With ON INTERRUPT it only checks for an interrupt condition and jumps to your interrupt
handler after each PBP command has finished. If you need fast reaction times to
interrupts, use DTs' interrupt code or use assembler interrupts.

FYI: With Timer1 in 16-bit read/write mode, you don't want to use EXT to create your word
to read/write Timer1. EXT works on the low byte first, then the high byte.

With RD16 set, to write to Timer1, you write to TMR1H first, then write to TMR1L. Reading
Timer1 with RD16 set, you read TMR1L first, then read TMR1H.

In 8-bit mode EXT will work fine. In 16-bit mode it won't since you're always writing to and
reading low byte first.