!pr2
Booting ProDOS with a Modified Monitor ROM........Jan Eugenides

You may have already figured this out, but ProDOS won't boot if you have installed S. Knouse's modified ROM in your Apple.  This can easily be fixed, as follows:

!lm+5
!pp-3
On track 1, sector C, change bytes B4-B6 from AE B3 FB to A2 EA EA.  This tells ProDOS your machine is a II+.  If it's a //e, make B5 an A0 instead.
On track 1, sector 9, change bytes 60-61 from A9 00 to A5 0C.  This defeats the ROM check routine.
!lm-5
!pp0

Ta daaa!  Now ProDOS works just fine with your modified ROM.
