Attempting to push any value comprised of two bytes which add to zero
in the 'calc' program doesn't work. For example: ff01, fe02, fd03,
etc.

This appears to resolve the issue:

--- a/projects/software/calc.tal
+++ b/projects/software/calc.tal
@@ -211,7 +211,7 @@ JMP2r

 @do-push ( -- )

-       .input/value LDZ2 ADD #00 GTH JMP JMP2r
+       .input/value LDZ2 #0000 GTH2 JMP JMP2r
        .stack/length LDZ #07 LTH JMP JMP2r
        #40 .Audio0/pitch DEO
        .input/value LDZ2 ;push JSR2

Regards.
Dave

Thanks Dave :)