[Coco] Superboard discussion: Serial EEPROM

[Bob]

--- Robert Gault <robert.gault at worldnet.att.net> wrote:
> Boisy G. Pitre wrote:
> > 
> ><snip>
> > I'm proposing an assembly language routine that will be in unused 
> > portions of the BASIC ROM.  One call will write a byte to a location, 
> > another call will read a byte from a location.
> > 
>  ><snip>
> 
> USR takes a single argument but don't confuse this with DEF FN. 
> USR/DEFUSR is able to take a variable or string as an argument as well 
> as a number. That means one could execute USR1("$FE20,$A0") and the 
> routine could then store $A0 at $FE20 or USR2("$FE20,$A0B0") and store 
> $A0B0 at $FE20.

Expanding on that, one could fill the entire ~190 available bytes by sending a
longer string. The USR routine can tell the length of the string as noted
below.

> 
> "Going Ahead with Extended Basic" indicates that, "For a string 
> argument, the X register points to a five-byte descriptor. The first 
> byte of the descriptor is the length of the string. The third and fourth 
> bytes of the descriptor contain the address of the first byte of the 
> string. The second and fifth bytes are reserved...." The reference gives 
> an example of returning a string to Basic so you could read as well as 
> write.
> 
> You could say A$="$FE20"+"" and USR3(A$) could return A$="$A0" or 
> perhaps A$="$FE20:$A0". The length of the string can be changed by the 
> USR call.

I'm thinking specify a start location and number of bytes to return in the
string.

> Using the +"" prevents the Basic program from being changed.
Now that is a fascinating tidbit that I was unaware of.
Thanks Robert, you da' man!

Bob



	
		
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