Increment a table in Assembly (68HC11)

Question

I have a program with two data tables and a result table

          ORG $B000
TABLE1    FCB        222, 37, ...
TABLE2    FCB        37, 100, ...

          ORG $B010
RESULT    RMB        8

My program loops through the values in each table and passes them to a function which finds their greatest common denominator. The returned value is passed over the stack and needs to be stored in the Result table. Here is what I have so far

          ORG $C000
          LDX         #TABLE1
          LDY         #TABLE2
WHILE     LDAA        0,X              WHILE (*TABLE1 != -1)          
          CMPA        #$FF                  
          BEQ         ENDWHILE                     
          LDAA        0,X                          PASS VARIABLES TO
          LDAB        0,Y                          SUBROUTINE IN REGISTER
          INX                                      TABLE1++
          INY                                      TABLE2++
          JSR         SUB
          PULA                                     
          STAA      RESULT             GET RETURN VALUE FROM STACK

The above code is working properly as is my subroutine. I can't figure out how to increment my RESULT variable. I tried using a loop counter as an offset but for some reason the below code stores at location $B019 instead of $B010

         STAA    COUNTER,RESULT
         INC     COUNTER

If anything is unclear or if you think the question is bad please comment and I will fix it.

tonypdmtr · Accepted Answer · 2017-04-05T13:59:39.007

There are several problems with your code. For example, STAA COUNTER,RESULT is not a valid assembly instruction format for 68HC11.

To store the answer in a table, you need a third index. Since 68HC11 has only X and Y for indexing, you need to use an additional variable to keep the extra index. If your table addresses are constants (fixed addresses) in the zero page range (0 to 255) you can even use a single register to access all tables. For example:

LDX #0
LDA TABLE1,x
LDB TABLE2,x
...
JSR SUB
STA RESULT,x
INX
...

Before I can provide a complete answer I'd like to see the SUB subroutine. For SUB to return something on the stack without first having allocated the needed space for it on the stack, it has to do some 'acrobatics' with the stack -- possible but not the normal way to handle this kind of problem.

The normal way would be to create the needed stack space for the returned result before calling SUB, and then SUB would write the answer into that reserved space. In this particular case, however, and unless it's required to be otherwise for academic purposes, it's simpler to just return the result in register A instead of allocating a single byte on stack, and have SUB write to it.

Finally, a PULA after calling SUB is destroying the stack balance as you pull without having pushed before (unless you do 'acrobatics' in SUB, which I haven't seen yet.)

Please provide additional information, and I will update my answer.

I figured it out last night on my own but you are correct. I created a separate 2 byte variable to hold the address of my RESULT table. This was for an assignment so I was required to send the return value back to the main program on the stack hence the PULA after the JSR. In my subroutine I made sure it was transparent so the X and Y registers were restored just before the RTS — Omega Collision, Apr 05 '17 at 22:25

Increment a table in Assembly (68HC11)

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