Hi Al,
PBP declares several internal variables as it needs them. Names are R0 - R8, T0 - T7, FLAGS, GOP, RM1, RM2, RR1, RR2, RS1, RS2, PRODL, TBLPTRU, TBLPTRH, TBLPTRL and perhaps some I've missed. Some get's declared in every program and some only depending on what your PBP code does.

This, of course, isn't something I've figured out myself - I've simply looked at Darrels Instant Interrupt code - specifically the ReEnterPBP18 file.

/Henrik.