[Coco] lbsr and rts

[L. Curtis Boyle]

The Nitros9 project, at least at one time, had a disassembly of  
basic09 that would help you greatly. I think (going on very faint  
memory here) that those may have been jsr function code style calls,  
where the RTS address on the stack would point to the 'non-sensical'  
byte so that the called routine would use that byte as either a  
parameter, or a table lookup offset, or two bytes as a value/address  
of some sort.

Sent from my iPhone
L. Curtis Boyle


On Sep 16, 2009, at 12:45 PM, Lothan <lothan at newsguy.com> wrote:

> If I'm reading this right, that block of code translates to:
>
> L010A JSR <$1E
> LSR <$9D
> EXG D,Y
>
> What's funny here is that L010D is pointing to the $9D in the LSR  
> instruction. If we ignore that, the block at L010D is:
>
> L010D JSR <$1E
> FCB $02 #invalid instruction
>
> And the block at L013A is just as goofy:
>
> L013A JSR <$2A
> NEG <$00
> FCB $72 # invalid instruction
> FCB $02 # invalid instruction
>
> Either way, neither of those two blocks seem to be valid code. This  
> may indicate the block at L08F8 isn't decoded correctly.
>
> Can you send me the bin you're looking at it so I can disassemble it  
> and try to make heads/tails out of it?
>
> --------------------------------------------------
> From: "Wayne Campbell" <asa.rand at yahoo.com>
> Sent: Wednesday, September 16, 2009 1:46 PM
> To: <coco at maltedmedia.com>
> Subject: [Coco] lbsr and rts
>
>> I have found, in the assembly code generated by disasm, that there  
>> are labels in the declarations (all those fcb's) that are being  
>> addressed in the code using lbsr op codes. Yet, there is no rts  
>> instruction anywhere in those fcb's. Can someone explain how this  
>> works? An example is:
>>
>> * The L010A label is addressed twice in the program code.
>> L010A    fcb   $9D
>>        fcb   $1E
>>        fcb   $04
>> * The L010D label is addressed ten times in the program code.
>> L010D    fcb   $9D
>>        fcb   $1E
>>        fcb   $02
>>
>> this series continues until:
>>
>> L013A    fcb   $9D
>>        fcb   $2A *
>>        fcb   $00
>>        fcb   $00
>>        fcb   $72 r
>>        fcb   $02
>>
>> In the code, the following statements address these labels:
>>
>> Both occurrences of the L010A label, and one of the L010D labels,  
>> occur in this routine:
>> L08F8    ldx   $02,s
>>        lda   #$80
>>        lbsr  L010A
>>        bne   L090F
>>        lbsr  L010D
>>        beq   L0915
>>        leax  $03,x
>>        lda   #$20
>>        lbsr  L010A
>>        beq   L0915
>>
>> I just don't understand.
>>
>> Wayne
>
>
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