Saturday, May 20, 2017

Building a 68000 Single Board Computer - Programming Examples


I earlier mentioned the book 68000 Assembly Language Programming, Second Edition, by Lance A. Leventhal, Doug Hawkins, Gerry Kane, and William D. Cramer. The book has many complete programming examples listed in it that help explain 68000 programming.

The book recommends entering and running the programs ona 68000-based system. I've been doing that, and it makes the code much clearer than simply reading the text. With the 68000 TUTOR software it is very easy to disassemble code in memory, display amd enter memory values, and run the program examples. I typically step through the code an instruction at a time using the trace function, looking at the values of the registers and selected memory locations.

Here is a typical disassembly of some example code:

TUTOR  1.3 > MD 4000 24 ;DI
004000    307C6001             MOVE.W  #24577,A0 
004004    7003                 MOVEQ.L #3,D0 
004006    4281                 CLR.L   D1 
004008    4282                 CLR.L   D2 
00400A    6008                 BRA.S   $004014 
00400C    D241                 ADD.W   D1,D1 
00400E    3601                 MOVE.W  D1,D3 
004010    E54B                 LSL.W   #2,D3 
004012    D243                 ADD.W   D3,D1 
004014    1418                 MOVE.B  (A0)+,D2 
004016    D242                 ADD.W   D2,D1 
004018    51C8FFF2             DBF.L   D0,$00400C 
00401C    33C100006004         MOVE.W  D1,$00006004 
004022    4E75                 RTS

While I could enter the programs as hex data from the text, or use TUTOR's built-in assembler, I have been entering the source code on a Linux computer and cross-assembling it using the VASM assembler. Then I can load the Motorola hex (run) file generated by the assembler into the TS2 computer over the serial port.

Here is the source code corresponding to the disassembly above:

DATA     EQU     $6000
PROGRAM  EQU     $4000
STRING   EQU     $6001           ADDRESS OF FOUR DIGIT BCD STRING
RESULT   EQU     $6004           ADDRESS OF RESULT
         ORG     PROGRAM
PGM_7_4A MOVEA.W #STRING,A0      POINTER TO FIRST BCD DIGIT
         MOVEQ   #4-1,D0         NUMBER OF DIGITS(-1) TO PROCESS
         CLR.L   D1              CLEAR FINAL RESULT - D1
         CLR.L   D2              CLEAR DIGIT REGISTER
         BRA.S   NOMULT          SKIP MULTIPLY FIRST TIME
LOOP     ADD.W   D1,D1           2X
         MOVE.W  D1,D3
         LSL.W   #2,D3           8X = 2X * 4
         ADD.W   D3,D1           10X = 8X + 2X
NOMULT   MOVE.B  (A0)+,D2        NEXT BCD DIGIT,(D2[15-8] UNCHANGED)
         ADD.W   D2,D1           ADD NEXT DIGIT
         DBRA    D0,LOOP         CONTINUE PROCESSING IF STILL DIGITS
         MOVE.W  D1,RESULT       STORE RESULT
         RTS
         END     PGM_7_4A

The VASM assembler is almost entirely compatible with the Motorola assembler and I have had to make only a very few changes to the code listed in the book. I did find a couple of errors, too.

So far I have entered almost four chapters worth of examples, just over thirty programs. I have placed the code on my github account. I'll continue doing so until I either get bored or finish the examples.

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