[Coco] DEF USR and strings?

[Sean Conner]

It was thus said that the Great Allen Huffman via Coco once stated:
> Is there a way to tell if a routine was called from USR versus EXEC?

  Maybe.

  Address $9D contains the address EXEC uses to jump to your code, so that
should be called address.  Also, X will also be this address (implementation
detail).

  For Color BASIC, you need to know you are running under Color BASIC. 
Address $112 is the address for USR, so this should point to your code. 
Also, upon calling, X should be equal to $AA2F and B should be 6 (both are
implementation details).

  For Extended Color BASIC, you need to know you are running under Extended
Color BASIC (16 bits at $8000 are $4558).  Addresses $013E through $0150
contain the USRn addresses, so one of these 10 addresses should point to
your code.  Also, A will equal the contents of address $06.  If A=0, then
X=$4F; if A=255, then X is pointing elsewhere (the string descriptor).

  For Disk Extended Color BASIC, you need to know you are running under Disk
Extended BASIC (16 bits at $C000 are $444B).  The USRn addresses are now
$095F through $0971, but other than that, it's the same as Extended Color
BASIC.

  Based on all that, I think the best method might be (completely untested):

	mycode		cmpx	#mycode
			beq	called_by_exec
		; otherwise, assume called by USR/USRn

  Good luck.

  -spc