansichar ($38) then begin
-                        if keypressed then begin
-                            Scan := ReadKey;
-                            Scan := ReadKey;
-                            Scan := ReadKey;
-                            Ch := ansichar (0);
-                        end;
-                    end;
-                    //Process the keypress
-                    Output; //Local echo
-                    R [X] := byte (Ch);
-                end
-                //Tape reader
-                {$ifdef tape}
-                else if Addr = $fffd then begin
-                    wait;
-                    assign (State, ExpandFileName ('~/.tapes.thingamajig'));
-                    //Check the reader state
-                    if FileExists (ExpandFileName ('~/.tapes.thingamajig')) then begin
-                        try
-                            reset (State);
-                            read (State, Reader);
-                            read (State, Punch);
-                            close (State);
-                        except
-                        end;
-                    end;
-                    //Read
-                    assign (TapeIn, Reader.Path);
-                    try
-                        reset (TapeIn);
-                        seek (TapeIn, Reader.Pos);
-                        read (TapeIn, R [X]);
-                        close (TapeIn);
-                        Reader.Pos := Reader.Pos + 1;
-                    except
-                        R [X] := $ff;
-                    end;
-                    //Save the reader state
-                    if FileExists (ExpandFileName ('~/.tapes.thingamajig')) then begin
-                        try
-                            rewrite (State);
-                            write (State, Reader);
-                            write (State, Punch);
-                            close (State);
-                        except
-                        end;
-                    end;
-                end
-                {$endif}
-                //Regular load
-                else R [X] := Mem [Addr];
-            end;
+            else R [X] := LoadByte (Addr);
         end
         //Store
-        else if Op = $b then begin
-            //Terminal output
-            if Addr = IO then begin
-                wait;
-                Ch := ansichar (R [Y]);
-                Output;
-            end
-            //Printer
-            {$ifdef printer}
-            else if Addr = $fffe then begin
-                wait;
-                assign (Prn, '/dev/usb/lp0');
-                try
-                    rewrite (Prn);
-                    write (Prn, R [Y]);
-                    close (Prn);
-                except
-                end;
-            end
-            {$endif}
-            //Tape punch
-            {$ifdef tape}
-            else if Addr = $fffd then begin
-                wait;
-                assign (State, ExpandFileName ('~/.tapes.thingamajig'));
-                //Check the punch state
-                if FileExists (ExpandFileName ('~/.tapes.thingamajig')) then begin
-                    try
-                        reset (State);
-                        read (State, Reader);
-                        read (State, Punch);
-                        close (State);
-                    except
-                    end;
-                end;
-                //Punch
-                if Punch.Path <> '' then begin
-                    assign (TapeOut, Punch.Path);
-                    if FileExists (Punch.Path) = false then begin
-                        try
-                            rewrite (TapeOut);
-                            write (TapeOut, R [Y]);
-                            close (TapeOut);
-                            Punch.Reset := false;
-                        except
-                        end;
-                    end
-                    else if Punch.Reset then begin
-                        try
-                            rewrite (TapeOut);
-                            write (TapeOut, R [Y]);
-                            close (TapeOut);
-                            Punch.Reset := false;
-                        except
-                        end;
-                    end
-                    else begin
-                        try
-                            reset (TapeOut);
-                            seek (TapeOut, FileSize (TapeOut));
-                            write (TapeOut, R [Y]);
-                            close (TapeOut);
-                        except
-                        end;
-                    end;
-                end;
-                //Save the punch state
-                if FileExists (ExpandFileName ('~/.tapes.thingamajig')) then begin
-                    try
-                        rewrite (State);
-                        write (State, Reader);
-                        write (State, Punch);
-                        close (State);
-                    except
-                    end;
-                end;
-            end
-            {$endif}
-            //Regular store
-            else Mem [Addr] := R [Y];
-        end
+        else if Op = $b then StoreByte (Addr, R [Y])
         //Breq
         else if Op = $c then begin
             if R [X] = R [Y] then IP := Addr;
@@ -393,53 +401,11 @@ begin
         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;
+            if R [X] = R [Y] then Call;
         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;
+            if R [X] <> R [Y] then Call;
         end;
 
         //Increment the instruction counter
diff --git a/readme.md b/readme.md
index ccf4dcf..2aed37d 100644
--- a/readme.md
+++ b/readme.md
@@ -14,12 +14,6 @@ assembler and a disassembler, all written in FreePascal. It also
 includes couple of simple example programs for Thingamajig written in
 Assembly.
 
-Speed
------
-
-Thingamajig does not have a prescribed speed. The emulator runs at
-roughly 500 KIPS.
-
 Registers and Memory
 --------------------
 
@@ -28,11 +22,10 @@ Registers and Memory
   * 8-bit general-purpose registers R0-R3
   * 8-bit memory locations 0-FFFF
 
-Multi-byte values are big-endian. Memory addresses FFF0-FFFF are
-reserved for memory mapped devices; the instruction and return pointers
-cannot have values higher than FFEF and FFF0 respectively to avoid them.
-The instruction and return pointers are initialised as 0 and FFF0
-respectively, while other registers and memory are unitialised.
+Multi-byte values are big-endian. Memory locations 0-FFEF are used for
+RAM while FFF0-FFFF are reserved for memory mapped devices. Input and
+output are mapped to address FFFF, while arbitrary devices can be mapped
+to the other reserved addresses.
 
 Instructions
 ------------
@@ -78,8 +71,7 @@ For the arguments of each instruction see the previous section.
 
 Address arguments can be either absolute addresses or references to or
 relative to a label. Relative references are of the form LABEL +/- N,
-the spacing being optional. Note that the assembler does not check for
-addresses or references to reserved addresses.
+the spacing being optional.
 
 In addition to the true instructions there are three
 pseudo-instructions. ORG defines the starting address of the program: it
@@ -88,26 +80,34 @@ not required if the starting address is 0. DATA introduces a byte of
 data. ADDR introduces two bytes of data containing the address of a
 reference to or relative to a label.
 
-Memory-Mapped Devices
----------------------
+Boot
+----
 
-Input and output are mapped to address FFFF; the emulator emulates a
-roughly 1000 CPS glass teletype with local echo.
+At boot the initial program loader (IPL) loads a program to the RAM
+starting at address 0 after which is cedes control to the CPU. The
+instruction and return pointers are initialised as 0 and the first
+address after RAM respectively, while other registers and RAM are
+uninitialised.
 
-Arbitrary devices can be mapped to the other reserved addresses.
+Emulator
+--------
+
+The emulator runs at roughly 500 KIPS and has the full 65520 bytes of
+RAM.
+
+Input and output are handled by an emulated roughly 1000 CPS glass
+teletype terminal with local echo. Note that of the control characters
+only bell, backspace, line feed, and carriage return are used by the
+terminal, and the backspace and delete keys are tied to their respective
+characters and non-character keys to null.
 
 In Linux the emulator can be compiled with support for a character
 printer and an emulated punched tape reader and punch with the arguments
 -dprinter and -dtape respectively. The printer is mapped to address FFFE
 and the tape reader and punch to FFFD. The printer prints into
 /dev/usb/lp0 and the tape files read from and punched to are (re)set
-using the program tapectl. If you wish to use a different setup you have
-to modify the code yourself.
+using the program tapectl.
 
-Initial Program Loader
-----------------------
-
-At boot the initial program loader (IPL) loads a program to the memory
-starting from address 0 after which is cedes control to the CPU. The
-emulator loads the program from a ROM file.
+The IPL loads a program from a file specified when launching the
+emulator.
 
\ No newline at end of file