going-flying.com gemini git repository
4499ba6b49165c30e909f2053422d9dd0620f002 - Matthew Ernisse - 1612493253
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+Title: Technomancy with retrocomputing begets retro programming
+Date: 2/4/2021 21:05
+
+Every so often a wave of boredom and nostalgia crash over me in a manner that
+causes a reaction that pushes me towards one abandoned project or another.
+For years I have been trying to build an infrastructure that would allow me
+to re-create the BBS I used to run without having to drag out a bunch of old
+computers, install DOS and some archaic network stack, and hang a pile of
+TCP/IP to serial converters about the place. Not that I wouldn't enjoy that,
+I just don't want to maintain it after I have it built.
+
+A while back I started the process by trying to put the whole BBS into
+DOSBox in a Docker container. I had built a DOSBox Docker container� years
+ago so I figured I'd be able to build on that. It turns out that the BBS
+software wants the old DOS file locking semantics and that doesn't seem to
+be emulated by DOSBox.
+
+=> https://www.going-flying.com/blog/i-made-some-containers-to-run-some-old-browsers.html [1] Necrocomputing in Docker Containers
+
+So that went on the shelf for a few years and after a few more projects
+popped on and off the stack I came back to this idea. Part of me wondered
+if it was important to have the whole BBS or if I really just wanted to play
+the old games I remembered. With this thought in mind I set out to see if I
+could get the venerable L.O.R.D. running in a container. Now for anyone who
+missed the BBS era L.O.R.D. is a text-based RPG (more of a menu based RPG,
+honestly) called Legend of the Red Dragon, written by a guy named Seth
+Robinson. It was probably the best known BBS game in the mid 1990's and was
+certainly the big one in my area. It turns out I still have a copy of the
+game in a backup archive from the early 2000's so I set about seeing if it
+would behave.
+
+After a day of fighting I was looking at the familiar loading screen via a
+telnet session but I couldn't help but be annoyed at one small bug. You see
+the way games worked on the old DOS BBS systems was pretty simple. DOS could
+only run one program at a time so your BBS (which answered the phone and did
+all the hard work getting a user connected to your computer's serial port)
+would mostly exit, usually leaving a stub in memory that would cause the BBS
+to load back in when the game exited, and then execute the game. The game
+could therefore assume that the user was already there and most of them would
+just start talking to the serial port without any prompting. In my case that
+meant that if I started the container and didn't connect to the telnet port
+(which DOSBox was connecting to an emulated serial port in DOS) within a second
+or two, I'd miss all the intro text of the game and it would appear that
+nothing was happening. This was obviously not acceptable. The thought occurred
+to me to see if there was a way to get DOSBox to tell DOS that someone was
+connected. It turns out that the answer is... sort of? DOSBox does emulate
+some of the control lines present in a serial port, though not the released
+version, so I had to update my Dockerfile to build DOSBox from source instead
+of using a packaged version. Armed with the proper version of DOSBox I can
+interrogate the CD signal (Carrier Detect, used to tell the host that the
+connected modem has someone connected to it) and wait for it before letting
+the game start. But... how? I looked around a bit but couldn't find any good
+way to report on the status of the CD signal, or any way to check for it in
+a batch file (the rudimentary DOS scripting language), so I decided to see
+if I could dust off my very first programming language and commit a bit of
+computational necromancy.
+
+I dusted off a copy of Turbo Pascal 7.0 from a backup that probably originated
+on floppy disks and started trying to re-learn just enough Pascal to get a
+bit out of a piece of hardware. I write a fair bit of C these days for various
+microcontrollers, usually to drive LEDs in ways that amuse me so I'm
+comfortable with talking to hardware at a low level. DOSBox emulates a
+16550 UART chip for the serial port well enough that I was able to find the
+registers I needed easily enough, it was just... remembering Pascal. After
+about 20 minutes I came up with the program below.
+
+```
+{* COMTEST.PAS (C) 2020 Matthew J. Ernisse <matt@going-flying.com>
+ * All Rights Reserved.
+ *
+ * Licensed under the Simplified BSD 2-Clause license.
+ *}
+program COMStatus;
+uses
+ Crt;
+const
+ { UART Registers }
+ DLL = 0;
+ DLM = 1;
+ RBR = 0;
+ THR = 0;
+ IER = 1;
+ IIR = 2;
+ FCR = 2;
+ LCR = 3;
+ MCR = 4;
+ LSR = 5;
+ MSR = 6;
+ SCR = 7;
+
+ { MSR Bits }
+ CD = 7;
+ RI = 6;
+ DSR = 5;
+ CTS = 4;
+
+ { MCR Bits }
+ RTS = 1;
+ DTR = 0;
+
+ { LSR Bits }
+ DA = 0;
+
+var
+ {* The DOS holds the address of the COM ports at $0040:$0000 ->
+ :$0006, so lean on an abolutely located array as a way to
+ access the IO port location without hard coding things like
+ $03F8 in here.
+ *}
+ ComPort: array[1..4] of Word absolute $0040:$0000;
+
+begin
+ Writeln('COMTEST (c) 2020 Matthew J. Ernisse <matt@going-flying.com>');
+ Writeln('All Rights Reserved.');
+
+ { Enable DTR / RTS so the modem knows we are here }
+ Port[ComPort[1] + MCR] := $0B;
+
+ Writeln('Waiting for CD on COM1');
+
+ { Watch the Modem Status Register on the UART }
+ while (Port[ComPort[1] + MSR] and (1 shl CD)) = 0 do begin
+ if KeyPressed then
+ if ReadKey = ^C then
+ Break;
+ end;
+
+ if (Port[ComPort[1] + MSR] and (1 shl CD)) > 0 then
+ Halt(1);
+end.
+```
+
+The program is simple, it pokes some bits on the Modem Control Register (MCR)
+to tell DOSBox that it is ready and then waits for the Carrier Detect (CD)
+bit in the Modem Status Register (MSR) which DOSBox will use to tell it that
+someone has connected to the telnet port. Once CD turns on the program
+exits and the game starts.
+
+I have to admit that this scratched a huge nostalgia itch. Not only did I get
+to play one of my favorite BBS games again, but I got to write some DOS batch
+files and even some Turbo Pascal. It made me remember late nights in front of
+an amber monitor, typing odd screed into a text editor to make the computer
+do new things. It's funny that one of the last things that I got my DOS
+computer to do was to blink LEDs and now I use microcontrollers that are
+slightly less sophisticated to do the same thing.
+
+If you are interested in the rest of the bits that make this work, you
+can find it in my git repository�.
+
+=> https://ssl.ub3rgeek.net/git/?p=containers.git;a=tree;f=lord;hb=HEAD [2] LORD Container Git Repository