[Coco] Returning a string to basic ?

[Arthur Flexser]

Strings under Basic need not be terminated by anything;  the length
byte tells Basic where the end is.

Art

On Tue, Sep 20, 2011 at 6:16 PM, Bob Devries <devries.bob at gmail.com> wrote:
> Don't forget to terminate the string with a $FF byte, and put the length of
> the string back into the first byte.
>
> Regards, Bob Devries
> Dalby, QLD, Australia
>
> ----- Original Message ----- From: "Rogelio Perea" <os9dude at gmail.com>
> To: "CoCoList for Color Computer Enthusiasts" <coco at maltedmedia.com>
> Sent: Wednesday, September 21, 2011 8:11 AM
> Subject: Re: [Coco] Returning a string to basic ?
>
>
>> On Tue, Sep 20, 2011 at 5:43 PM, Arthur Flexser <flexser at fiu.edu> wrote:
>>
>> I'm going from hazy memory on this...
>>>
>>> I'm assuming the string is at a known address and has a known length.
>>>
>>> What you need to do is this:
>>>
>>> Define a dummy string in the Basic program with some statement like
>>> S$="A".
>>>
>>> Then use the Basic VARPTR function to find the address of Basic's
>>
>> pointer to the string:  ST=VARPTR(S$).
>>>
>>> At the address now contained in ST is a 5-byte string specifier.  The
>>> first (I think) byte is the string length, and the 3rd and 4th (?)
>>> contain the address of the string data.
>>> (The other 2 bytes of the 5-byte identifier are unused.)
>>>
>>> Using POKEs, you can modify these 3 bytes to insert the length and
>>> address of the string you want Basic to know about, which will then be
>>> referenced by S$.
>>>
>>> Hope that does the trick for you!
>>>
>>> Art
>>>
>>
>> Elaborating:
>>
>> PEEK(ST) will return the length of the string variable
>> PEEK(ST+2) will be the MSB of the variable first character address in
>> memory
>> PEEK(ST+3) will be the LSB of the variable first character address in
>> memory
>>
>> Examining RAM locations after the MSB & LSB are calculated up to the
>> length
>> of the string will reveal the variable contents.
>>
>> As noted, manipulating these values once a string variable has been
>> defined
>> we can put whatever we want in the string variable... as long as it is at
>> most 255 bytes long ;-)
>>
>>
>> -- RP
>>
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