Building a 68000 Single Board Computer - The TS2 Monitor

The original Teesside TS2 computer included a small machine language monitor program called TS2MON or TSBUG. There is a source listing for it in the book as well as on the included CD-ROM.

As I posted here earlier, I ported the monitor so it could be cross-compiled with the GNU assembler for the 68000. I now have it running on my prototype board. It is a very rudimentary monitor program, running out of ROM, and is a little over 3 Kilobytes in size.

I programmed it into two 27C64 64KB EPROMs. I've also verified that the board works with 28C64 EEPROMs.

The monitor provides twelve commands offering some basic features:

• Examine and change memory.
• Display and change registers.
• Start program execution.
• Load and save memory though a serial port using Motorola hex (S record) format.
• Basic support for breakpoints, allow the setting and clearing of breakpoints and program execution to be halted and continued after a breakpoint.

I've documented the commands here. Here are a few examples.

?TSBUG 2 Version 23.07.86
?

Display and change some memory:

?MEM 1000
?
?00001000 0000  1234
?00001002 1234  2345
?00001004 2345  6789
?00001006 0000 -
?00001004 6789 N
?00001006 0000 N
?00001008 0000 

Display registers:

?DISP
?
?  Data reg       Address reg
?0 00000000        00000000
?1 00000000        00000000
?2 00000000        00000000
?3 00000000        00000000
?4 00000000        00000008
?5 00000040        00000000
?6 00000000        00000000
?7 00000000        00000000
?
? SS  =  00000000
? SR  =  2700
? PC  =  00001000

Change a couple of registers:

?REG PC 1000
?00001000
?
?REG D0 12345678
?00000000
?
?DISP
?
?  Data reg       Address reg
?0 12345678        00000000
?1 00000000        00000000
?2 00000000        00000000
?3 00000000        00000000
?4 00000000        00000008
?5 00000040        00000000
?6 00000000        00000000
?7 00000000        00000000
?
? SS  =  00000000
? SR  =  2700
? PC  =  00001000

Dump a range of memory to an S record file:

?DUMP 8000 8080
?
S113800000000800000080084DF80C0042AE004A51
S1138010422E0048422E00496136610005D86100B5
S1138020044E49FA09D06164207C0000C00020108D
S11380300C80524F4D3266044EA800084E714E71AA
S113804042876128614C61000080610000BE60F0DD
S113805041F90001004010BC0003117C0003000141
S113806010BC0015117C001500014E7548E700088E
S113807049FA099C61064CDF10004E752F00101C54
S10480806794
S9

The code was designed to be simple and readable. Despite its simplicity, it does have some more advanced features. It uses device control blocks (DCBs) in RAM to allow modifying the device input/output routines to redirect them. It can also be extended by code in the second set of ROMs to add more commands.

I've now wired up the second 6850 UART chip. The board has two serial ports. The rationale at the time was that users would typically have a dumb serial terminal connected to one port to use as a console, and the other port would connect to a development system computer for uploading or downloading programs. The TSMON LOAD and DUMP commands use the second serial port for this reason.

One of the simplifications in my design is to use an FTDI USB to serial (TTL-level) converter for the serial ports, avoiding the need for some additional chips and +/- 12 volt power supplies, and allowing it to be connected to a USB port. You can also power the board from USB if desired.

Typical usage today would be to connect to one computer using a terminal emulator (I'm using a laptop running Ubuntu Linux and use the minicom program). In that case it is probably more convenient to use only one serial port both as a console and for file transfers. While the monitor source code could be modified to use the monitor serial port for LOAD and DUMP, you can patch the DCB in RAM to direct the commands to use the console port rather than the auxiliary port. It is simply a matter of changing two words in the DCB. Addresses $00000D0E and $00000D26 need to be changed from $0041 to $0040. This needs to be done each time monitor is entered, i.e. after a reset.

Having done that, I can upload an S record file from my Linux laptop to the TS2 by cating the file to /dev/ttyUSB0 after running a monitor LOAD command. You can then run the program from the monitor. A simple test program can be found in git here.

In a future blog post I'll cover a more sophisticated monitor program, the TUTOR software, which was developed by Motorola for the 68000 Educational Computer Board (ECB), but also runs on the TS2 since it was designed to have a memory map compatible with the ECB.