[Coco] DEF USR and strings?

[Sean Conner]

It was thus said that the Great Allen Huffman via Coco once stated:
> > On Oct 29, 2025, at 8:52 PM, Sean Conner via Coco <coco at maltedmedia.com> wrote:
> > 
> > It was thus said that the Great Bruce Borer once stated:
> >> I do not know what can and cannot be done (yet) but can you pass the
> >> address (number) of the string and use the pointer in USRn to do what you
> >> want?
> > 
> >  Is the page I linked to:
> > 
> > 	https://boston.conman.org/2024/11/26.2-3
> > 
> > not clear enough?  It goes into extensive detail.
> > 
> >  -spc
> 
> This is excellent.
> 
> The sad thing is, I’ve seen this page before, but was not working on
> anything so I didn’t dive too deep.
> 
> Your research into these ROM routines is interesting. It sure makes things
> easy for you to do stuff like:
> 
> 110 X$ = USR0("ABCXYZ")
> 120 PRINT X$
> 
> I am surprised at how little code is needed thanks to those ROM routines
> already existing.

  Not only that, but the registers upon calling USRn (ECB) are (from my
notes):

	       USRn    Entry point to user-callable machine language subroutine
	Entry: A - 0   if paramater in FP0 is a number
	         - 255 if parameter in FP0 is a string descriptor
	       B - length if string descriptor
	       X - FP0EXP if A=0
	         - string descriptor if A=255
	       Zf - 1 if A = 0
                  - 0 if A = 255
	       Zn - 1 if A = 255
                  - 0 if A = 0
	       FP0 + _STRPTR - string descriptor if A=$FF
	       VALTYP - same as A
	
	Note:  ECB/DECB allow returning a string value.
	
	       Changing the characters of the string pointer is undefined; changing
	       the contents of the string descriptor is allowed.

  Because the flags reflect the value in A, there's no need to CMPA or TSTA
before jumping if supporting both a string or numeric input.  Also, INTCVT
does a type check, so calls to CHKNUM (a name I came up with) isn't required
if calling INTCVT.

  USR (from Color BASIC) doesn't set the registers with any meaningful
values, just FYI.

  -spc