Friday, January 6, 2017

Building a 68000 Single Board Computer - The TUTOR Monitor

When the 68000 chip was introduced, Motorola offered the MEX68KECB Educational Computer Board (ECB). It included a monitor program called TUTOR. I remember briefly having access to one where I worked back in the mid 1980s when some products were moving to this new processor and staff was becoming familiar with it.

The Teesside TS2, while similar, is a simpler design using static rather than dynamic RAM, and lacks some features like the parallel port and timer (PIT) and cassette tape interfaces. The TS2 was designed to be compatible with the ECB in terms of the memory map for ROM and RAM and serial ports. This allows it to run the TUTOR monitor program.

I was able to find source and binary code for TUTOR here. I ported it to the GNU assembler so that I could modify it if desired.

It is 16KB in size (unlike the TS2 monitor which is about 3Kb) and will fit in the first two 8K EPROMs of the TS2.

I wasn't sure if TUTOR would run as is on my design. The Clements book implied that it should, but did not say so explicitly. After programming the two EEPROMs and inserting them in the board I was pleased to see that it came up and accepted commands.

The Motorola ECB included a parallel printer port and cassette tape interface using the 68230 parallel Interface/Timer (PIT) chip. That chip is not included in the TS2. However, other than commands specific to the printer port and cassette tape, everything else works.

There is a copy of the review of the Motorola ECB from Byte magazine in 1983 here. At US$495 it would be equivalent to about US$1200 today. I believe one of the reasons that the University of Teesside designed their own educational board rather than the ECB was that they could significantly reduce the cost.

The TUTOR monitor is well documented in chapter 3 of the M68000 Educational Computer Board User's Manual. It is quite sophisticated, even including a disassembler and assembler. I'll just cover a few highlights and examples of what it offers.

It provides the following features:

  1. Display and modification of memory as byte, word, longwords, strings, characters, or disassembled instructions.
  2. Display and modification of registers.
  3. Memory fill, move, search, and test functions.
  4. Number conversion between decimal and hexadecimal.
  5. The ability to set and clear breakpoints.
  6. Ability to run programs with breakpoints or line by line tracing.
  7. Output to either of two serial ports, parallel printer port, or cassette tape.
  8. Loading and saving of memory in S record format.
  9. An assembler which allows entering assembly language mnemonics.

Here is a example of displaying memory, first as hex and ASCII data, then as a disassembly:

TUTOR  1.3 > MD 8008 80
008008    60 00 0C B0 41 F8 04 4C  20 3C 00 00 02 0E 42 81  `..0Ax.L <....B.
008018    10 C1 53 80 66 FA 48 7A  00 10 21 DF 00 08 48 7A  .AS.fzHz..!_..Hz
008028    00 12 21 DF 00 0C 4E 75  21 FC 42 55 53 20 00 30  ..!_..Nu!|BUS .0
008038    60 08 21 FC 41 44 44 52  00 30 21 DF 04 CA 21 DF  `.!|ADDR.0!_.J!_
008048    04 CE 21 CF 04 44 4F FA  00 0A 21 CF 04 D6 60 00  .N!O.DOz..!O.V`.
008058    0C 34 61 00 1C 3A 3C FC  0D 0A 30 38 04 CA 61 00  .4a..:<|..08.Ja.
008068    19 48 1C FC 00 20 20 38  04 CC 61 00 19 2E 1C FC  .H.|.  8.La....|
008078    00 20 30 38 04 D0 61 00  19 30 61 00 1B 86 60 00  . 08.Pa..0a...`.

TUTOR  1.3 > MD 8008 80 ;DI
008008    60000CB0             BRA.L   $008CBA 
00800C    41F8044C             LEA.L   $0000044C,A0 
008010    203C0000020E         MOVE.L  #526,D0 
008016    4281                 CLR.L   D1 
008018    10C1                 MOVE.B  D1,(A0)+ 
00801A    5380                 SUBQ.L  #1,D0 
00801C    66FA                 BNE.S   $008018 
00801E    487A0010             PEA.L   $00008030(PC) 
008022    21DF0008             MOVE.L  (A7)+,$00000008 
008026    487A0012             PEA.L   $0000803A(PC) 
00802A    21DF000C             MOVE.L  (A7)+,$0000000C 
00802E    4E75                 RTS      
008030    21FC425553200030     MOVE.L  #1112888096,$00000030 
008038    6008                 BRA.S   $008042 
00803A    21FC414444520030     MOVE.L  #1094992978,$00000030 
008042    21DF04CA             MOVE.L  (A7)+,$000004CA 
008046    21DF04CE             MOVE.L  (A7)+,$000004CE 
00804A    21CF0444             MOVE.L  A7,$00000444 
00804E    4FFA000A             LEA.L   $0000805A(PC),A7 
008052    21CF04D6             MOVE.L  A7,$000004D6 
008056    60000C34             BRA.L   $008C8C 
00805A    61001C3A             BSR.L   $009C96 
00805E    3CFC0D0A             MOVE.W  #3338,(A6)+ 
008062    303804CA             MOVE.W  $000004CA,D0 
008066    61001948             BSR.L   $0099B0 
00806A    1CFC0020             MOVE.B  #32,(A6)+ 
00806E    203804CC             MOVE.L  $000004CC,D0 
008072    6100192E             BSR.L   $0099A2 
008076    1CFC0020             MOVE.B  #32,(A6)+ 
00807A    303804D0             MOVE.W  $000004D0,D0 
00807E    61001930             BSR.L   $0099B0 
008082    61001B86             BSR.L   $009C0A 
008086    600018F6             BRA.L   $00997E 

TUTOR  1.3 > 

The assembler is quite powerful, mostly compatible with Motorola's cross-assembler but lacking support for editing, line numbers, and labels. In pinch, if you could not afford a development system with a cross-compiler, you could use TUTOR's assembler for development and upload the disassembled source and assembled S record file.

Using TUTOR I can easily cross-compile code on a Linux laptop, generate a Motorola hex file, and then transfer it to the TS2 over the serial port.

I'm shortly going to wire up the second 16K of RAM on the board. I can use the memory test command to verify that the new memory is working.

One quirk of TUTOR is that you need to enter all commands in upper case.

Also, some commands can be interrupted by typing BREAK. This is a special serial port sequence and not a character. From minicom it can be sent using F although this doesn't seem to work if you are using a USB to serial convertor.

Overall I see little reason to use the TS2 monitor as TUTOR is much more powerful. The features like breakpoints, tracing, and disassembler make it much easier to debug test programs.

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