[Coco] Assembler Help

[William Astle]

Robert Gault wrote:
> Charles Shrader wrote:
>> Hi all!  I thought I'd break into something other than the recent 
>> copyright
>> discussion and ask for some help with an odd Assembler language question.
>>
>> I'm using EDTASM++.BIN.  I've written a very simple program that 
>> clears the
>> screen, then waits for the user to type in eight characters.  Once eight
>> characters have been typed in, the program outputs to the screen a space,
>> then outputs a repeat of the string.  Very simple.
>>
>> When I assemble and run this in the Z-Debugger, it runs fine, although 
>> the
>> RTS at the end of the code causes unusual results.  But I can see 
>> before the
>> system goes hay-wire that on the top line, my entry is repeated:
>>
>> ABCDEFGH ABCDEFGH.
>>
>> This runs fine also if I change the RTS line to a line that just 
>> brances to
>> itself over and over.  This keeps the system from going nuts.
>>
>> Now if I compile the program to disk and execute the .BIN file, my output
>> is:
>>
>> ABCDEFGH BCDEFGH
>>
>> (Where did the first A go in the echoed second string??)
>>
>> I'm totally perplexed as to why this is happening.  It's like my x 
>> index is
>> not pointing to the correct start byte of STMP.  (See below)
>>
>> I'll include the code below; perhaps someone here more knowledgeable 
>> than I
>> may be able to figure out what's going on.  I'm doing this in the 
>> David Keil
>> emulator for the Coco3 with 128K RAM:
>>
>> 00100                ORG             $3F00   ;Start location of code
>> 00110 START          JSR             43304   ;Clear Screen
>> 00120                LDB             #0      ;Initialize register B
>> 00130                LDA             #0      ;Initialize Accumulator
>> 00135                STA             DEVNUM  ;Output to screen
>> 00140                LDX             STMP    ;Load start address of 
>> storage
>> location STMP
>> 00150 LOOP           JSR             $A1C1   ;Poll for keypress
>> 00160                CMPA            #0      ;Compare vs 0.  If 0, no key
>> was pressed.
>> 00170                BEQ             LOOP    ;No keypress so keep polling
>> 00180                STA             ,X+     ;Store keypress in STMP and
>> move byte pointer
>> 00190                JSR             [40962] ;Output keypress to screen
>> 00192                INCB                    ;Increment counter
>> 00194                CMPB            #8      ;See if 8 keys have been
>> pressed
>> 00200                BNE             LOOP    ;Keep polling until 8 
>> keys have
>> been pressed
>> 00210                LDX             STMP    ;Point to start of STMP 
>> so we
>> can echo output
>> 00220                LDA             #32     ;Load A with ASCII code for
>> 'Space'
>> 00230                JSR             [40962] ;Output character to screen
>> 00240 LOOP2          LDA             ,X      ;Load A with first 
>> character of
>> stored string
>> 00250                JSR             [40962] ;Output to screen
>> 00260                CMPX            #8      ;Have we displayed all 8
>> characters?
>> 00270                BNE             LOOP2   ;If not, get next 
>> character in
>> string
>> 00280                RTS                     ;Return to caller
>> 00290 STMP           RMB             $09     ;Define storage area STMP
>> 00300 DEVNUM         EQU             $6F     ;Location where output is
>> designated
>> 00310                END             START
>>
>> Thanks for any help,
>>
>> Chuck
>>
> 
> There is some code that can be cleaned up but that is a matter of style. 
> What I see wrong is in loop2.
> 
> lda ,x   should be lda ,x+
> cmpx #8  should be cmpb #8  preceded by an incb
> regB should be cleared   231  clrb
> 
> When I made these changes, the program worked for me under Basic. To get 
> it to work in EDTASM the rts should be replaced with swi. You should 
> also make sure the program does not assemble over and corrupt EDTASM.

There's one more problem. Both instances of LDX STMP should be LDX 
#STMP. The latter actually loads the address of STMP while the former 
loads the contents at that address. So, if the memory contains $0000, 
your buffer is actually at address 0 (which might by why CMPX #8 worked 
for you?).

-- 
William Astle
finger lost at l-w.ca for further information

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