Building a 68000 Single Board Computer - Enhanced Basic

The TUTOR monitor is quite powerful, but a high-level language makes programming much easier. In the 1980s the standard programming language for home computers was BASIC.

One of the options for BASIC on the 68000 is Enhanced Basic. Written by Lee Davison, Enhanced BASIC is a BASIC interpreter that he wrote from scratch, for both the 6502 and 68000 processors. It is designed to be easy to port to different systems and is free for non-commercial use.

I earlier looked at Enhanced Basic on my 6502-based Briel Replica 1 computer. The 68000 version looked interesting and feasible to port to the TS2.

Unfortunately, Lee Davison passed away in 2013 and his web site is no longer on-line. I was able to find the source for version 3.5.2, one version older than the latest 3.5.3 release.

It was originally written to compile using the Windows-based Easy68K assembler. While I was able to run this on Linux using the Wine software, it found that it would assemble with only a couple of trivial changes using the VASM assembler, which runs natively on Linux.

To port it to the TS2 I only needed to write routines for character input and output. I wrote them to talk directly to the 6850 console UART. I also needed to adjust the program addresses to work within the memory map of the TS2.

Enhanced Basic is almost 16K in size and and needs at least 16K of RAM for programs. The TS2 has 32K, so it was just enough for it to run out of RAM. It takes about 30 seconds to download the S record file over the serial port at 9600 bps.

Another machine dependent feature is loading and saving. As there is no easy way to implement the LOAD and SAVE commands, I modified it to display an unimplemented error if these commands are used. One could save programs by running LIST and capturing the output to a file from the terminal emulator program, and load using the reverse method.

Once I got it working from RAM, I then assembled it to run out of ROM. I first had to wire up the additional 16K ROM sockets on the TS2. Enhanced Basic conveniently fits in the second pair of 16K ROM, with TUTOR in the first 16K.

Running from ROM, it can now use all of the RAM. About 23K is available to BASIC programs when it starts up. Here is as sample session, starting from reset into the TUTOR monitor:

TUTOR  1.3 > GO C000
PHYSICAL ADDRESS=0000C000

23056 Bytes free
Enhanced 68k BASIC Version 3.52

Ready
10 FOR I = 1 TO 100
20 PRINT I;
30 NEXT I

RUN
 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 3
0 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 
57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83
 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100
Ready

20 PRINT HEX$(PEEK(I),2);" ";
LIST
10 FOR I = 1 TO 100
20 PRINT HEX$(PEEK(I),2);" ";
30 NEXT I

Ready
RUN
00 04 44 00 00 81 46 00 00 80 30 00 00 80 3A 00 00 8D E4 00 00 83 AC 00 00 83 B6
 00 00 83 C0 00 00 83 CA 00 00 8C BA 00 00 83 DE 00 00 83 E8 00 00 99 62 00 00 9
9 62 00 00 99 62 00 00 99 62 00 00 99 62 00 00 99 62 00 00 99 62 00 00 99 62 00 
00 99 62 00 00 99 62 00 00 99 62 00 00 99 62 00 00 83 F2 00 
Ready

The source code for the port can be found here on github.

Enhanced Basic has floating point and integer variables and over 100 keywords. It includes advanced functions like conversion to hexadecimal and binary. Keywords must be in uppercase.

It is not totally compatible with other BASICs so you will typically need to port programs to it. I tried the classic Hamurabi program that was in a book published by Creative Computing and found that it worked without changes, although there were some formatting issues in the output due to differences in the way the PRINT command functions. Here is a sample run of the game:

                                HAMURABI
               CREATIVE COMPUTING  MORRISTOWN, NEW JERSEY

TRY YOUR HAND AT GOVERNING ANCIENT SUMERIA
FOR A TEN-YEAR TERM OF OFFICE.

HAMURABI:  I BEG TO REPORT TO YOU,
IN YEAR 1, 0PEOPLE STARVED, 5CAME TO THE CITY,
POPULATION IS NOW 100
THE CITY NOW OWNS  1000ACRES.
YOU HARVESTED 3BUSHELS PER ACRE.
THE RATS ATE 200BUSHELS.
YOU NOW HAVE  2800BUSHELS IN STORE.

LAND IS TRADING AT 17BUSHELS PER ACRE.
HOW MANY ACRES DO YOU WISH TO BUY? 0
HOW MANY ACRES DO YOU WISH TO SELL? 0

HOW MANY BUSHELS DO YOU WISH TO FEED YOUR PEOPLE? 2000
HOW MANY ACRES DO YOU WISH TO PLANT WITH SEED? 20

HAMURABI:  I BEG TO REPORT TO YOU,
IN YEAR 2, 0PEOPLE STARVED, 3CAME TO THE CITY,
A HORRIBLE PLAGUE STRUCK!  HALF THE PEOPLE DIED.
POPULATION IS NOW 51
THE CITY NOW OWNS  1000ACRES.
YOU HARVESTED 1BUSHELS PER ACRE.
THE RATS ATE 0BUSHELS.
YOU NOW HAVE  810BUSHELS IN STORE.

LAND IS TRADING AT 17BUSHELS PER ACRE.
HOW MANY ACRES DO YOU WISH TO BUY? 10
HOW MANY BUSHELS DO YOU WISH TO FEED YOUR PEOPLE? 600
HOW MANY ACRES DO YOU WISH TO PLANT WITH SEED? 10

HAMURABI:  I BEG TO REPORT TO YOU,
IN YEAR 3, 21PEOPLE STARVED, 4CAME TO THE CITY,
A HORRIBLE PLAGUE STRUCK!  HALF THE PEOPLE DIED.
POPULATION IS NOW 17
THE CITY NOW OWNS  1010ACRES.
YOU HARVESTED 1BUSHELS PER ACRE.
THE RATS ATE 0BUSHELS.
YOU NOW HAVE  45BUSHELS IN STORE.

LAND IS TRADING AT 17BUSHELS PER ACRE.
HOW MANY ACRES DO YOU WISH TO BUY? 0
HOW MANY ACRES DO YOU WISH TO SELL? 0

HOW MANY BUSHELS DO YOU WISH TO FEED YOUR PEOPLE? 45
HOW MANY ACRES DO YOU WISH TO PLANT WITH SEED? 0

YOU STARVED 15PEOPLE IN ONE YEAR!!!
DUE TO THIS EXTREME MISMANAGEMENT YOU HAVE NOT ONLY
BEEN IMPEACHED AND THROWN OUT OF OFFICE BUT YOU HAVE
ALSO BEEN DECLARED NATIONAL FINK!!!!

SO LONG FOR NOW.

Maybe in the future I will try porting the classic Star Trek game, one of my favourite BASIC programs of that era.